TOMOYO Linux Cross Reference
Linux/net/ipv4/tcp_output.c

   // SPDX-License-Identifier: GPL-2.0-only
   /*
    * INET         An implementation of the TCP/IP protocol suite for the LINUX
    *              operating system.  INET is implemented using the  BSD Socket
    *              interface as the means of communication with the user level.
    *
    *              Implementation of the Transmission Control Protocol(TCP).
    *
    * Authors:     Ross Biro
   *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
   *              Mark Evans, <evansmp@uhura.aston.ac.uk>
   *              Corey Minyard <wf-rch!minyard@relay.EU.net>
   *              Florian La Roche, <flla@stud.uni-sb.de>
   *              Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
   *              Linus Torvalds, <torvalds@cs.helsinki.fi>
   *              Alan Cox, <gw4pts@gw4pts.ampr.org>
   *              Matthew Dillon, <dillon@apollo.west.oic.com>
   *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
   *              Jorge Cwik, <jorge@laser.satlink.net>
   */
  
  /*
   * Changes:     Pedro Roque     :       Retransmit queue handled by TCP.
   *                              :       Fragmentation on mtu decrease
   *                              :       Segment collapse on retransmit
   *                              :       AF independence
   *
   *              Linus Torvalds  :       send_delayed_ack
   *              David S. Miller :       Charge memory using the right skb
   *                                      during syn/ack processing.
   *              David S. Miller :       Output engine completely rewritten.
   *              Andrea Arcangeli:       SYNACK carry ts_recent in tsecr.
   *              Cacophonix Gaul :       draft-minshall-nagle-01
   *              J Hadi Salim    :       ECN support
   *
   */
  
  #define pr_fmt(fmt) "TCP: " fmt
  
  #include <net/tcp.h>
  #include <net/mptcp.h>
  #include <net/proto_memory.h>
  
  #include <linux/compiler.h>
  #include <linux/gfp.h>
  #include <linux/module.h>
  #include <linux/static_key.h>
  #include <linux/skbuff_ref.h>
  
  #include <trace/events/tcp.h>
  
  /* Refresh clocks of a TCP socket,
   * ensuring monotically increasing values.
   */
  void tcp_mstamp_refresh(struct tcp_sock *tp)
  {
          u64 val = tcp_clock_ns();
  
          tp->tcp_clock_cache = val;
          tp->tcp_mstamp = div_u64(val, NSEC_PER_USEC);
  }
  
  static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                             int push_one, gfp_t gfp);
  
  /* Account for new data that has been sent to the network. */
  static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
  {
          struct inet_connection_sock *icsk = inet_csk(sk);
          struct tcp_sock *tp = tcp_sk(sk);
          unsigned int prior_packets = tp->packets_out;
  
          WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(skb)->end_seq);
  
          __skb_unlink(skb, &sk->sk_write_queue);
          tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);
  
          if (tp->highest_sack == NULL)
                  tp->highest_sack = skb;
  
          tp->packets_out += tcp_skb_pcount(skb);
          if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
                  tcp_rearm_rto(sk);
  
          NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT,
                        tcp_skb_pcount(skb));
          tcp_check_space(sk);
  }
  
  /* SND.NXT, if window was not shrunk or the amount of shrunk was less than one
   * window scaling factor due to loss of precision.
   * If window has been shrunk, what should we make? It is not clear at all.
   * Using SND.UNA we will fail to open window, SND.NXT is out of window. :-(
   * Anything in between SND.UNA...SND.UNA+SND.WND also can be already
   * invalid. OK, let's make this for now:
   */
  static inline __u32 tcp_acceptable_seq(const struct sock *sk)
  {
          const struct tcp_sock *tp = tcp_sk(sk);
 
         if (!before(tcp_wnd_end(tp), tp->snd_nxt) ||
             (tp->rx_opt.wscale_ok &&
              ((tp->snd_nxt - tcp_wnd_end(tp)) < (1 << tp->rx_opt.rcv_wscale))))
                 return tp->snd_nxt;
         else
                 return tcp_wnd_end(tp);
 }
 
 /* Calculate mss to advertise in SYN segment.
  * RFC1122, RFC1063, draft-ietf-tcpimpl-pmtud-01 state that:
  *
  * 1. It is independent of path mtu.
  * 2. Ideally, it is maximal possible segment size i.e. 65535-40.
  * 3. For IPv4 it is reasonable to calculate it from maximal MTU of
  *    attached devices, because some buggy hosts are confused by
  *    large MSS.
  * 4. We do not make 3, we advertise MSS, calculated from first
  *    hop device mtu, but allow to raise it to ip_rt_min_advmss.
  *    This may be overridden via information stored in routing table.
  * 5. Value 65535 for MSS is valid in IPv6 and means "as large as possible,
  *    probably even Jumbo".
  */
 static __u16 tcp_advertise_mss(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         const struct dst_entry *dst = __sk_dst_get(sk);
         int mss = tp->advmss;
 
         if (dst) {
                 unsigned int metric = dst_metric_advmss(dst);
 
                 if (metric < mss) {
                         mss = metric;
                         tp->advmss = mss;
                 }
         }
 
         return (__u16)mss;
 }
 
 /* RFC2861. Reset CWND after idle period longer RTO to "restart window".
  * This is the first part of cwnd validation mechanism.
  */
 void tcp_cwnd_restart(struct sock *sk, s32 delta)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         u32 restart_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
         u32 cwnd = tcp_snd_cwnd(tp);
 
         tcp_ca_event(sk, CA_EVENT_CWND_RESTART);
 
         tp->snd_ssthresh = tcp_current_ssthresh(sk);
         restart_cwnd = min(restart_cwnd, cwnd);
 
         while ((delta -= inet_csk(sk)->icsk_rto) > 0 && cwnd > restart_cwnd)
                 cwnd >>= 1;
         tcp_snd_cwnd_set(tp, max(cwnd, restart_cwnd));
         tp->snd_cwnd_stamp = tcp_jiffies32;
         tp->snd_cwnd_used = 0;
 }
 
 /* Congestion state accounting after a packet has been sent. */
 static void tcp_event_data_sent(struct tcp_sock *tp,
                                 struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         const u32 now = tcp_jiffies32;
 
         if (tcp_packets_in_flight(tp) == 0)
                 tcp_ca_event(sk, CA_EVENT_TX_START);
 
         tp->lsndtime = now;
 
         /* If it is a reply for ato after last received
          * packet, increase pingpong count.
          */
         if ((u32)(now - icsk->icsk_ack.lrcvtime) < icsk->icsk_ack.ato)
                 inet_csk_inc_pingpong_cnt(sk);
 }
 
 /* Account for an ACK we sent. */
 static inline void tcp_event_ack_sent(struct sock *sk, u32 rcv_nxt)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (unlikely(tp->compressed_ack)) {
                 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPACKCOMPRESSED,
                               tp->compressed_ack);
                 tp->compressed_ack = 0;
                 if (hrtimer_try_to_cancel(&tp->compressed_ack_timer) == 1)
                         __sock_put(sk);
         }
 
         if (unlikely(rcv_nxt != tp->rcv_nxt))
                 return;  /* Special ACK sent by DCTCP to reflect ECN */
         tcp_dec_quickack_mode(sk);
         inet_csk_clear_xmit_timer(sk, ICSK_TIME_DACK);
 }
 
 /* Determine a window scaling and initial window to offer.
  * Based on the assumption that the given amount of space
  * will be offered. Store the results in the tp structure.
  * NOTE: for smooth operation initial space offering should
  * be a multiple of mss if possible. We assume here that mss >= 1.
  * This MUST be enforced by all callers.
  */
 void tcp_select_initial_window(const struct sock *sk, int __space, __u32 mss,
                                __u32 *rcv_wnd, __u32 *__window_clamp,
                                int wscale_ok, __u8 *rcv_wscale,
                                __u32 init_rcv_wnd)
 {
         unsigned int space = (__space < 0 ? 0 : __space);
         u32 window_clamp = READ_ONCE(*__window_clamp);
 
         /* If no clamp set the clamp to the max possible scaled window */
         if (window_clamp == 0)
                 window_clamp = (U16_MAX << TCP_MAX_WSCALE);
         space = min(window_clamp, space);
 
         /* Quantize space offering to a multiple of mss if possible. */
         if (space > mss)
                 space = rounddown(space, mss);
 
         /* NOTE: offering an initial window larger than 32767
          * will break some buggy TCP stacks. If the admin tells us
          * it is likely we could be speaking with such a buggy stack
          * we will truncate our initial window offering to 32K-1
          * unless the remote has sent us a window scaling option,
          * which we interpret as a sign the remote TCP is not
          * misinterpreting the window field as a signed quantity.
          */
         if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_workaround_signed_windows))
                 (*rcv_wnd) = min(space, MAX_TCP_WINDOW);
         else
                 (*rcv_wnd) = space;
 
         if (init_rcv_wnd)
                 *rcv_wnd = min(*rcv_wnd, init_rcv_wnd * mss);
 
         *rcv_wscale = 0;
         if (wscale_ok) {
                 /* Set window scaling on max possible window */
                 space = max_t(u32, space, READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_rmem[2]));
                 space = max_t(u32, space, READ_ONCE(sysctl_rmem_max));
                 space = min_t(u32, space, window_clamp);
                 *rcv_wscale = clamp_t(int, ilog2(space) - 15,
                                       0, TCP_MAX_WSCALE);
         }
         /* Set the clamp no higher than max representable value */
         WRITE_ONCE(*__window_clamp,
                    min_t(__u32, U16_MAX << (*rcv_wscale), window_clamp));
 }
 EXPORT_SYMBOL(tcp_select_initial_window);
 
 /* Chose a new window to advertise, update state in tcp_sock for the
  * socket, and return result with RFC1323 scaling applied.  The return
  * value can be stuffed directly into th->window for an outgoing
  * frame.
  */
 static u16 tcp_select_window(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct net *net = sock_net(sk);
         u32 old_win = tp->rcv_wnd;
         u32 cur_win, new_win;
 
         /* Make the window 0 if we failed to queue the data because we
          * are out of memory. The window is temporary, so we don't store
          * it on the socket.
          */
         if (unlikely(inet_csk(sk)->icsk_ack.pending & ICSK_ACK_NOMEM))
                 return 0;
 
         cur_win = tcp_receive_window(tp);
         new_win = __tcp_select_window(sk);
         if (new_win < cur_win) {
                 /* Danger Will Robinson!
                  * Don't update rcv_wup/rcv_wnd here or else
                  * we will not be able to advertise a zero
                  * window in time.  --DaveM
                  *
                  * Relax Will Robinson.
                  */
                 if (!READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) || !tp->rx_opt.rcv_wscale) {
                         /* Never shrink the offered window */
                         if (new_win == 0)
                                 NET_INC_STATS(net, LINUX_MIB_TCPWANTZEROWINDOWADV);
                         new_win = ALIGN(cur_win, 1 << tp->rx_opt.rcv_wscale);
                 }
         }
 
         tp->rcv_wnd = new_win;
         tp->rcv_wup = tp->rcv_nxt;
 
         /* Make sure we do not exceed the maximum possible
          * scaled window.
          */
         if (!tp->rx_opt.rcv_wscale &&
             READ_ONCE(net->ipv4.sysctl_tcp_workaround_signed_windows))
                 new_win = min(new_win, MAX_TCP_WINDOW);
         else
                 new_win = min(new_win, (65535U << tp->rx_opt.rcv_wscale));
 
         /* RFC1323 scaling applied */
         new_win >>= tp->rx_opt.rcv_wscale;
 
         /* If we advertise zero window, disable fast path. */
         if (new_win == 0) {
                 tp->pred_flags = 0;
                 if (old_win)
                         NET_INC_STATS(net, LINUX_MIB_TCPTOZEROWINDOWADV);
         } else if (old_win == 0) {
                 NET_INC_STATS(net, LINUX_MIB_TCPFROMZEROWINDOWADV);
         }
 
         return new_win;
 }
 
 /* Packet ECN state for a SYN-ACK */
 static void tcp_ecn_send_synack(struct sock *sk, struct sk_buff *skb)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
 
         TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_CWR;
         if (!(tp->ecn_flags & TCP_ECN_OK))
                 TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_ECE;
         else if (tcp_ca_needs_ecn(sk) ||
                  tcp_bpf_ca_needs_ecn(sk))
                 INET_ECN_xmit(sk);
 }
 
 /* Packet ECN state for a SYN.  */
 static void tcp_ecn_send_syn(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         bool bpf_needs_ecn = tcp_bpf_ca_needs_ecn(sk);
         bool use_ecn = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn) == 1 ||
                 tcp_ca_needs_ecn(sk) || bpf_needs_ecn;
 
         if (!use_ecn) {
                 const struct dst_entry *dst = __sk_dst_get(sk);
 
                 if (dst && dst_feature(dst, RTAX_FEATURE_ECN))
                         use_ecn = true;
         }
 
         tp->ecn_flags = 0;
 
         if (use_ecn) {
                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ECE | TCPHDR_CWR;
                 tp->ecn_flags = TCP_ECN_OK;
                 if (tcp_ca_needs_ecn(sk) || bpf_needs_ecn)
                         INET_ECN_xmit(sk);
         }
 }
 
 static void tcp_ecn_clear_syn(struct sock *sk, struct sk_buff *skb)
 {
         if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_ecn_fallback))
                 /* tp->ecn_flags are cleared at a later point in time when
                  * SYN ACK is ultimatively being received.
                  */
                 TCP_SKB_CB(skb)->tcp_flags &= ~(TCPHDR_ECE | TCPHDR_CWR);
 }
 
 static void
 tcp_ecn_make_synack(const struct request_sock *req, struct tcphdr *th)
 {
         if (inet_rsk(req)->ecn_ok)
                 th->ece = 1;
 }
 
 /* Set up ECN state for a packet on a ESTABLISHED socket that is about to
  * be sent.
  */
 static void tcp_ecn_send(struct sock *sk, struct sk_buff *skb,
                          struct tcphdr *th, int tcp_header_len)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (tp->ecn_flags & TCP_ECN_OK) {
                 /* Not-retransmitted data segment: set ECT and inject CWR. */
                 if (skb->len != tcp_header_len &&
                     !before(TCP_SKB_CB(skb)->seq, tp->snd_nxt)) {
                         INET_ECN_xmit(sk);
                         if (tp->ecn_flags & TCP_ECN_QUEUE_CWR) {
                                 tp->ecn_flags &= ~TCP_ECN_QUEUE_CWR;
                                 th->cwr = 1;
                                 skb_shinfo(skb)->gso_type |= SKB_GSO_TCP_ECN;
                         }
                 } else if (!tcp_ca_needs_ecn(sk)) {
                         /* ACK or retransmitted segment: clear ECT|CE */
                         INET_ECN_dontxmit(sk);
                 }
                 if (tp->ecn_flags & TCP_ECN_DEMAND_CWR)
                         th->ece = 1;
         }
 }
 
 /* Constructs common control bits of non-data skb. If SYN/FIN is present,
  * auto increment end seqno.
  */
 static void tcp_init_nondata_skb(struct sk_buff *skb, u32 seq, u8 flags)
 {
         skb->ip_summed = CHECKSUM_PARTIAL;
 
         TCP_SKB_CB(skb)->tcp_flags = flags;
 
         tcp_skb_pcount_set(skb, 1);
 
         TCP_SKB_CB(skb)->seq = seq;
         if (flags & (TCPHDR_SYN | TCPHDR_FIN))
                 seq++;
         TCP_SKB_CB(skb)->end_seq = seq;
 }
 
 static inline bool tcp_urg_mode(const struct tcp_sock *tp)
 {
         return tp->snd_una != tp->snd_up;
 }
 
 #define OPTION_SACK_ADVERTISE   BIT(0)
 #define OPTION_TS               BIT(1)
 #define OPTION_MD5              BIT(2)
 #define OPTION_WSCALE           BIT(3)
 #define OPTION_FAST_OPEN_COOKIE BIT(8)
 #define OPTION_SMC              BIT(9)
 #define OPTION_MPTCP            BIT(10)
 #define OPTION_AO               BIT(11)
 
 static void smc_options_write(__be32 *ptr, u16 *options)
 {
 #if IS_ENABLED(CONFIG_SMC)
         if (static_branch_unlikely(&tcp_have_smc)) {
                 if (unlikely(OPTION_SMC & *options)) {
                         *ptr++ = htonl((TCPOPT_NOP  << 24) |
                                        (TCPOPT_NOP  << 16) |
                                        (TCPOPT_EXP <<  8) |
                                        (TCPOLEN_EXP_SMC_BASE));
                         *ptr++ = htonl(TCPOPT_SMC_MAGIC);
                 }
         }
 #endif
 }
 
 struct tcp_out_options {
         u16 options;            /* bit field of OPTION_* */
         u16 mss;                /* 0 to disable */
         u8 ws;                  /* window scale, 0 to disable */
         u8 num_sack_blocks;     /* number of SACK blocks to include */
         u8 hash_size;           /* bytes in hash_location */
         u8 bpf_opt_len;         /* length of BPF hdr option */
         __u8 *hash_location;    /* temporary pointer, overloaded */
         __u32 tsval, tsecr;     /* need to include OPTION_TS */
         struct tcp_fastopen_cookie *fastopen_cookie;    /* Fast open cookie */
         struct mptcp_out_options mptcp;
 };
 
 static void mptcp_options_write(struct tcphdr *th, __be32 *ptr,
                                 struct tcp_sock *tp,
                                 struct tcp_out_options *opts)
 {
 #if IS_ENABLED(CONFIG_MPTCP)
         if (unlikely(OPTION_MPTCP & opts->options))
                 mptcp_write_options(th, ptr, tp, &opts->mptcp);
 #endif
 }
 
 #ifdef CONFIG_CGROUP_BPF
 static int bpf_skops_write_hdr_opt_arg0(struct sk_buff *skb,
                                         enum tcp_synack_type synack_type)
 {
         if (unlikely(!skb))
                 return BPF_WRITE_HDR_TCP_CURRENT_MSS;
 
         if (unlikely(synack_type == TCP_SYNACK_COOKIE))
                 return BPF_WRITE_HDR_TCP_SYNACK_COOKIE;
 
         return 0;
 }
 
 /* req, syn_skb and synack_type are used when writing synack */
 static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
                                   struct request_sock *req,
                                   struct sk_buff *syn_skb,
                                   enum tcp_synack_type synack_type,
                                   struct tcp_out_options *opts,
                                   unsigned int *remaining)
 {
         struct bpf_sock_ops_kern sock_ops;
         int err;
 
         if (likely(!BPF_SOCK_OPS_TEST_FLAG(tcp_sk(sk),
                                            BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG)) ||
             !*remaining)
                 return;
 
         /* *remaining has already been aligned to 4 bytes, so *remaining >= 4 */
 
         /* init sock_ops */
         memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 
         sock_ops.op = BPF_SOCK_OPS_HDR_OPT_LEN_CB;
 
         if (req) {
                 /* The listen "sk" cannot be passed here because
                  * it is not locked.  It would not make too much
                  * sense to do bpf_setsockopt(listen_sk) based
                  * on individual connection request also.
                  *
                  * Thus, "req" is passed here and the cgroup-bpf-progs
                  * of the listen "sk" will be run.
                  *
                  * "req" is also used here for fastopen even the "sk" here is
                  * a fullsock "child" sk.  It is to keep the behavior
                  * consistent between fastopen and non-fastopen on
                  * the bpf programming side.
                  */
                 sock_ops.sk = (struct sock *)req;
                 sock_ops.syn_skb = syn_skb;
         } else {
                 sock_owned_by_me(sk);
 
                 sock_ops.is_fullsock = 1;
                 sock_ops.sk = sk;
         }
 
         sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
         sock_ops.remaining_opt_len = *remaining;
         /* tcp_current_mss() does not pass a skb */
         if (skb)
                 bpf_skops_init_skb(&sock_ops, skb, 0);
 
         err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
 
         if (err || sock_ops.remaining_opt_len == *remaining)
                 return;
 
         opts->bpf_opt_len = *remaining - sock_ops.remaining_opt_len;
         /* round up to 4 bytes */
         opts->bpf_opt_len = (opts->bpf_opt_len + 3) & ~3;
 
         *remaining -= opts->bpf_opt_len;
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
                                     struct request_sock *req,
                                     struct sk_buff *syn_skb,
                                     enum tcp_synack_type synack_type,
                                     struct tcp_out_options *opts)
 {
         u8 first_opt_off, nr_written, max_opt_len = opts->bpf_opt_len;
         struct bpf_sock_ops_kern sock_ops;
         int err;
 
         if (likely(!max_opt_len))
                 return;
 
         memset(&sock_ops, 0, offsetof(struct bpf_sock_ops_kern, temp));
 
         sock_ops.op = BPF_SOCK_OPS_WRITE_HDR_OPT_CB;
 
         if (req) {
                 sock_ops.sk = (struct sock *)req;
                 sock_ops.syn_skb = syn_skb;
         } else {
                 sock_owned_by_me(sk);
 
                 sock_ops.is_fullsock = 1;
                 sock_ops.sk = sk;
         }
 
         sock_ops.args[0] = bpf_skops_write_hdr_opt_arg0(skb, synack_type);
         sock_ops.remaining_opt_len = max_opt_len;
         first_opt_off = tcp_hdrlen(skb) - max_opt_len;
         bpf_skops_init_skb(&sock_ops, skb, first_opt_off);
 
         err = BPF_CGROUP_RUN_PROG_SOCK_OPS_SK(&sock_ops, sk);
 
         if (err)
                 nr_written = 0;
         else
                 nr_written = max_opt_len - sock_ops.remaining_opt_len;
 
         if (nr_written < max_opt_len)
                 memset(skb->data + first_opt_off + nr_written, TCPOPT_NOP,
                        max_opt_len - nr_written);
 }
 #else
 static void bpf_skops_hdr_opt_len(struct sock *sk, struct sk_buff *skb,
                                   struct request_sock *req,
                                   struct sk_buff *syn_skb,
                                   enum tcp_synack_type synack_type,
                                   struct tcp_out_options *opts,
                                   unsigned int *remaining)
 {
 }
 
 static void bpf_skops_write_hdr_opt(struct sock *sk, struct sk_buff *skb,
                                     struct request_sock *req,
                                     struct sk_buff *syn_skb,
                                     enum tcp_synack_type synack_type,
                                     struct tcp_out_options *opts)
 {
 }
 #endif
 
 static __be32 *process_tcp_ao_options(struct tcp_sock *tp,
                                       const struct tcp_request_sock *tcprsk,
                                       struct tcp_out_options *opts,
                                       struct tcp_key *key, __be32 *ptr)
 {
 #ifdef CONFIG_TCP_AO
         u8 maclen = tcp_ao_maclen(key->ao_key);
 
         if (tcprsk) {
                 u8 aolen = maclen + sizeof(struct tcp_ao_hdr);
 
                 *ptr++ = htonl((TCPOPT_AO << 24) | (aolen << 16) |
                                (tcprsk->ao_keyid << 8) |
                                (tcprsk->ao_rcv_next));
         } else {
                 struct tcp_ao_key *rnext_key;
                 struct tcp_ao_info *ao_info;
 
                 ao_info = rcu_dereference_check(tp->ao_info,
                         lockdep_sock_is_held(&tp->inet_conn.icsk_inet.sk));
                 rnext_key = READ_ONCE(ao_info->rnext_key);
                 if (WARN_ON_ONCE(!rnext_key))
                         return ptr;
                 *ptr++ = htonl((TCPOPT_AO << 24) |
                                (tcp_ao_len(key->ao_key) << 16) |
                                (key->ao_key->sndid << 8) |
                                (rnext_key->rcvid));
         }
         opts->hash_location = (__u8 *)ptr;
         ptr += maclen / sizeof(*ptr);
         if (unlikely(maclen % sizeof(*ptr))) {
                 memset(ptr, TCPOPT_NOP, sizeof(*ptr));
                 ptr++;
         }
 #endif
         return ptr;
 }
 
 /* Write previously computed TCP options to the packet.
  *
  * Beware: Something in the Internet is very sensitive to the ordering of
  * TCP options, we learned this through the hard way, so be careful here.
  * Luckily we can at least blame others for their non-compliance but from
  * inter-operability perspective it seems that we're somewhat stuck with
  * the ordering which we have been using if we want to keep working with
  * those broken things (not that it currently hurts anybody as there isn't
  * particular reason why the ordering would need to be changed).
  *
  * At least SACK_PERM as the first option is known to lead to a disaster
  * (but it may well be that other scenarios fail similarly).
  */
 static void tcp_options_write(struct tcphdr *th, struct tcp_sock *tp,
                               const struct tcp_request_sock *tcprsk,
                               struct tcp_out_options *opts,
                               struct tcp_key *key)
 {
         __be32 *ptr = (__be32 *)(th + 1);
         u16 options = opts->options;    /* mungable copy */
 
         if (tcp_key_is_md5(key)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) | (TCPOPT_NOP << 16) |
                                (TCPOPT_MD5SIG << 8) | TCPOLEN_MD5SIG);
                 /* overload cookie hash location */
                 opts->hash_location = (__u8 *)ptr;
                 ptr += 4;
         } else if (tcp_key_is_ao(key)) {
                 ptr = process_tcp_ao_options(tp, tcprsk, opts, key, ptr);
         }
         if (unlikely(opts->mss)) {
                 *ptr++ = htonl((TCPOPT_MSS << 24) |
                                (TCPOLEN_MSS << 16) |
                                opts->mss);
         }
 
         if (likely(OPTION_TS & options)) {
                 if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                         *ptr++ = htonl((TCPOPT_SACK_PERM << 24) |
                                        (TCPOLEN_SACK_PERM << 16) |
                                        (TCPOPT_TIMESTAMP << 8) |
                                        TCPOLEN_TIMESTAMP);
                         options &= ~OPTION_SACK_ADVERTISE;
                 } else {
                         *ptr++ = htonl((TCPOPT_NOP << 24) |
                                        (TCPOPT_NOP << 16) |
                                        (TCPOPT_TIMESTAMP << 8) |
                                        TCPOLEN_TIMESTAMP);
                 }
                 *ptr++ = htonl(opts->tsval);
                 *ptr++ = htonl(opts->tsecr);
         }
 
         if (unlikely(OPTION_SACK_ADVERTISE & options)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |
                                (TCPOPT_NOP << 16) |
                                (TCPOPT_SACK_PERM << 8) |
                                TCPOLEN_SACK_PERM);
         }
 
         if (unlikely(OPTION_WSCALE & options)) {
                 *ptr++ = htonl((TCPOPT_NOP << 24) |
                                (TCPOPT_WINDOW << 16) |
                                (TCPOLEN_WINDOW << 8) |
                                opts->ws);
         }
 
         if (unlikely(opts->num_sack_blocks)) {
                 struct tcp_sack_block *sp = tp->rx_opt.dsack ?
                         tp->duplicate_sack : tp->selective_acks;
                 int this_sack;
 
                 *ptr++ = htonl((TCPOPT_NOP  << 24) |
                                (TCPOPT_NOP  << 16) |
                                (TCPOPT_SACK <<  8) |
                                (TCPOLEN_SACK_BASE + (opts->num_sack_blocks *
                                                      TCPOLEN_SACK_PERBLOCK)));
 
                 for (this_sack = 0; this_sack < opts->num_sack_blocks;
                      ++this_sack) {
                         *ptr++ = htonl(sp[this_sack].start_seq);
                         *ptr++ = htonl(sp[this_sack].end_seq);
                 }
 
                 tp->rx_opt.dsack = 0;
         }
 
         if (unlikely(OPTION_FAST_OPEN_COOKIE & options)) {
                 struct tcp_fastopen_cookie *foc = opts->fastopen_cookie;
                 u8 *p = (u8 *)ptr;
                 u32 len; /* Fast Open option length */
 
                 if (foc->exp) {
                         len = TCPOLEN_EXP_FASTOPEN_BASE + foc->len;
                         *ptr = htonl((TCPOPT_EXP << 24) | (len << 16) |
                                      TCPOPT_FASTOPEN_MAGIC);
                         p += TCPOLEN_EXP_FASTOPEN_BASE;
                 } else {
                         len = TCPOLEN_FASTOPEN_BASE + foc->len;
                         *p++ = TCPOPT_FASTOPEN;
                         *p++ = len;
                 }
 
                 memcpy(p, foc->val, foc->len);
                 if ((len & 3) == 2) {
                         p[foc->len] = TCPOPT_NOP;
                         p[foc->len + 1] = TCPOPT_NOP;
                 }
                 ptr += (len + 3) >> 2;
         }
 
         smc_options_write(ptr, &options);
 
         mptcp_options_write(th, ptr, tp, opts);
 }
 
 static void smc_set_option(const struct tcp_sock *tp,
                            struct tcp_out_options *opts,
                            unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
         if (static_branch_unlikely(&tcp_have_smc)) {
                 if (tp->syn_smc) {
                         if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
                                 opts->options |= OPTION_SMC;
                                 *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
                         }
                 }
         }
 #endif
 }
 
 static void smc_set_option_cond(const struct tcp_sock *tp,
                                 const struct inet_request_sock *ireq,
                                 struct tcp_out_options *opts,
                                 unsigned int *remaining)
 {
 #if IS_ENABLED(CONFIG_SMC)
         if (static_branch_unlikely(&tcp_have_smc)) {
                 if (tp->syn_smc && ireq->smc_ok) {
                         if (*remaining >= TCPOLEN_EXP_SMC_BASE_ALIGNED) {
                                 opts->options |= OPTION_SMC;
                                 *remaining -= TCPOLEN_EXP_SMC_BASE_ALIGNED;
                         }
                 }
         }
 #endif
 }
 
 static void mptcp_set_option_cond(const struct request_sock *req,
                                   struct tcp_out_options *opts,
                                   unsigned int *remaining)
 {
         if (rsk_is_mptcp(req)) {
                 unsigned int size;
 
                 if (mptcp_synack_options(req, &size, &opts->mptcp)) {
                         if (*remaining >= size) {
                                 opts->options |= OPTION_MPTCP;
                                 *remaining -= size;
                         }
                 }
         }
 }
 
 /* Compute TCP options for SYN packets. This is not the final
  * network wire format yet.
  */
 static unsigned int tcp_syn_options(struct sock *sk, struct sk_buff *skb,
                                 struct tcp_out_options *opts,
                                 struct tcp_key *key)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         unsigned int remaining = MAX_TCP_OPTION_SPACE;
         struct tcp_fastopen_request *fastopen = tp->fastopen_req;
         bool timestamps;
 
         /* Better than switch (key.type) as it has static branches */
         if (tcp_key_is_md5(key)) {
                 timestamps = false;
                 opts->options |= OPTION_MD5;
                 remaining -= TCPOLEN_MD5SIG_ALIGNED;
         } else {
                 timestamps = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps);
                 if (tcp_key_is_ao(key)) {
                         opts->options |= OPTION_AO;
                         remaining -= tcp_ao_len_aligned(key->ao_key);
                 }
         }
 
         /* We always get an MSS option.  The option bytes which will be seen in
          * normal data packets should timestamps be used, must be in the MSS
          * advertised.  But we subtract them from tp->mss_cache so that
          * calculations in tcp_sendmsg are simpler etc.  So account for this
          * fact here if necessary.  If we don't do this correctly, as a
          * receiver we won't recognize data packets as being full sized when we
          * should, and thus we won't abide by the delayed ACK rules correctly.
          * SACKs don't matter, we never delay an ACK when we have any of those
          * going out.  */
         opts->mss = tcp_advertise_mss(sk);
         remaining -= TCPOLEN_MSS_ALIGNED;
 
         if (likely(timestamps)) {
                 opts->options |= OPTION_TS;
                 opts->tsval = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) + tp->tsoffset;
                 opts->tsecr = tp->rx_opt.ts_recent;
                 remaining -= TCPOLEN_TSTAMP_ALIGNED;
         }
         if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling))) {
                 opts->ws = tp->rx_opt.rcv_wscale;
                 opts->options |= OPTION_WSCALE;
                 remaining -= TCPOLEN_WSCALE_ALIGNED;
         }
         if (likely(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_sack))) {
                 opts->options |= OPTION_SACK_ADVERTISE;
                 if (unlikely(!(OPTION_TS & opts->options)))
                         remaining -= TCPOLEN_SACKPERM_ALIGNED;
         }
 
         if (fastopen && fastopen->cookie.len >= 0) {
                 u32 need = fastopen->cookie.len;
 
                 need += fastopen->cookie.exp ? TCPOLEN_EXP_FASTOPEN_BASE :
                                                TCPOLEN_FASTOPEN_BASE;
                 need = (need + 3) & ~3U;  /* Align to 32 bits */
                 if (remaining >= need) {
                         opts->options |= OPTION_FAST_OPEN_COOKIE;
                         opts->fastopen_cookie = &fastopen->cookie;
                         remaining -= need;
                         tp->syn_fastopen = 1;
                         tp->syn_fastopen_exp = fastopen->cookie.exp ? 1 : 0;
                 }
         }
 
         smc_set_option(tp, opts, &remaining);
 
         if (sk_is_mptcp(sk)) {
                 unsigned int size;
 
                 if (mptcp_syn_options(sk, skb, &size, &opts->mptcp)) {
                         opts->options |= OPTION_MPTCP;
                         remaining -= size;
                 }
         }
 
         bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
 
         return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Set up TCP options for SYN-ACKs. */
 static unsigned int tcp_synack_options(const struct sock *sk,
                                        struct request_sock *req,
                                        unsigned int mss, struct sk_buff *skb,
                                        struct tcp_out_options *opts,
                                        const struct tcp_key *key,
                                        struct tcp_fastopen_cookie *foc,
                                        enum tcp_synack_type synack_type,
                                        struct sk_buff *syn_skb)
 {
         struct inet_request_sock *ireq = inet_rsk(req);
         unsigned int remaining = MAX_TCP_OPTION_SPACE;
 
         if (tcp_key_is_md5(key)) {
                 opts->options |= OPTION_MD5;
                 remaining -= TCPOLEN_MD5SIG_ALIGNED;
 
                 /* We can't fit any SACK blocks in a packet with MD5 + TS
                  * options. There was discussion about disabling SACK
                  * rather than TS in order to fit in better with old,
                  * buggy kernels, but that was deemed to be unnecessary.
                  */
                 if (synack_type != TCP_SYNACK_COOKIE)
                         ireq->tstamp_ok &= !ireq->sack_ok;
         } else if (tcp_key_is_ao(key)) {
                 opts->options |= OPTION_AO;
                 remaining -= tcp_ao_len_aligned(key->ao_key);
                 ireq->tstamp_ok &= !ireq->sack_ok;
         }
 
         /* We always send an MSS option. */
         opts->mss = mss;
         remaining -= TCPOLEN_MSS_ALIGNED;
 
         if (likely(ireq->wscale_ok)) {
                 opts->ws = ireq->rcv_wscale;
                 opts->options |= OPTION_WSCALE;
                 remaining -= TCPOLEN_WSCALE_ALIGNED;
         }
         if (likely(ireq->tstamp_ok)) {
                 opts->options |= OPTION_TS;
                 opts->tsval = tcp_skb_timestamp_ts(tcp_rsk(req)->req_usec_ts, skb) +
                               tcp_rsk(req)->ts_off;
                 opts->tsecr = READ_ONCE(req->ts_recent);
                 remaining -= TCPOLEN_TSTAMP_ALIGNED;
         }
         if (likely(ireq->sack_ok)) {
                 opts->options |= OPTION_SACK_ADVERTISE;
                 if (unlikely(!ireq->tstamp_ok))
                         remaining -= TCPOLEN_SACKPERM_ALIGNED;
         }
         if (foc != NULL && foc->len >= 0) {
                 u32 need = foc->len;
 
                 need += foc->exp ? TCPOLEN_EXP_FASTOPEN_BASE :
                                    TCPOLEN_FASTOPEN_BASE;
                 need = (need + 3) & ~3U;  /* Align to 32 bits */
                 if (remaining >= need) {
                         opts->options |= OPTION_FAST_OPEN_COOKIE;
                         opts->fastopen_cookie = foc;
                         remaining -= need;
                 }
         }
 
         mptcp_set_option_cond(req, opts, &remaining);
 
         smc_set_option_cond(tcp_sk(sk), ireq, opts, &remaining);
 
         bpf_skops_hdr_opt_len((struct sock *)sk, skb, req, syn_skb,
                               synack_type, opts, &remaining);
 
         return MAX_TCP_OPTION_SPACE - remaining;
 }
 
 /* Compute TCP options for ESTABLISHED sockets. This is not the
  * final wire format yet.
  */
 static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb,
                                         struct tcp_out_options *opts,
                                         struct tcp_key *key)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         unsigned int size = 0;
         unsigned int eff_sacks;
 
         opts->options = 0;
 
         /* Better than switch (key.type) as it has static branches */
         if (tcp_key_is_md5(key)) {
                 opts->options |= OPTION_MD5;
                 size += TCPOLEN_MD5SIG_ALIGNED;
         } else if (tcp_key_is_ao(key)) {
                 opts->options |= OPTION_AO;
                 size += tcp_ao_len_aligned(key->ao_key);
         }
 
         if (likely(tp->rx_opt.tstamp_ok)) {
                 opts->options |= OPTION_TS;
                 opts->tsval = skb ? tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb) +
                                 tp->tsoffset : 0;
                 opts->tsecr = tp->rx_opt.ts_recent;
                 size += TCPOLEN_TSTAMP_ALIGNED;
         }
 
         /* MPTCP options have precedence over SACK for the limited TCP
          * option space because a MPTCP connection would be forced to
          * fall back to regular TCP if a required multipath option is
          * missing. SACK still gets a chance to use whatever space is
          * left.
          */
         if (sk_is_mptcp(sk)) {
                 unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
                 unsigned int opt_size = 0;
 
                 if (mptcp_established_options(sk, skb, &opt_size, remaining,
                                               &opts->mptcp)) {
                         opts->options |= OPTION_MPTCP;
                         size += opt_size;
                 }
         }
 
         eff_sacks = tp->rx_opt.num_sacks + tp->rx_opt.dsack;
         if (unlikely(eff_sacks)) {
                 const unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
                 if (unlikely(remaining < TCPOLEN_SACK_BASE_ALIGNED +
                                          TCPOLEN_SACK_PERBLOCK))
                         return size;
 
                 opts->num_sack_blocks =
                         min_t(unsigned int, eff_sacks,
                               (remaining - TCPOLEN_SACK_BASE_ALIGNED) /
                               TCPOLEN_SACK_PERBLOCK);
 
                 size += TCPOLEN_SACK_BASE_ALIGNED +
                         opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
         }
 
         if (unlikely(BPF_SOCK_OPS_TEST_FLAG(tp,
                                             BPF_SOCK_OPS_WRITE_HDR_OPT_CB_FLAG))) {
                 unsigned int remaining = MAX_TCP_OPTION_SPACE - size;
 
                 bpf_skops_hdr_opt_len(sk, skb, NULL, NULL, 0, opts, &remaining);
 
                 size = MAX_TCP_OPTION_SPACE - remaining;
         }
 
         return size;
 }
 
 
 /* TCP SMALL QUEUES (TSQ)
  *
  * TSQ goal is to keep small amount of skbs per tcp flow in tx queues (qdisc+dev)
  * to reduce RTT and bufferbloat.
  * We do this using a special skb destructor (tcp_wfree).
  *
  * Its important tcp_wfree() can be replaced by sock_wfree() in the event skb
  * needs to be reallocated in a driver.
  * The invariant being skb->truesize subtracted from sk->sk_wmem_alloc
  *
  * Since transmit from skb destructor is forbidden, we use a tasklet
  * to process all sockets that eventually need to send more skbs.
  * We use one tasklet per cpu, with its own queue of sockets.
  */
 struct tsq_tasklet {
         struct tasklet_struct   tasklet;
         struct list_head        head; /* queue of tcp sockets */
 };
 static DEFINE_PER_CPU(struct tsq_tasklet, tsq_tasklet);
 
 static void tcp_tsq_write(struct sock *sk)
 {
         if ((1 << sk->sk_state) &
             (TCPF_ESTABLISHED | TCPF_FIN_WAIT1 | TCPF_CLOSING |
              TCPF_CLOSE_WAIT  | TCPF_LAST_ACK)) {
                 struct tcp_sock *tp = tcp_sk(sk);
 
                 if (tp->lost_out > tp->retrans_out &&
                     tcp_snd_cwnd(tp) > tcp_packets_in_flight(tp)) {
                         tcp_mstamp_refresh(tp);
                         tcp_xmit_retransmit_queue(sk);
                 }
 
                 tcp_write_xmit(sk, tcp_current_mss(sk), tp->nonagle,
                                0, GFP_ATOMIC);
         }
 }
 
 static void tcp_tsq_handler(struct sock *sk)
 {
         bh_lock_sock(sk);
         if (!sock_owned_by_user(sk))
                 tcp_tsq_write(sk);
         else if (!test_and_set_bit(TCP_TSQ_DEFERRED, &sk->sk_tsq_flags))
                 sock_hold(sk);
         bh_unlock_sock(sk);
 }
 /*
  * One tasklet per cpu tries to send more skbs.
  * We run in tasklet context but need to disable irqs when
  * transferring tsq->head because tcp_wfree() might
  * interrupt us (non NAPI drivers)
  */
 static void tcp_tasklet_func(struct tasklet_struct *t)
 {
         struct tsq_tasklet *tsq = from_tasklet(tsq,  t, tasklet);
         LIST_HEAD(list);
         unsigned long flags;
         struct list_head *q, *n;
         struct tcp_sock *tp;
         struct sock *sk;
 
         local_irq_save(flags);
         list_splice_init(&tsq->head, &list);
         local_irq_restore(flags);
 
         list_for_each_safe(q, n, &list) {
                 tp = list_entry(q, struct tcp_sock, tsq_node);
                 list_del(&tp->tsq_node);
 
                 sk = (struct sock *)tp;
                 smp_mb__before_atomic();
                 clear_bit(TSQ_QUEUED, &sk->sk_tsq_flags);
 
                 tcp_tsq_handler(sk);
                 sk_free(sk);
         }
 }
 
 #define TCP_DEFERRED_ALL (TCPF_TSQ_DEFERRED |           \
                           TCPF_WRITE_TIMER_DEFERRED |   \
                           TCPF_DELACK_TIMER_DEFERRED |  \
                           TCPF_MTU_REDUCED_DEFERRED |   \
                           TCPF_ACK_DEFERRED)
 /**
  * tcp_release_cb - tcp release_sock() callback
  * @sk: socket
  *
  * called from release_sock() to perform protocol dependent
  * actions before socket release.
  */
 void tcp_release_cb(struct sock *sk)
 {
         unsigned long flags = smp_load_acquire(&sk->sk_tsq_flags);
         unsigned long nflags;
 
         /* perform an atomic operation only if at least one flag is set */
         do {
                 if (!(flags & TCP_DEFERRED_ALL))
                         return;
                 nflags = flags & ~TCP_DEFERRED_ALL;
         } while (!try_cmpxchg(&sk->sk_tsq_flags, &flags, nflags));
 
         if (flags & TCPF_TSQ_DEFERRED) {
                 tcp_tsq_write(sk);
                 __sock_put(sk);
         }
 
         if (flags & TCPF_WRITE_TIMER_DEFERRED) {
                 tcp_write_timer_handler(sk);
                 __sock_put(sk);
         }
         if (flags & TCPF_DELACK_TIMER_DEFERRED) {
                 tcp_delack_timer_handler(sk);
                 __sock_put(sk);
         }
         if (flags & TCPF_MTU_REDUCED_DEFERRED) {
                 inet_csk(sk)->icsk_af_ops->mtu_reduced(sk);
                 __sock_put(sk);
         }
         if ((flags & TCPF_ACK_DEFERRED) && inet_csk_ack_scheduled(sk))
                 tcp_send_ack(sk);
 }
 EXPORT_SYMBOL(tcp_release_cb);
 
 void __init tcp_tasklet_init(void)
 {
         int i;
 
         for_each_possible_cpu(i) {
                 struct tsq_tasklet *tsq = &per_cpu(tsq_tasklet, i);
 
                 INIT_LIST_HEAD(&tsq->head);
                 tasklet_setup(&tsq->tasklet, tcp_tasklet_func);
         }
 }
 
 /*
  * Write buffer destructor automatically called from kfree_skb.
  * We can't xmit new skbs from this context, as we might already
  * hold qdisc lock.
  */
 void tcp_wfree(struct sk_buff *skb)
 {
         struct sock *sk = skb->sk;
         struct tcp_sock *tp = tcp_sk(sk);
         unsigned long flags, nval, oval;
         struct tsq_tasklet *tsq;
         bool empty;
 
         /* Keep one reference on sk_wmem_alloc.
          * Will be released by sk_free() from here or tcp_tasklet_func()
          */
         WARN_ON(refcount_sub_and_test(skb->truesize - 1, &sk->sk_wmem_alloc));
 
         /* If this softirq is serviced by ksoftirqd, we are likely under stress.
          * Wait until our queues (qdisc + devices) are drained.
          * This gives :
          * - less callbacks to tcp_write_xmit(), reducing stress (batches)
          * - chance for incoming ACK (processed by another cpu maybe)
          *   to migrate this flow (skb->ooo_okay will be eventually set)
          */
         if (refcount_read(&sk->sk_wmem_alloc) >= SKB_TRUESIZE(1) && this_cpu_ksoftirqd() == current)
                 goto out;
 
         oval = smp_load_acquire(&sk->sk_tsq_flags);
         do {
                 if (!(oval & TSQF_THROTTLED) || (oval & TSQF_QUEUED))
                         goto out;
 
                 nval = (oval & ~TSQF_THROTTLED) | TSQF_QUEUED;
         } while (!try_cmpxchg(&sk->sk_tsq_flags, &oval, nval));
 
         /* queue this socket to tasklet queue */
         local_irq_save(flags);
         tsq = this_cpu_ptr(&tsq_tasklet);
         empty = list_empty(&tsq->head);
         list_add(&tp->tsq_node, &tsq->head);
         if (empty)
                 tasklet_schedule(&tsq->tasklet);
         local_irq_restore(flags);
         return;
 out:
         sk_free(sk);
 }
 
 /* Note: Called under soft irq.
  * We can call TCP stack right away, unless socket is owned by user.
  */
 enum hrtimer_restart tcp_pace_kick(struct hrtimer *timer)
 {
         struct tcp_sock *tp = container_of(timer, struct tcp_sock, pacing_timer);
         struct sock *sk = (struct sock *)tp;
 
         tcp_tsq_handler(sk);
         sock_put(sk);
 
         return HRTIMER_NORESTART;
 }
 
 static void tcp_update_skb_after_send(struct sock *sk, struct sk_buff *skb,
                                       u64 prior_wstamp)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (sk->sk_pacing_status != SK_PACING_NONE) {
                 unsigned long rate = READ_ONCE(sk->sk_pacing_rate);
 
                 /* Original sch_fq does not pace first 10 MSS
                  * Note that tp->data_segs_out overflows after 2^32 packets,
                  * this is a minor annoyance.
                  */
                 if (rate != ~0UL && rate && tp->data_segs_out >= 10) {
                         u64 len_ns = div64_ul((u64)skb->len * NSEC_PER_SEC, rate);
                         u64 credit = tp->tcp_wstamp_ns - prior_wstamp;
 
                         /* take into account OS jitter */
                         len_ns -= min_t(u64, len_ns / 2, credit);
                         tp->tcp_wstamp_ns += len_ns;
                 }
         }
         list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
 }
 
 INDIRECT_CALLABLE_DECLARE(int ip_queue_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
 INDIRECT_CALLABLE_DECLARE(int inet6_csk_xmit(struct sock *sk, struct sk_buff *skb, struct flowi *fl));
 INDIRECT_CALLABLE_DECLARE(void tcp_v4_send_check(struct sock *sk, struct sk_buff *skb));
 
 /* This routine actually transmits TCP packets queued in by
  * tcp_do_sendmsg().  This is used by both the initial
  * transmission and possible later retransmissions.
  * All SKB's seen here are completely headerless.  It is our
  * job to build the TCP header, and pass the packet down to
  * IP so it can do the same plus pass the packet off to the
  * device.
  *
  * We are working here with either a clone of the original
  * SKB, or a fresh unique copy made by the retransmit engine.
  */
 static int __tcp_transmit_skb(struct sock *sk, struct sk_buff *skb,
                               int clone_it, gfp_t gfp_mask, u32 rcv_nxt)
 {
         const struct inet_connection_sock *icsk = inet_csk(sk);
         struct inet_sock *inet;
         struct tcp_sock *tp;
         struct tcp_skb_cb *tcb;
         struct tcp_out_options opts;
         unsigned int tcp_options_size, tcp_header_size;
         struct sk_buff *oskb = NULL;
         struct tcp_key key;
         struct tcphdr *th;
         u64 prior_wstamp;
         int err;
 
         BUG_ON(!skb || !tcp_skb_pcount(skb));
         tp = tcp_sk(sk);
         prior_wstamp = tp->tcp_wstamp_ns;
         tp->tcp_wstamp_ns = max(tp->tcp_wstamp_ns, tp->tcp_clock_cache);
         skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
         if (clone_it) {
                 oskb = skb;
 
                 tcp_skb_tsorted_save(oskb) {
                         if (unlikely(skb_cloned(oskb)))
                                 skb = pskb_copy(oskb, gfp_mask);
                         else
                                 skb = skb_clone(oskb, gfp_mask);
                 } tcp_skb_tsorted_restore(oskb);
 
                 if (unlikely(!skb))
                         return -ENOBUFS;
                 /* retransmit skbs might have a non zero value in skb->dev
                  * because skb->dev is aliased with skb->rbnode.rb_left
                  */
                 skb->dev = NULL;
         }
 
         inet = inet_sk(sk);
         tcb = TCP_SKB_CB(skb);
         memset(&opts, 0, sizeof(opts));
 
         tcp_get_current_key(sk, &key);
         if (unlikely(tcb->tcp_flags & TCPHDR_SYN)) {
                 tcp_options_size = tcp_syn_options(sk, skb, &opts, &key);
         } else {
                 tcp_options_size = tcp_established_options(sk, skb, &opts, &key);
                 /* Force a PSH flag on all (GSO) packets to expedite GRO flush
                  * at receiver : This slightly improve GRO performance.
                  * Note that we do not force the PSH flag for non GSO packets,
                  * because they might be sent under high congestion events,
                  * and in this case it is better to delay the delivery of 1-MSS
                  * packets and thus the corresponding ACK packet that would
                  * release the following packet.
                  */
                 if (tcp_skb_pcount(skb) > 1)
                         tcb->tcp_flags |= TCPHDR_PSH;
         }
         tcp_header_size = tcp_options_size + sizeof(struct tcphdr);
 
         /* We set skb->ooo_okay to one if this packet can select
          * a different TX queue than prior packets of this flow,
          * to avoid self inflicted reorders.
          * The 'other' queue decision is based on current cpu number
          * if XPS is enabled, or sk->sk_txhash otherwise.
          * We can switch to another (and better) queue if:
          * 1) No packet with payload is in qdisc/device queues.
          *    Delays in TX completion can defeat the test
          *    even if packets were already sent.
          * 2) Or rtx queue is empty.
          *    This mitigates above case if ACK packets for
          *    all prior packets were already processed.
          */
         skb->ooo_okay = sk_wmem_alloc_get(sk) < SKB_TRUESIZE(1) ||
                         tcp_rtx_queue_empty(sk);
 
         /* If we had to use memory reserve to allocate this skb,
          * this might cause drops if packet is looped back :
          * Other socket might not have SOCK_MEMALLOC.
          * Packets not looped back do not care about pfmemalloc.
          */
         skb->pfmemalloc = 0;
 
         skb_push(skb, tcp_header_size);
         skb_reset_transport_header(skb);
 
         skb_orphan(skb);
         skb->sk = sk;
         skb->destructor = skb_is_tcp_pure_ack(skb) ? __sock_wfree : tcp_wfree;
         refcount_add(skb->truesize, &sk->sk_wmem_alloc);
 
         skb_set_dst_pending_confirm(skb, READ_ONCE(sk->sk_dst_pending_confirm));
 
         /* Build TCP header and checksum it. */
         th = (struct tcphdr *)skb->data;
         th->source              = inet->inet_sport;
         th->dest                = inet->inet_dport;
         th->seq                 = htonl(tcb->seq);
         th->ack_seq             = htonl(rcv_nxt);
         *(((__be16 *)th) + 6)   = htons(((tcp_header_size >> 2) << 12) |
                                         tcb->tcp_flags);
 
         th->check               = 0;
         th->urg_ptr             = 0;
 
         /* The urg_mode check is necessary during a below snd_una win probe */
         if (unlikely(tcp_urg_mode(tp) && before(tcb->seq, tp->snd_up))) {
                 if (before(tp->snd_up, tcb->seq + 0x10000)) {
                         th->urg_ptr = htons(tp->snd_up - tcb->seq);
                         th->urg = 1;
                 } else if (after(tcb->seq + 0xFFFF, tp->snd_nxt)) {
                         th->urg_ptr = htons(0xFFFF);
                         th->urg = 1;
                 }
         }
 
         skb_shinfo(skb)->gso_type = sk->sk_gso_type;
         if (likely(!(tcb->tcp_flags & TCPHDR_SYN))) {
                 th->window      = htons(tcp_select_window(sk));
                 tcp_ecn_send(sk, skb, th, tcp_header_size);
         } else {
                 /* RFC1323: The window in SYN & SYN/ACK segments
                  * is never scaled.
                  */
                 th->window      = htons(min(tp->rcv_wnd, 65535U));
         }
 
         tcp_options_write(th, tp, NULL, &opts, &key);
 
         if (tcp_key_is_md5(&key)) {
 #ifdef CONFIG_TCP_MD5SIG
                 /* Calculate the MD5 hash, as we have all we need now */
                 sk_gso_disable(sk);
                 tp->af_specific->calc_md5_hash(opts.hash_location,
                                                key.md5_key, sk, skb);
 #endif
         } else if (tcp_key_is_ao(&key)) {
                 int err;
 
                 err = tcp_ao_transmit_skb(sk, skb, key.ao_key, th,
                                           opts.hash_location);
                 if (err) {
                         kfree_skb_reason(skb, SKB_DROP_REASON_NOT_SPECIFIED);
                         return -ENOMEM;
                 }
         }
 
         /* BPF prog is the last one writing header option */
         bpf_skops_write_hdr_opt(sk, skb, NULL, NULL, 0, &opts);
 
         INDIRECT_CALL_INET(icsk->icsk_af_ops->send_check,
                            tcp_v6_send_check, tcp_v4_send_check,
                            sk, skb);
 
         if (likely(tcb->tcp_flags & TCPHDR_ACK))
                 tcp_event_ack_sent(sk, rcv_nxt);
 
         if (skb->len != tcp_header_size) {
                 tcp_event_data_sent(tp, sk);
                 tp->data_segs_out += tcp_skb_pcount(skb);
                 tp->bytes_sent += skb->len - tcp_header_size;
         }
 
         if (after(tcb->end_seq, tp->snd_nxt) || tcb->seq == tcb->end_seq)
                 TCP_ADD_STATS(sock_net(sk), TCP_MIB_OUTSEGS,
                               tcp_skb_pcount(skb));
 
         tp->segs_out += tcp_skb_pcount(skb);
         skb_set_hash_from_sk(skb, sk);
         /* OK, its time to fill skb_shinfo(skb)->gso_{segs|size} */
         skb_shinfo(skb)->gso_segs = tcp_skb_pcount(skb);
         skb_shinfo(skb)->gso_size = tcp_skb_mss(skb);
 
         /* Leave earliest departure time in skb->tstamp (skb->skb_mstamp_ns) */
 
         /* Cleanup our debris for IP stacks */
         memset(skb->cb, 0, max(sizeof(struct inet_skb_parm),
                                sizeof(struct inet6_skb_parm)));
 
         tcp_add_tx_delay(skb, tp);
 
         err = INDIRECT_CALL_INET(icsk->icsk_af_ops->queue_xmit,
                                  inet6_csk_xmit, ip_queue_xmit,
                                  sk, skb, &inet->cork.fl);
 
         if (unlikely(err > 0)) {
                 tcp_enter_cwr(sk);
                 err = net_xmit_eval(err);
         }
         if (!err && oskb) {
                 tcp_update_skb_after_send(sk, oskb, prior_wstamp);
                 tcp_rate_skb_sent(sk, oskb);
         }
         return err;
 }
 
 static int tcp_transmit_skb(struct sock *sk, struct sk_buff *skb, int clone_it,
                             gfp_t gfp_mask)
 {
         return __tcp_transmit_skb(sk, skb, clone_it, gfp_mask,
                                   tcp_sk(sk)->rcv_nxt);
 }
 
 /* This routine just queues the buffer for sending.
  *
  * NOTE: probe0 timer is not checked, do not forget tcp_push_pending_frames,
  * otherwise socket can stall.
  */
 static void tcp_queue_skb(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         /* Advance write_seq and place onto the write_queue. */
         WRITE_ONCE(tp->write_seq, TCP_SKB_CB(skb)->end_seq);
         __skb_header_release(skb);
         tcp_add_write_queue_tail(sk, skb);
         sk_wmem_queued_add(sk, skb->truesize);
         sk_mem_charge(sk, skb->truesize);
 }
 
 /* Initialize TSO segments for a packet. */
 static int tcp_set_skb_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
         int tso_segs;
 
         if (skb->len <= mss_now) {
                 /* Avoid the costly divide in the normal
                  * non-TSO case.
                  */
                 TCP_SKB_CB(skb)->tcp_gso_size = 0;
                 tcp_skb_pcount_set(skb, 1);
                 return 1;
         }
         TCP_SKB_CB(skb)->tcp_gso_size = mss_now;
         tso_segs = DIV_ROUND_UP(skb->len, mss_now);
         tcp_skb_pcount_set(skb, tso_segs);
         return tso_segs;
 }
 
 /* Pcount in the middle of the write queue got changed, we need to do various
  * tweaks to fix counters
  */
 static void tcp_adjust_pcount(struct sock *sk, const struct sk_buff *skb, int decr)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         tp->packets_out -= decr;
 
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                 tp->sacked_out -= decr;
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS)
                 tp->retrans_out -= decr;
         if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
                 tp->lost_out -= decr;
 
         /* Reno case is special. Sigh... */
         if (tcp_is_reno(tp) && decr > 0)
                 tp->sacked_out -= min_t(u32, tp->sacked_out, decr);
 
         if (tp->lost_skb_hint &&
             before(TCP_SKB_CB(skb)->seq, TCP_SKB_CB(tp->lost_skb_hint)->seq) &&
             (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
                 tp->lost_cnt_hint -= decr;
 
         tcp_verify_left_out(tp);
 }
 
 static bool tcp_has_tx_tstamp(const struct sk_buff *skb)
 {
         return TCP_SKB_CB(skb)->txstamp_ack ||
                 (skb_shinfo(skb)->tx_flags & SKBTX_ANY_TSTAMP);
 }
 
 static void tcp_fragment_tstamp(struct sk_buff *skb, struct sk_buff *skb2)
 {
         struct skb_shared_info *shinfo = skb_shinfo(skb);
 
         if (unlikely(tcp_has_tx_tstamp(skb)) &&
             !before(shinfo->tskey, TCP_SKB_CB(skb2)->seq)) {
                 struct skb_shared_info *shinfo2 = skb_shinfo(skb2);
                 u8 tsflags = shinfo->tx_flags & SKBTX_ANY_TSTAMP;
 
                 shinfo->tx_flags &= ~tsflags;
                 shinfo2->tx_flags |= tsflags;
                 swap(shinfo->tskey, shinfo2->tskey);
                 TCP_SKB_CB(skb2)->txstamp_ack = TCP_SKB_CB(skb)->txstamp_ack;
                 TCP_SKB_CB(skb)->txstamp_ack = 0;
         }
 }
 
 static void tcp_skb_fragment_eor(struct sk_buff *skb, struct sk_buff *skb2)
 {
         TCP_SKB_CB(skb2)->eor = TCP_SKB_CB(skb)->eor;
         TCP_SKB_CB(skb)->eor = 0;
 }
 
 /* Insert buff after skb on the write or rtx queue of sk.  */
 static void tcp_insert_write_queue_after(struct sk_buff *skb,
                                          struct sk_buff *buff,
                                          struct sock *sk,
                                          enum tcp_queue tcp_queue)
 {
         if (tcp_queue == TCP_FRAG_IN_WRITE_QUEUE)
                 __skb_queue_after(&sk->sk_write_queue, skb, buff);
         else
                 tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
 }
 
 /* Function to create two new TCP segments.  Shrinks the given segment
  * to the specified size and appends a new segment with the rest of the
  * packet to the list.  This won't be called frequently, I hope.
  * Remember, these are still headerless SKBs at this point.
  */
 int tcp_fragment(struct sock *sk, enum tcp_queue tcp_queue,
                  struct sk_buff *skb, u32 len,
                  unsigned int mss_now, gfp_t gfp)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *buff;
         int old_factor;
         long limit;
         int nlen;
         u8 flags;
 
         if (WARN_ON(len > skb->len))
                 return -EINVAL;
 
         DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
 
         /* tcp_sendmsg() can overshoot sk_wmem_queued by one full size skb.
          * We need some allowance to not penalize applications setting small
          * SO_SNDBUF values.
          * Also allow first and last skb in retransmit queue to be split.
          */
         limit = sk->sk_sndbuf + 2 * SKB_TRUESIZE(GSO_LEGACY_MAX_SIZE);
         if (unlikely((sk->sk_wmem_queued >> 1) > limit &&
                      tcp_queue != TCP_FRAG_IN_WRITE_QUEUE &&
                      skb != tcp_rtx_queue_head(sk) &&
                      skb != tcp_rtx_queue_tail(sk))) {
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPWQUEUETOOBIG);
                 return -ENOMEM;
         }
 
         if (skb_unclone_keeptruesize(skb, gfp))
                 return -ENOMEM;
 
         /* Get a new skb... force flag on. */
         buff = tcp_stream_alloc_skb(sk, gfp, true);
         if (!buff)
                 return -ENOMEM; /* We'll just try again later. */
         skb_copy_decrypted(buff, skb);
         mptcp_skb_ext_copy(buff, skb);
 
         sk_wmem_queued_add(sk, buff->truesize);
         sk_mem_charge(sk, buff->truesize);
         nlen = skb->len - len;
         buff->truesize += nlen;
         skb->truesize -= nlen;
 
         /* Correct the sequence numbers. */
         TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
         TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
 
         /* PSH and FIN should only be set in the second packet. */
         flags = TCP_SKB_CB(skb)->tcp_flags;
         TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
         TCP_SKB_CB(buff)->tcp_flags = flags;
         TCP_SKB_CB(buff)->sacked = TCP_SKB_CB(skb)->sacked;
         tcp_skb_fragment_eor(skb, buff);
 
         skb_split(skb, buff, len);
 
         skb_set_delivery_time(buff, skb->tstamp, SKB_CLOCK_MONOTONIC);
         tcp_fragment_tstamp(skb, buff);
 
         old_factor = tcp_skb_pcount(skb);
 
         /* Fix up tso_factor for both original and new SKB.  */
         tcp_set_skb_tso_segs(skb, mss_now);
         tcp_set_skb_tso_segs(buff, mss_now);
 
         /* Update delivered info for the new segment */
         TCP_SKB_CB(buff)->tx = TCP_SKB_CB(skb)->tx;
 
         /* If this packet has been sent out already, we must
          * adjust the various packet counters.
          */
         if (!before(tp->snd_nxt, TCP_SKB_CB(buff)->end_seq)) {
                 int diff = old_factor - tcp_skb_pcount(skb) -
                         tcp_skb_pcount(buff);
 
                 if (diff)
                         tcp_adjust_pcount(sk, skb, diff);
         }
 
         /* Link BUFF into the send queue. */
         __skb_header_release(buff);
         tcp_insert_write_queue_after(skb, buff, sk, tcp_queue);
         if (tcp_queue == TCP_FRAG_IN_RTX_QUEUE)
                 list_add(&buff->tcp_tsorted_anchor, &skb->tcp_tsorted_anchor);
 
         return 0;
 }
 
 /* This is similar to __pskb_pull_tail(). The difference is that pulled
  * data is not copied, but immediately discarded.
  */
 static int __pskb_trim_head(struct sk_buff *skb, int len)
 {
         struct skb_shared_info *shinfo;
         int i, k, eat;
 
         DEBUG_NET_WARN_ON_ONCE(skb_headlen(skb));
         eat = len;
         k = 0;
         shinfo = skb_shinfo(skb);
         for (i = 0; i < shinfo->nr_frags; i++) {
                 int size = skb_frag_size(&shinfo->frags[i]);
 
                 if (size <= eat) {
                         skb_frag_unref(skb, i);
                         eat -= size;
                 } else {
                         shinfo->frags[k] = shinfo->frags[i];
                         if (eat) {
                                 skb_frag_off_add(&shinfo->frags[k], eat);
                                 skb_frag_size_sub(&shinfo->frags[k], eat);
                                 eat = 0;
                         }
                         k++;
                 }
         }
         shinfo->nr_frags = k;
 
         skb->data_len -= len;
         skb->len = skb->data_len;
         return len;
 }
 
 /* Remove acked data from a packet in the transmit queue. */
 int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
 {
         u32 delta_truesize;
 
         if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
                 return -ENOMEM;
 
         delta_truesize = __pskb_trim_head(skb, len);
 
         TCP_SKB_CB(skb)->seq += len;
 
         skb->truesize      -= delta_truesize;
         sk_wmem_queued_add(sk, -delta_truesize);
         if (!skb_zcopy_pure(skb))
                 sk_mem_uncharge(sk, delta_truesize);
 
         /* Any change of skb->len requires recalculation of tso factor. */
         if (tcp_skb_pcount(skb) > 1)
                 tcp_set_skb_tso_segs(skb, tcp_skb_mss(skb));
 
         return 0;
 }
 
 /* Calculate MSS not accounting any TCP options.  */
 static inline int __tcp_mtu_to_mss(struct sock *sk, int pmtu)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
         const struct inet_connection_sock *icsk = inet_csk(sk);
         int mss_now;
 
         /* Calculate base mss without TCP options:
            It is MMS_S - sizeof(tcphdr) of rfc1122
          */
         mss_now = pmtu - icsk->icsk_af_ops->net_header_len - sizeof(struct tcphdr);
 
         /* Clamp it (mss_clamp does not include tcp options) */
         if (mss_now > tp->rx_opt.mss_clamp)
                 mss_now = tp->rx_opt.mss_clamp;
 
         /* Now subtract optional transport overhead */
         mss_now -= icsk->icsk_ext_hdr_len;
 
         /* Then reserve room for full set of TCP options and 8 bytes of data */
         mss_now = max(mss_now,
                       READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_snd_mss));
         return mss_now;
 }
 
 /* Calculate MSS. Not accounting for SACKs here.  */
 int tcp_mtu_to_mss(struct sock *sk, int pmtu)
 {
         /* Subtract TCP options size, not including SACKs */
         return __tcp_mtu_to_mss(sk, pmtu) -
                (tcp_sk(sk)->tcp_header_len - sizeof(struct tcphdr));
 }
 EXPORT_SYMBOL(tcp_mtu_to_mss);
 
 /* Inverse of above */
 int tcp_mss_to_mtu(struct sock *sk, int mss)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
         const struct inet_connection_sock *icsk = inet_csk(sk);
 
         return mss +
               tp->tcp_header_len +
               icsk->icsk_ext_hdr_len +
               icsk->icsk_af_ops->net_header_len;
 }
 EXPORT_SYMBOL(tcp_mss_to_mtu);
 
 /* MTU probing init per socket */
 void tcp_mtup_init(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct net *net = sock_net(sk);
 
         icsk->icsk_mtup.enabled = READ_ONCE(net->ipv4.sysctl_tcp_mtu_probing) > 1;
         icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp + sizeof(struct tcphdr) +
                                icsk->icsk_af_ops->net_header_len;
         icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, READ_ONCE(net->ipv4.sysctl_tcp_base_mss));
         icsk->icsk_mtup.probe_size = 0;
         if (icsk->icsk_mtup.enabled)
                 icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
 }
 EXPORT_SYMBOL(tcp_mtup_init);
 
 /* This function synchronize snd mss to current pmtu/exthdr set.
 
    tp->rx_opt.user_mss is mss set by user by TCP_MAXSEG. It does NOT counts
    for TCP options, but includes only bare TCP header.
 
    tp->rx_opt.mss_clamp is mss negotiated at connection setup.
    It is minimum of user_mss and mss received with SYN.
    It also does not include TCP options.
 
    inet_csk(sk)->icsk_pmtu_cookie is last pmtu, seen by this function.
 
    tp->mss_cache is current effective sending mss, including
    all tcp options except for SACKs. It is evaluated,
    taking into account current pmtu, but never exceeds
    tp->rx_opt.mss_clamp.
 
    NOTE1. rfc1122 clearly states that advertised MSS
    DOES NOT include either tcp or ip options.
 
    NOTE2. inet_csk(sk)->icsk_pmtu_cookie and tp->mss_cache
    are READ ONLY outside this function.         --ANK (980731)
  */
 unsigned int tcp_sync_mss(struct sock *sk, u32 pmtu)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct inet_connection_sock *icsk = inet_csk(sk);
         int mss_now;
 
         if (icsk->icsk_mtup.search_high > pmtu)
                 icsk->icsk_mtup.search_high = pmtu;
 
         mss_now = tcp_mtu_to_mss(sk, pmtu);
         mss_now = tcp_bound_to_half_wnd(tp, mss_now);
 
         /* And store cached results */
         icsk->icsk_pmtu_cookie = pmtu;
         if (icsk->icsk_mtup.enabled)
                 mss_now = min(mss_now, tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low));
         tp->mss_cache = mss_now;
 
         return mss_now;
 }
 EXPORT_SYMBOL(tcp_sync_mss);
 
 /* Compute the current effective MSS, taking SACKs and IP options,
  * and even PMTU discovery events into account.
  */
 unsigned int tcp_current_mss(struct sock *sk)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
         const struct dst_entry *dst = __sk_dst_get(sk);
         u32 mss_now;
         unsigned int header_len;
         struct tcp_out_options opts;
         struct tcp_key key;
 
         mss_now = tp->mss_cache;
 
         if (dst) {
                 u32 mtu = dst_mtu(dst);
                 if (mtu != inet_csk(sk)->icsk_pmtu_cookie)
                         mss_now = tcp_sync_mss(sk, mtu);
         }
         tcp_get_current_key(sk, &key);
         header_len = tcp_established_options(sk, NULL, &opts, &key) +
                      sizeof(struct tcphdr);
         /* The mss_cache is sized based on tp->tcp_header_len, which assumes
          * some common options. If this is an odd packet (because we have SACK
          * blocks etc) then our calculated header_len will be different, and
          * we have to adjust mss_now correspondingly */
         if (header_len != tp->tcp_header_len) {
                 int delta = (int) header_len - tp->tcp_header_len;
                 mss_now -= delta;
         }
 
         return mss_now;
 }
 
 /* RFC2861, slow part. Adjust cwnd, after it was not full during one rto.
  * As additional protections, we do not touch cwnd in retransmission phases,
  * and if application hit its sndbuf limit recently.
  */
 static void tcp_cwnd_application_limited(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (inet_csk(sk)->icsk_ca_state == TCP_CA_Open &&
             sk->sk_socket && !test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
                 /* Limited by application or receiver window. */
                 u32 init_win = tcp_init_cwnd(tp, __sk_dst_get(sk));
                 u32 win_used = max(tp->snd_cwnd_used, init_win);
                 if (win_used < tcp_snd_cwnd(tp)) {
                         tp->snd_ssthresh = tcp_current_ssthresh(sk);
                         tcp_snd_cwnd_set(tp, (tcp_snd_cwnd(tp) + win_used) >> 1);
                 }
                 tp->snd_cwnd_used = 0;
         }
         tp->snd_cwnd_stamp = tcp_jiffies32;
 }
 
 static void tcp_cwnd_validate(struct sock *sk, bool is_cwnd_limited)
 {
         const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
         struct tcp_sock *tp = tcp_sk(sk);
 
         /* Track the strongest available signal of the degree to which the cwnd
          * is fully utilized. If cwnd-limited then remember that fact for the
          * current window. If not cwnd-limited then track the maximum number of
          * outstanding packets in the current window. (If cwnd-limited then we
          * chose to not update tp->max_packets_out to avoid an extra else
          * clause with no functional impact.)
          */
         if (!before(tp->snd_una, tp->cwnd_usage_seq) ||
             is_cwnd_limited ||
             (!tp->is_cwnd_limited &&
              tp->packets_out > tp->max_packets_out)) {
                 tp->is_cwnd_limited = is_cwnd_limited;
                 tp->max_packets_out = tp->packets_out;
                 tp->cwnd_usage_seq = tp->snd_nxt;
         }
 
         if (tcp_is_cwnd_limited(sk)) {
                 /* Network is feed fully. */
                 tp->snd_cwnd_used = 0;
                 tp->snd_cwnd_stamp = tcp_jiffies32;
         } else {
                 /* Network starves. */
                 if (tp->packets_out > tp->snd_cwnd_used)
                         tp->snd_cwnd_used = tp->packets_out;
 
                 if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle) &&
                     (s32)(tcp_jiffies32 - tp->snd_cwnd_stamp) >= inet_csk(sk)->icsk_rto &&
                     !ca_ops->cong_control)
                         tcp_cwnd_application_limited(sk);
 
                 /* The following conditions together indicate the starvation
                  * is caused by insufficient sender buffer:
                  * 1) just sent some data (see tcp_write_xmit)
                  * 2) not cwnd limited (this else condition)
                  * 3) no more data to send (tcp_write_queue_empty())
                  * 4) application is hitting buffer limit (SOCK_NOSPACE)
                  */
                 if (tcp_write_queue_empty(sk) && sk->sk_socket &&
                     test_bit(SOCK_NOSPACE, &sk->sk_socket->flags) &&
                     (1 << sk->sk_state) & (TCPF_ESTABLISHED | TCPF_CLOSE_WAIT))
                         tcp_chrono_start(sk, TCP_CHRONO_SNDBUF_LIMITED);
         }
 }
 
 /* Minshall's variant of the Nagle send check. */
 static bool tcp_minshall_check(const struct tcp_sock *tp)
 {
         return after(tp->snd_sml, tp->snd_una) &&
                 !after(tp->snd_sml, tp->snd_nxt);
 }
 
 /* Update snd_sml if this skb is under mss
  * Note that a TSO packet might end with a sub-mss segment
  * The test is really :
  * if ((skb->len % mss) != 0)
  *        tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
  * But we can avoid doing the divide again given we already have
  *  skb_pcount = skb->len / mss_now
  */
 static void tcp_minshall_update(struct tcp_sock *tp, unsigned int mss_now,
                                 const struct sk_buff *skb)
 {
         if (skb->len < tcp_skb_pcount(skb) * mss_now)
                 tp->snd_sml = TCP_SKB_CB(skb)->end_seq;
 }
 
 /* Return false, if packet can be sent now without violation Nagle's rules:
  * 1. It is full sized. (provided by caller in %partial bool)
  * 2. Or it contains FIN. (already checked by caller)
  * 3. Or TCP_CORK is not set, and TCP_NODELAY is set.
  * 4. Or TCP_CORK is not set, and all sent packets are ACKed.
  *    With Minshall's modification: all sent small packets are ACKed.
  */
 static bool tcp_nagle_check(bool partial, const struct tcp_sock *tp,
                             int nonagle)
 {
         return partial &&
                 ((nonagle & TCP_NAGLE_CORK) ||
                  (!nonagle && tp->packets_out && tcp_minshall_check(tp)));
 }
 
 /* Return how many segs we'd like on a TSO packet,
  * depending on current pacing rate, and how close the peer is.
  *
  * Rationale is:
  * - For close peers, we rather send bigger packets to reduce
  *   cpu costs, because occasional losses will be repaired fast.
  * - For long distance/rtt flows, we would like to get ACK clocking
  *   with 1 ACK per ms.
  *
  * Use min_rtt to help adapt TSO burst size, with smaller min_rtt resulting
  * in bigger TSO bursts. We we cut the RTT-based allowance in half
  * for every 2^9 usec (aka 512 us) of RTT, so that the RTT-based allowance
  * is below 1500 bytes after 6 * ~500 usec = 3ms.
  */
 static u32 tcp_tso_autosize(const struct sock *sk, unsigned int mss_now,
                             int min_tso_segs)
 {
         unsigned long bytes;
         u32 r;
 
         bytes = READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift);
 
         r = tcp_min_rtt(tcp_sk(sk)) >> READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_rtt_log);
         if (r < BITS_PER_TYPE(sk->sk_gso_max_size))
                 bytes += sk->sk_gso_max_size >> r;
 
         bytes = min_t(unsigned long, bytes, sk->sk_gso_max_size);
 
         return max_t(u32, bytes / mss_now, min_tso_segs);
 }
 
 /* Return the number of segments we want in the skb we are transmitting.
  * See if congestion control module wants to decide; otherwise, autosize.
  */
 static u32 tcp_tso_segs(struct sock *sk, unsigned int mss_now)
 {
         const struct tcp_congestion_ops *ca_ops = inet_csk(sk)->icsk_ca_ops;
         u32 min_tso, tso_segs;
 
         min_tso = ca_ops->min_tso_segs ?
                         ca_ops->min_tso_segs(sk) :
                         READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_min_tso_segs);
 
         tso_segs = tcp_tso_autosize(sk, mss_now, min_tso);
         return min_t(u32, tso_segs, sk->sk_gso_max_segs);
 }
 
 /* Returns the portion of skb which can be sent right away */
 static unsigned int tcp_mss_split_point(const struct sock *sk,
                                         const struct sk_buff *skb,
                                         unsigned int mss_now,
                                         unsigned int max_segs,
                                         int nonagle)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
         u32 partial, needed, window, max_len;
 
         window = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
         max_len = mss_now * max_segs;
 
         if (likely(max_len <= window && skb != tcp_write_queue_tail(sk)))
                 return max_len;
 
         needed = min(skb->len, window);
 
         if (max_len <= needed)
                 return max_len;
 
         partial = needed % mss_now;
         /* If last segment is not a full MSS, check if Nagle rules allow us
          * to include this last segment in this skb.
          * Otherwise, we'll split the skb at last MSS boundary
          */
         if (tcp_nagle_check(partial != 0, tp, nonagle))
                 return needed - partial;
 
         return needed;
 }
 
 /* Can at least one segment of SKB be sent right now, according to the
  * congestion window rules?  If so, return how many segments are allowed.
  */
 static u32 tcp_cwnd_test(const struct tcp_sock *tp)
 {
         u32 in_flight, cwnd, halfcwnd;
 
         in_flight = tcp_packets_in_flight(tp);
         cwnd = tcp_snd_cwnd(tp);
         if (in_flight >= cwnd)
                 return 0;
 
         /* For better scheduling, ensure we have at least
          * 2 GSO packets in flight.
          */
         halfcwnd = max(cwnd >> 1, 1U);
         return min(halfcwnd, cwnd - in_flight);
 }
 
 /* Initialize TSO state of a skb.
  * This must be invoked the first time we consider transmitting
  * SKB onto the wire.
  */
 static int tcp_init_tso_segs(struct sk_buff *skb, unsigned int mss_now)
 {
         int tso_segs = tcp_skb_pcount(skb);
 
         if (!tso_segs || (tso_segs > 1 && tcp_skb_mss(skb) != mss_now))
                 return tcp_set_skb_tso_segs(skb, mss_now);
 
         return tso_segs;
 }
 
 
 /* Return true if the Nagle test allows this packet to be
  * sent now.
  */
 static inline bool tcp_nagle_test(const struct tcp_sock *tp, const struct sk_buff *skb,
                                   unsigned int cur_mss, int nonagle)
 {
         /* Nagle rule does not apply to frames, which sit in the middle of the
          * write_queue (they have no chances to get new data).
          *
          * This is implemented in the callers, where they modify the 'nonagle'
          * argument based upon the location of SKB in the send queue.
          */
         if (nonagle & TCP_NAGLE_PUSH)
                 return true;
 
         /* Don't use the nagle rule for urgent data (or for the final FIN). */
         if (tcp_urg_mode(tp) || (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN))
                 return true;
 
         if (!tcp_nagle_check(skb->len < cur_mss, tp, nonagle))
                 return true;
 
         return false;
 }
 
 /* Does at least the first segment of SKB fit into the send window? */
 static bool tcp_snd_wnd_test(const struct tcp_sock *tp,
                              const struct sk_buff *skb,
                              unsigned int cur_mss)
 {
         u32 end_seq = TCP_SKB_CB(skb)->end_seq;
 
         if (skb->len > cur_mss)
                 end_seq = TCP_SKB_CB(skb)->seq + cur_mss;
 
         return !after(end_seq, tcp_wnd_end(tp));
 }
 
 /* Trim TSO SKB to LEN bytes, put the remaining data into a new packet
  * which is put after SKB on the list.  It is very much like
  * tcp_fragment() except that it may make several kinds of assumptions
  * in order to speed up the splitting operation.  In particular, we
  * know that all the data is in scatter-gather pages, and that the
  * packet has never been sent out before (and thus is not cloned).
  */
 static int tso_fragment(struct sock *sk, struct sk_buff *skb, unsigned int len,
                         unsigned int mss_now, gfp_t gfp)
 {
         int nlen = skb->len - len;
         struct sk_buff *buff;
         u8 flags;
 
         /* All of a TSO frame must be composed of paged data.  */
         DEBUG_NET_WARN_ON_ONCE(skb->len != skb->data_len);
 
         buff = tcp_stream_alloc_skb(sk, gfp, true);
         if (unlikely(!buff))
                 return -ENOMEM;
         skb_copy_decrypted(buff, skb);
         mptcp_skb_ext_copy(buff, skb);
 
         sk_wmem_queued_add(sk, buff->truesize);
         sk_mem_charge(sk, buff->truesize);
         buff->truesize += nlen;
         skb->truesize -= nlen;
 
         /* Correct the sequence numbers. */
         TCP_SKB_CB(buff)->seq = TCP_SKB_CB(skb)->seq + len;
         TCP_SKB_CB(buff)->end_seq = TCP_SKB_CB(skb)->end_seq;
         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(buff)->seq;
 
         /* PSH and FIN should only be set in the second packet. */
         flags = TCP_SKB_CB(skb)->tcp_flags;
         TCP_SKB_CB(skb)->tcp_flags = flags & ~(TCPHDR_FIN | TCPHDR_PSH);
         TCP_SKB_CB(buff)->tcp_flags = flags;
 
         tcp_skb_fragment_eor(skb, buff);
 
         skb_split(skb, buff, len);
         tcp_fragment_tstamp(skb, buff);
 
         /* Fix up tso_factor for both original and new SKB.  */
         tcp_set_skb_tso_segs(skb, mss_now);
         tcp_set_skb_tso_segs(buff, mss_now);
 
         /* Link BUFF into the send queue. */
         __skb_header_release(buff);
         tcp_insert_write_queue_after(skb, buff, sk, TCP_FRAG_IN_WRITE_QUEUE);
 
         return 0;
 }
 
 /* Try to defer sending, if possible, in order to minimize the amount
  * of TSO splitting we do.  View it as a kind of TSO Nagle test.
  *
  * This algorithm is from John Heffner.
  */
 static bool tcp_tso_should_defer(struct sock *sk, struct sk_buff *skb,
                                  bool *is_cwnd_limited,
                                  bool *is_rwnd_limited,
                                  u32 max_segs)
 {
         const struct inet_connection_sock *icsk = inet_csk(sk);
         u32 send_win, cong_win, limit, in_flight;
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *head;
         int win_divisor;
         s64 delta;
 
         if (icsk->icsk_ca_state >= TCP_CA_Recovery)
                 goto send_now;
 
         /* Avoid bursty behavior by allowing defer
          * only if the last write was recent (1 ms).
          * Note that tp->tcp_wstamp_ns can be in the future if we have
          * packets waiting in a qdisc or device for EDT delivery.
          */
         delta = tp->tcp_clock_cache - tp->tcp_wstamp_ns - NSEC_PER_MSEC;
         if (delta > 0)
                 goto send_now;
 
         in_flight = tcp_packets_in_flight(tp);
 
         BUG_ON(tcp_skb_pcount(skb) <= 1);
         BUG_ON(tcp_snd_cwnd(tp) <= in_flight);
 
         send_win = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
         /* From in_flight test above, we know that cwnd > in_flight.  */
         cong_win = (tcp_snd_cwnd(tp) - in_flight) * tp->mss_cache;
 
         limit = min(send_win, cong_win);
 
         /* If a full-sized TSO skb can be sent, do it. */
         if (limit >= max_segs * tp->mss_cache)
                 goto send_now;
 
         /* Middle in queue won't get any more data, full sendable already? */
         if ((skb != tcp_write_queue_tail(sk)) && (limit >= skb->len))
                 goto send_now;
 
         win_divisor = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_tso_win_divisor);
         if (win_divisor) {
                 u32 chunk = min(tp->snd_wnd, tcp_snd_cwnd(tp) * tp->mss_cache);
 
                 /* If at least some fraction of a window is available,
                  * just use it.
                  */
                 chunk /= win_divisor;
                 if (limit >= chunk)
                         goto send_now;
         } else {
                 /* Different approach, try not to defer past a single
                  * ACK.  Receiver should ACK every other full sized
                  * frame, so if we have space for more than 3 frames
                  * then send now.
                  */
                 if (limit > tcp_max_tso_deferred_mss(tp) * tp->mss_cache)
                         goto send_now;
         }
 
         /* TODO : use tsorted_sent_queue ? */
         head = tcp_rtx_queue_head(sk);
         if (!head)
                 goto send_now;
         delta = tp->tcp_clock_cache - head->tstamp;
         /* If next ACK is likely to come too late (half srtt), do not defer */
         if ((s64)(delta - (u64)NSEC_PER_USEC * (tp->srtt_us >> 4)) < 0)
                 goto send_now;
 
         /* Ok, it looks like it is advisable to defer.
          * Three cases are tracked :
          * 1) We are cwnd-limited
          * 2) We are rwnd-limited
          * 3) We are application limited.
          */
         if (cong_win < send_win) {
                 if (cong_win <= skb->len) {
                         *is_cwnd_limited = true;
                         return true;
                 }
         } else {
                 if (send_win <= skb->len) {
                         *is_rwnd_limited = true;
                         return true;
                 }
         }
 
         /* If this packet won't get more data, do not wait. */
         if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) ||
             TCP_SKB_CB(skb)->eor)
                 goto send_now;
 
         return true;
 
 send_now:
         return false;
 }
 
 static inline void tcp_mtu_check_reprobe(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct net *net = sock_net(sk);
         u32 interval;
         s32 delta;
 
         interval = READ_ONCE(net->ipv4.sysctl_tcp_probe_interval);
         delta = tcp_jiffies32 - icsk->icsk_mtup.probe_timestamp;
         if (unlikely(delta >= interval * HZ)) {
                 int mss = tcp_current_mss(sk);
 
                 /* Update current search range */
                 icsk->icsk_mtup.probe_size = 0;
                 icsk->icsk_mtup.search_high = tp->rx_opt.mss_clamp +
                         sizeof(struct tcphdr) +
                         icsk->icsk_af_ops->net_header_len;
                 icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
 
                 /* Update probe time stamp */
                 icsk->icsk_mtup.probe_timestamp = tcp_jiffies32;
         }
 }
 
 static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
 {
         struct sk_buff *skb, *next;
 
         skb = tcp_send_head(sk);
         tcp_for_write_queue_from_safe(skb, next, sk) {
                 if (len <= skb->len)
                         break;
 
                 if (unlikely(TCP_SKB_CB(skb)->eor) ||
                     tcp_has_tx_tstamp(skb) ||
                     !skb_pure_zcopy_same(skb, next))
                         return false;
 
                 len -= skb->len;
         }
 
         return true;
 }
 
 static int tcp_clone_payload(struct sock *sk, struct sk_buff *to,
                              int probe_size)
 {
         skb_frag_t *lastfrag = NULL, *fragto = skb_shinfo(to)->frags;
         int i, todo, len = 0, nr_frags = 0;
         const struct sk_buff *skb;
 
         if (!sk_wmem_schedule(sk, to->truesize + probe_size))
                 return -ENOMEM;
 
         skb_queue_walk(&sk->sk_write_queue, skb) {
                 const skb_frag_t *fragfrom = skb_shinfo(skb)->frags;
 
                 if (skb_headlen(skb))
                         return -EINVAL;
 
                 for (i = 0; i < skb_shinfo(skb)->nr_frags; i++, fragfrom++) {
                         if (len >= probe_size)
                                 goto commit;
                         todo = min_t(int, skb_frag_size(fragfrom),
                                      probe_size - len);
                         len += todo;
                         if (lastfrag &&
                             skb_frag_page(fragfrom) == skb_frag_page(lastfrag) &&
                             skb_frag_off(fragfrom) == skb_frag_off(lastfrag) +
                                                       skb_frag_size(lastfrag)) {
                                 skb_frag_size_add(lastfrag, todo);
                                 continue;
                         }
                         if (unlikely(nr_frags == MAX_SKB_FRAGS))
                                 return -E2BIG;
                         skb_frag_page_copy(fragto, fragfrom);
                         skb_frag_off_copy(fragto, fragfrom);
                         skb_frag_size_set(fragto, todo);
                         nr_frags++;
                         lastfrag = fragto++;
                 }
         }
 commit:
         WARN_ON_ONCE(len != probe_size);
         for (i = 0; i < nr_frags; i++)
                 skb_frag_ref(to, i);
 
         skb_shinfo(to)->nr_frags = nr_frags;
         to->truesize += probe_size;
         to->len += probe_size;
         to->data_len += probe_size;
         __skb_header_release(to);
         return 0;
 }
 
 /* tcp_mtu_probe() and tcp_grow_skb() can both eat an skb (src) if
  * all its payload was moved to another one (dst).
  * Make sure to transfer tcp_flags, eor, and tstamp.
  */
 static void tcp_eat_one_skb(struct sock *sk,
                             struct sk_buff *dst,
                             struct sk_buff *src)
 {
         TCP_SKB_CB(dst)->tcp_flags |= TCP_SKB_CB(src)->tcp_flags;
         TCP_SKB_CB(dst)->eor = TCP_SKB_CB(src)->eor;
         tcp_skb_collapse_tstamp(dst, src);
         tcp_unlink_write_queue(src, sk);
         tcp_wmem_free_skb(sk, src);
 }
 
 /* Create a new MTU probe if we are ready.
  * MTU probe is regularly attempting to increase the path MTU by
  * deliberately sending larger packets.  This discovers routing
  * changes resulting in larger path MTUs.
  *
  * Returns 0 if we should wait to probe (no cwnd available),
  *         1 if a probe was sent,
  *         -1 otherwise
  */
 static int tcp_mtu_probe(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb, *nskb, *next;
         struct net *net = sock_net(sk);
         int probe_size;
         int size_needed;
         int copy, len;
         int mss_now;
         int interval;
 
         /* Not currently probing/verifying,
          * not in recovery,
          * have enough cwnd, and
          * not SACKing (the variable headers throw things off)
          */
         if (likely(!icsk->icsk_mtup.enabled ||
                    icsk->icsk_mtup.probe_size ||
                    inet_csk(sk)->icsk_ca_state != TCP_CA_Open ||
                    tcp_snd_cwnd(tp) < 11 ||
                    tp->rx_opt.num_sacks || tp->rx_opt.dsack))
                 return -1;
 
         /* Use binary search for probe_size between tcp_mss_base,
          * and current mss_clamp. if (search_high - search_low)
          * smaller than a threshold, backoff from probing.
          */
         mss_now = tcp_current_mss(sk);
         probe_size = tcp_mtu_to_mss(sk, (icsk->icsk_mtup.search_high +
                                     icsk->icsk_mtup.search_low) >> 1);
         size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
         interval = icsk->icsk_mtup.search_high - icsk->icsk_mtup.search_low;
         /* When misfortune happens, we are reprobing actively,
          * and then reprobe timer has expired. We stick with current
          * probing process by not resetting search range to its orignal.
          */
         if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high) ||
             interval < READ_ONCE(net->ipv4.sysctl_tcp_probe_threshold)) {
                 /* Check whether enough time has elaplased for
                  * another round of probing.
                  */
                 tcp_mtu_check_reprobe(sk);
                 return -1;
         }
 
         /* Have enough data in the send queue to probe? */
         if (tp->write_seq - tp->snd_nxt < size_needed)
                 return -1;
 
         if (tp->snd_wnd < size_needed)
                 return -1;
         if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp)))
                 return 0;
 
         /* Do we need to wait to drain cwnd? With none in flight, don't stall */
         if (tcp_packets_in_flight(tp) + 2 > tcp_snd_cwnd(tp)) {
                 if (!tcp_packets_in_flight(tp))
                         return -1;
                 else
                         return 0;
         }
 
         if (!tcp_can_coalesce_send_queue_head(sk, probe_size))
                 return -1;
 
         /* We're allowed to probe.  Build it now. */
         nskb = tcp_stream_alloc_skb(sk, GFP_ATOMIC, false);
         if (!nskb)
                 return -1;
 
         /* build the payload, and be prepared to abort if this fails. */
         if (tcp_clone_payload(sk, nskb, probe_size)) {
                 tcp_skb_tsorted_anchor_cleanup(nskb);
                 consume_skb(nskb);
                 return -1;
         }
         sk_wmem_queued_add(sk, nskb->truesize);
         sk_mem_charge(sk, nskb->truesize);
 
         skb = tcp_send_head(sk);
         skb_copy_decrypted(nskb, skb);
         mptcp_skb_ext_copy(nskb, skb);
 
         TCP_SKB_CB(nskb)->seq = TCP_SKB_CB(skb)->seq;
         TCP_SKB_CB(nskb)->end_seq = TCP_SKB_CB(skb)->seq + probe_size;
         TCP_SKB_CB(nskb)->tcp_flags = TCPHDR_ACK;
 
         tcp_insert_write_queue_before(nskb, skb, sk);
         tcp_highest_sack_replace(sk, skb, nskb);
 
         len = 0;
         tcp_for_write_queue_from_safe(skb, next, sk) {
                 copy = min_t(int, skb->len, probe_size - len);
 
                 if (skb->len <= copy) {
                         tcp_eat_one_skb(sk, nskb, skb);
                 } else {
                         TCP_SKB_CB(nskb)->tcp_flags |= TCP_SKB_CB(skb)->tcp_flags &
                                                    ~(TCPHDR_FIN|TCPHDR_PSH);
                         __pskb_trim_head(skb, copy);
                         tcp_set_skb_tso_segs(skb, mss_now);
                         TCP_SKB_CB(skb)->seq += copy;
                 }
 
                 len += copy;
 
                 if (len >= probe_size)
                         break;
         }
         tcp_init_tso_segs(nskb, nskb->len);
 
         /* We're ready to send.  If this fails, the probe will
          * be resegmented into mss-sized pieces by tcp_write_xmit().
          */
         if (!tcp_transmit_skb(sk, nskb, 1, GFP_ATOMIC)) {
                 /* Decrement cwnd here because we are sending
                  * effectively two packets. */
                 tcp_snd_cwnd_set(tp, tcp_snd_cwnd(tp) - 1);
                 tcp_event_new_data_sent(sk, nskb);
 
                 icsk->icsk_mtup.probe_size = tcp_mss_to_mtu(sk, nskb->len);
                 tp->mtu_probe.probe_seq_start = TCP_SKB_CB(nskb)->seq;
                 tp->mtu_probe.probe_seq_end = TCP_SKB_CB(nskb)->end_seq;
 
                 return 1;
         }
 
         return -1;
 }
 
 static bool tcp_pacing_check(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         if (!tcp_needs_internal_pacing(sk))
                 return false;
 
         if (tp->tcp_wstamp_ns <= tp->tcp_clock_cache)
                 return false;
 
         if (!hrtimer_is_queued(&tp->pacing_timer)) {
                 hrtimer_start(&tp->pacing_timer,
                               ns_to_ktime(tp->tcp_wstamp_ns),
                               HRTIMER_MODE_ABS_PINNED_SOFT);
                 sock_hold(sk);
         }
         return true;
 }
 
 static bool tcp_rtx_queue_empty_or_single_skb(const struct sock *sk)
 {
         const struct rb_node *node = sk->tcp_rtx_queue.rb_node;
 
         /* No skb in the rtx queue. */
         if (!node)
                 return true;
 
         /* Only one skb in rtx queue. */
         return !node->rb_left && !node->rb_right;
 }
 
 /* TCP Small Queues :
  * Control number of packets in qdisc/devices to two packets / or ~1 ms.
  * (These limits are doubled for retransmits)
  * This allows for :
  *  - better RTT estimation and ACK scheduling
  *  - faster recovery
  *  - high rates
  * Alas, some drivers / subsystems require a fair amount
  * of queued bytes to ensure line rate.
  * One example is wifi aggregation (802.11 AMPDU)
  */
 static bool tcp_small_queue_check(struct sock *sk, const struct sk_buff *skb,
                                   unsigned int factor)
 {
         unsigned long limit;
 
         limit = max_t(unsigned long,
                       2 * skb->truesize,
                       READ_ONCE(sk->sk_pacing_rate) >> READ_ONCE(sk->sk_pacing_shift));
         if (sk->sk_pacing_status == SK_PACING_NONE)
                 limit = min_t(unsigned long, limit,
                               READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_limit_output_bytes));
         limit <<= factor;
 
         if (static_branch_unlikely(&tcp_tx_delay_enabled) &&
             tcp_sk(sk)->tcp_tx_delay) {
                 u64 extra_bytes = (u64)READ_ONCE(sk->sk_pacing_rate) *
                                   tcp_sk(sk)->tcp_tx_delay;
 
                 /* TSQ is based on skb truesize sum (sk_wmem_alloc), so we
                  * approximate our needs assuming an ~100% skb->truesize overhead.
                  * USEC_PER_SEC is approximated by 2^20.
                  * do_div(extra_bytes, USEC_PER_SEC/2) is replaced by a right shift.
                  */
                 extra_bytes >>= (20 - 1);
                 limit += extra_bytes;
         }
         if (refcount_read(&sk->sk_wmem_alloc) > limit) {
                 /* Always send skb if rtx queue is empty or has one skb.
                  * No need to wait for TX completion to call us back,
                  * after softirq/tasklet schedule.
                  * This helps when TX completions are delayed too much.
                  */
                 if (tcp_rtx_queue_empty_or_single_skb(sk))
                         return false;
 
                 set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
                 /* It is possible TX completion already happened
                  * before we set TSQ_THROTTLED, so we must
                  * test again the condition.
                  */
                 smp_mb__after_atomic();
                 if (refcount_read(&sk->sk_wmem_alloc) > limit)
                         return true;
         }
         return false;
 }
 
 static void tcp_chrono_set(struct tcp_sock *tp, const enum tcp_chrono new)
 {
         const u32 now = tcp_jiffies32;
         enum tcp_chrono old = tp->chrono_type;
 
         if (old > TCP_CHRONO_UNSPEC)
                 tp->chrono_stat[old - 1] += now - tp->chrono_start;
         tp->chrono_start = now;
         tp->chrono_type = new;
 }
 
 void tcp_chrono_start(struct sock *sk, const enum tcp_chrono type)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
         /* If there are multiple conditions worthy of tracking in a
          * chronograph then the highest priority enum takes precedence
          * over the other conditions. So that if something "more interesting"
          * starts happening, stop the previous chrono and start a new one.
          */
         if (type > tp->chrono_type)
                 tcp_chrono_set(tp, type);
 }
 
 void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type)
 {
         struct tcp_sock *tp = tcp_sk(sk);
 
 
         /* There are multiple conditions worthy of tracking in a
          * chronograph, so that the highest priority enum takes
          * precedence over the other conditions (see tcp_chrono_start).
          * If a condition stops, we only stop chrono tracking if
          * it's the "most interesting" or current chrono we are
          * tracking and starts busy chrono if we have pending data.
          */
         if (tcp_rtx_and_write_queues_empty(sk))
                 tcp_chrono_set(tp, TCP_CHRONO_UNSPEC);
         else if (type == tp->chrono_type)
                 tcp_chrono_set(tp, TCP_CHRONO_BUSY);
 }
 
 /* First skb in the write queue is smaller than ideal packet size.
  * Check if we can move payload from the second skb in the queue.
  */
 static void tcp_grow_skb(struct sock *sk, struct sk_buff *skb, int amount)
 {
         struct sk_buff *next_skb = skb->next;
         unsigned int nlen;
 
         if (tcp_skb_is_last(sk, skb))
                 return;
 
         if (!tcp_skb_can_collapse(skb, next_skb))
                 return;
 
         nlen = min_t(u32, amount, next_skb->len);
         if (!nlen || !skb_shift(skb, next_skb, nlen))
                 return;
 
         TCP_SKB_CB(skb)->end_seq += nlen;
         TCP_SKB_CB(next_skb)->seq += nlen;
 
         if (!next_skb->len) {
                 /* In case FIN is set, we need to update end_seq */
                 TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
 
                 tcp_eat_one_skb(sk, skb, next_skb);
         }
 }
 
 /* This routine writes packets to the network.  It advances the
  * send_head.  This happens as incoming acks open up the remote
  * window for us.
  *
  * LARGESEND note: !tcp_urg_mode is overkill, only frames between
  * snd_up-64k-mss .. snd_up cannot be large. However, taking into
  * account rare use of URG, this is not a big flaw.
  *
  * Send at most one packet when push_one > 0. Temporarily ignore
  * cwnd limit to force at most one packet out when push_one == 2.
 
  * Returns true, if no segments are in flight and we have queued segments,
  * but cannot send anything now because of SWS or another problem.
  */
 static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle,
                            int push_one, gfp_t gfp)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
         unsigned int tso_segs, sent_pkts;
         u32 cwnd_quota, max_segs;
         int result;
         bool is_cwnd_limited = false, is_rwnd_limited = false;
 
         sent_pkts = 0;
 
         tcp_mstamp_refresh(tp);
         if (!push_one) {
                 /* Do MTU probing. */
                 result = tcp_mtu_probe(sk);
                 if (!result) {
                         return false;
                 } else if (result > 0) {
                         sent_pkts = 1;
                 }
         }
 
         max_segs = tcp_tso_segs(sk, mss_now);
         while ((skb = tcp_send_head(sk))) {
                 unsigned int limit;
                 int missing_bytes;
 
                 if (unlikely(tp->repair) && tp->repair_queue == TCP_SEND_QUEUE) {
                         /* "skb_mstamp_ns" is used as a start point for the retransmit timer */
                         tp->tcp_wstamp_ns = tp->tcp_clock_cache;
                         skb_set_delivery_time(skb, tp->tcp_wstamp_ns, SKB_CLOCK_MONOTONIC);
                         list_move_tail(&skb->tcp_tsorted_anchor, &tp->tsorted_sent_queue);
                         tcp_init_tso_segs(skb, mss_now);
                         goto repair; /* Skip network transmission */
                 }
 
                 if (tcp_pacing_check(sk))
                         break;
 
                 cwnd_quota = tcp_cwnd_test(tp);
                 if (!cwnd_quota) {
                         if (push_one == 2)
                                 /* Force out a loss probe pkt. */
                                 cwnd_quota = 1;
                         else
                                 break;
                 }
                 cwnd_quota = min(cwnd_quota, max_segs);
                 missing_bytes = cwnd_quota * mss_now - skb->len;
                 if (missing_bytes > 0)
                         tcp_grow_skb(sk, skb, missing_bytes);
 
                 tso_segs = tcp_set_skb_tso_segs(skb, mss_now);
 
                 if (unlikely(!tcp_snd_wnd_test(tp, skb, mss_now))) {
                         is_rwnd_limited = true;
                         break;
                 }
 
                 if (tso_segs == 1) {
                         if (unlikely(!tcp_nagle_test(tp, skb, mss_now,
                                                      (tcp_skb_is_last(sk, skb) ?
                                                       nonagle : TCP_NAGLE_PUSH))))
                                 break;
                 } else {
                         if (!push_one &&
                             tcp_tso_should_defer(sk, skb, &is_cwnd_limited,
                                                  &is_rwnd_limited, max_segs))
                                 break;
                 }
 
                 limit = mss_now;
                 if (tso_segs > 1 && !tcp_urg_mode(tp))
                         limit = tcp_mss_split_point(sk, skb, mss_now,
                                                     cwnd_quota,
                                                     nonagle);
 
                 if (skb->len > limit &&
                     unlikely(tso_fragment(sk, skb, limit, mss_now, gfp)))
                         break;
 
                 if (tcp_small_queue_check(sk, skb, 0))
                         break;
 
                 /* Argh, we hit an empty skb(), presumably a thread
                  * is sleeping in sendmsg()/sk_stream_wait_memory().
                  * We do not want to send a pure-ack packet and have
                  * a strange looking rtx queue with empty packet(s).
                  */
                 if (TCP_SKB_CB(skb)->end_seq == TCP_SKB_CB(skb)->seq)
                         break;
 
                 if (unlikely(tcp_transmit_skb(sk, skb, 1, gfp)))
                         break;
 
 repair:
                 /* Advance the send_head.  This one is sent out.
                  * This call will increment packets_out.
                  */
                 tcp_event_new_data_sent(sk, skb);
 
                 tcp_minshall_update(tp, mss_now, skb);
                 sent_pkts += tcp_skb_pcount(skb);
 
                 if (push_one)
                         break;
         }
 
         if (is_rwnd_limited)
                 tcp_chrono_start(sk, TCP_CHRONO_RWND_LIMITED);
         else
                 tcp_chrono_stop(sk, TCP_CHRONO_RWND_LIMITED);
 
         is_cwnd_limited |= (tcp_packets_in_flight(tp) >= tcp_snd_cwnd(tp));
         if (likely(sent_pkts || is_cwnd_limited))
                 tcp_cwnd_validate(sk, is_cwnd_limited);
 
         if (likely(sent_pkts)) {
                 if (tcp_in_cwnd_reduction(sk))
                         tp->prr_out += sent_pkts;
 
                 /* Send one loss probe per tail loss episode. */
                 if (push_one != 2)
                         tcp_schedule_loss_probe(sk, false);
                 return false;
         }
         return !tp->packets_out && !tcp_write_queue_empty(sk);
 }
 
 bool tcp_schedule_loss_probe(struct sock *sk, bool advancing_rto)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         u32 timeout, timeout_us, rto_delta_us;
         int early_retrans;
 
         /* Don't do any loss probe on a Fast Open connection before 3WHS
          * finishes.
          */
         if (rcu_access_pointer(tp->fastopen_rsk))
                 return false;
 
         early_retrans = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_early_retrans);
         /* Schedule a loss probe in 2*RTT for SACK capable connections
          * not in loss recovery, that are either limited by cwnd or application.
          */
         if ((early_retrans != 3 && early_retrans != 4) ||
             !tp->packets_out || !tcp_is_sack(tp) ||
             (icsk->icsk_ca_state != TCP_CA_Open &&
              icsk->icsk_ca_state != TCP_CA_CWR))
                 return false;
 
         /* Probe timeout is 2*rtt. Add minimum RTO to account
          * for delayed ack when there's one outstanding packet. If no RTT
          * sample is available then probe after TCP_TIMEOUT_INIT.
          */
         if (tp->srtt_us) {
                 timeout_us = tp->srtt_us >> 2;
                 if (tp->packets_out == 1)
                         timeout_us += tcp_rto_min_us(sk);
                 else
                         timeout_us += TCP_TIMEOUT_MIN_US;
                 timeout = usecs_to_jiffies(timeout_us);
         } else {
                 timeout = TCP_TIMEOUT_INIT;
         }
 
         /* If the RTO formula yields an earlier time, then use that time. */
         rto_delta_us = advancing_rto ?
                         jiffies_to_usecs(inet_csk(sk)->icsk_rto) :
                         tcp_rto_delta_us(sk);  /* How far in future is RTO? */
         if (rto_delta_us > 0)
                 timeout = min_t(u32, timeout, usecs_to_jiffies(rto_delta_us));
 
         tcp_reset_xmit_timer(sk, ICSK_TIME_LOSS_PROBE, timeout, TCP_RTO_MAX);
         return true;
 }
 
 /* Thanks to skb fast clones, we can detect if a prior transmit of
  * a packet is still in a qdisc or driver queue.
  * In this case, there is very little point doing a retransmit !
  */
 static bool skb_still_in_host_queue(struct sock *sk,
                                     const struct sk_buff *skb)
 {
         if (unlikely(skb_fclone_busy(sk, skb))) {
                 set_bit(TSQ_THROTTLED, &sk->sk_tsq_flags);
                 smp_mb__after_atomic();
                 if (skb_fclone_busy(sk, skb)) {
                         NET_INC_STATS(sock_net(sk),
                                       LINUX_MIB_TCPSPURIOUS_RTX_HOSTQUEUES);
                         return true;
                 }
         }
         return false;
 }
 
 /* When probe timeout (PTO) fires, try send a new segment if possible, else
  * retransmit the last segment.
  */
 void tcp_send_loss_probe(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
         int pcount;
         int mss = tcp_current_mss(sk);
 
         /* At most one outstanding TLP */
         if (tp->tlp_high_seq)
                 goto rearm_timer;
 
         tp->tlp_retrans = 0;
         skb = tcp_send_head(sk);
         if (skb && tcp_snd_wnd_test(tp, skb, mss)) {
                 pcount = tp->packets_out;
                 tcp_write_xmit(sk, mss, TCP_NAGLE_OFF, 2, GFP_ATOMIC);
                 if (tp->packets_out > pcount)
                         goto probe_sent;
                 goto rearm_timer;
         }
         skb = skb_rb_last(&sk->tcp_rtx_queue);
         if (unlikely(!skb)) {
                 WARN_ONCE(tp->packets_out,
                           "invalid inflight: %u state %u cwnd %u mss %d\n",
                           tp->packets_out, sk->sk_state, tcp_snd_cwnd(tp), mss);
                 inet_csk(sk)->icsk_pending = 0;
                 return;
         }
 
         if (skb_still_in_host_queue(sk, skb))
                 goto rearm_timer;
 
         pcount = tcp_skb_pcount(skb);
         if (WARN_ON(!pcount))
                 goto rearm_timer;
 
         if ((pcount > 1) && (skb->len > (pcount - 1) * mss)) {
                 if (unlikely(tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                                           (pcount - 1) * mss, mss,
                                           GFP_ATOMIC)))
                         goto rearm_timer;
                 skb = skb_rb_next(skb);
         }
 
         if (WARN_ON(!skb || !tcp_skb_pcount(skb)))
                 goto rearm_timer;
 
         if (__tcp_retransmit_skb(sk, skb, 1))
                 goto rearm_timer;
 
         tp->tlp_retrans = 1;
 
 probe_sent:
         /* Record snd_nxt for loss detection. */
         tp->tlp_high_seq = tp->snd_nxt;
 
         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPLOSSPROBES);
         /* Reset s.t. tcp_rearm_rto will restart timer from now */
         inet_csk(sk)->icsk_pending = 0;
 rearm_timer:
         tcp_rearm_rto(sk);
 }
 
 /* Push out any pending frames which were held back due to
  * TCP_CORK or attempt at coalescing tiny packets.
  * The socket must be locked by the caller.
  */
 void __tcp_push_pending_frames(struct sock *sk, unsigned int cur_mss,
                                int nonagle)
 {
         /* If we are closed, the bytes will have to remain here.
          * In time closedown will finish, we empty the write queue and
          * all will be happy.
          */
         if (unlikely(sk->sk_state == TCP_CLOSE))
                 return;
 
         if (tcp_write_xmit(sk, cur_mss, nonagle, 0,
                            sk_gfp_mask(sk, GFP_ATOMIC)))
                 tcp_check_probe_timer(sk);
 }
 
 /* Send _single_ skb sitting at the send head. This function requires
  * true push pending frames to setup probe timer etc.
  */
 void tcp_push_one(struct sock *sk, unsigned int mss_now)
 {
         struct sk_buff *skb = tcp_send_head(sk);
 
         BUG_ON(!skb || skb->len < mss_now);
 
         tcp_write_xmit(sk, mss_now, TCP_NAGLE_PUSH, 1, sk->sk_allocation);
 }
 
 /* This function returns the amount that we can raise the
  * usable window based on the following constraints
  *
  * 1. The window can never be shrunk once it is offered (RFC 793)
  * 2. We limit memory per socket
  *
  * RFC 1122:
  * "the suggested [SWS] avoidance algorithm for the receiver is to keep
  *  RECV.NEXT + RCV.WIN fixed until:
  *  RCV.BUFF - RCV.USER - RCV.WINDOW >= min(1/2 RCV.BUFF, MSS)"
  *
  * i.e. don't raise the right edge of the window until you can raise
  * it at least MSS bytes.
  *
  * Unfortunately, the recommended algorithm breaks header prediction,
  * since header prediction assumes th->window stays fixed.
  *
  * Strictly speaking, keeping th->window fixed violates the receiver
  * side SWS prevention criteria. The problem is that under this rule
  * a stream of single byte packets will cause the right side of the
  * window to always advance by a single byte.
  *
  * Of course, if the sender implements sender side SWS prevention
  * then this will not be a problem.
  *
  * BSD seems to make the following compromise:
  *
  *      If the free space is less than the 1/4 of the maximum
  *      space available and the free space is less than 1/2 mss,
  *      then set the window to 0.
  *      [ Actually, bsd uses MSS and 1/4 of maximal _window_ ]
  *      Otherwise, just prevent the window from shrinking
  *      and from being larger than the largest representable value.
  *
  * This prevents incremental opening of the window in the regime
  * where TCP is limited by the speed of the reader side taking
  * data out of the TCP receive queue. It does nothing about
  * those cases where the window is constrained on the sender side
  * because the pipeline is full.
  *
  * BSD also seems to "accidentally" limit itself to windows that are a
  * multiple of MSS, at least until the free space gets quite small.
  * This would appear to be a side effect of the mbuf implementation.
  * Combining these two algorithms results in the observed behavior
  * of having a fixed window size at almost all times.
  *
  * Below we obtain similar behavior by forcing the offered window to
  * a multiple of the mss when it is feasible to do so.
  *
  * Note, we don't "adjust" for TIMESTAMP or SACK option bytes.
  * Regular options like TIMESTAMP are taken into account.
  */
 u32 __tcp_select_window(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct net *net = sock_net(sk);
         /* MSS for the peer's data.  Previous versions used mss_clamp
          * here.  I don't know if the value based on our guesses
          * of peer's MSS is better for the performance.  It's more correct
          * but may be worse for the performance because of rcv_mss
          * fluctuations.  --SAW  1998/11/1
          */
         int mss = icsk->icsk_ack.rcv_mss;
         int free_space = tcp_space(sk);
         int allowed_space = tcp_full_space(sk);
         int full_space, window;
 
         if (sk_is_mptcp(sk))
                 mptcp_space(sk, &free_space, &allowed_space);
 
         full_space = min_t(int, tp->window_clamp, allowed_space);
 
         if (unlikely(mss > full_space)) {
                 mss = full_space;
                 if (mss <= 0)
                         return 0;
         }
 
         /* Only allow window shrink if the sysctl is enabled and we have
          * a non-zero scaling factor in effect.
          */
         if (READ_ONCE(net->ipv4.sysctl_tcp_shrink_window) && tp->rx_opt.rcv_wscale)
                 goto shrink_window_allowed;
 
         /* do not allow window to shrink */
 
         if (free_space < (full_space >> 1)) {
                 icsk->icsk_ack.quick = 0;
 
                 if (tcp_under_memory_pressure(sk))
                         tcp_adjust_rcv_ssthresh(sk);
 
                 /* free_space might become our new window, make sure we don't
                  * increase it due to wscale.
                  */
                 free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
 
                 /* if free space is less than mss estimate, or is below 1/16th
                  * of the maximum allowed, try to move to zero-window, else
                  * tcp_clamp_window() will grow rcv buf up to tcp_rmem[2], and
                  * new incoming data is dropped due to memory limits.
                  * With large window, mss test triggers way too late in order
                  * to announce zero window in time before rmem limit kicks in.
                  */
                 if (free_space < (allowed_space >> 4) || free_space < mss)
                         return 0;
         }
 
         if (free_space > tp->rcv_ssthresh)
                 free_space = tp->rcv_ssthresh;
 
         /* Don't do rounding if we are using window scaling, since the
          * scaled window will not line up with the MSS boundary anyway.
          */
         if (tp->rx_opt.rcv_wscale) {
                 window = free_space;
 
                 /* Advertise enough space so that it won't get scaled away.
                  * Import case: prevent zero window announcement if
                  * 1<<rcv_wscale > mss.
                  */
                 window = ALIGN(window, (1 << tp->rx_opt.rcv_wscale));
         } else {
                 window = tp->rcv_wnd;
                 /* Get the largest window that is a nice multiple of mss.
                  * Window clamp already applied above.
                  * If our current window offering is within 1 mss of the
                  * free space we just keep it. This prevents the divide
                  * and multiply from happening most of the time.
                  * We also don't do any window rounding when the free space
                  * is too small.
                  */
                 if (window <= free_space - mss || window > free_space)
                         window = rounddown(free_space, mss);
                 else if (mss == full_space &&
                          free_space > window + (full_space >> 1))
                         window = free_space;
         }
 
         return window;
 
 shrink_window_allowed:
         /* new window should always be an exact multiple of scaling factor */
         free_space = round_down(free_space, 1 << tp->rx_opt.rcv_wscale);
 
         if (free_space < (full_space >> 1)) {
                 icsk->icsk_ack.quick = 0;
 
                 if (tcp_under_memory_pressure(sk))
                         tcp_adjust_rcv_ssthresh(sk);
 
                 /* if free space is too low, return a zero window */
                 if (free_space < (allowed_space >> 4) || free_space < mss ||
                         free_space < (1 << tp->rx_opt.rcv_wscale))
                         return 0;
         }
 
         if (free_space > tp->rcv_ssthresh) {
                 free_space = tp->rcv_ssthresh;
                 /* new window should always be an exact multiple of scaling factor
                  *
                  * For this case, we ALIGN "up" (increase free_space) because
                  * we know free_space is not zero here, it has been reduced from
                  * the memory-based limit, and rcv_ssthresh is not a hard limit
                  * (unlike sk_rcvbuf).
                  */
                 free_space = ALIGN(free_space, (1 << tp->rx_opt.rcv_wscale));
         }
 
         return free_space;
 }
 
 void tcp_skb_collapse_tstamp(struct sk_buff *skb,
                              const struct sk_buff *next_skb)
 {
         if (unlikely(tcp_has_tx_tstamp(next_skb))) {
                 const struct skb_shared_info *next_shinfo =
                         skb_shinfo(next_skb);
                 struct skb_shared_info *shinfo = skb_shinfo(skb);
 
                 shinfo->tx_flags |= next_shinfo->tx_flags & SKBTX_ANY_TSTAMP;
                 shinfo->tskey = next_shinfo->tskey;
                 TCP_SKB_CB(skb)->txstamp_ack |=
                         TCP_SKB_CB(next_skb)->txstamp_ack;
         }
 }
 
 /* Collapses two adjacent SKB's during retransmission. */
 static bool tcp_collapse_retrans(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *next_skb = skb_rb_next(skb);
         int next_skb_size;
 
         next_skb_size = next_skb->len;
 
         BUG_ON(tcp_skb_pcount(skb) != 1 || tcp_skb_pcount(next_skb) != 1);
 
         if (next_skb_size && !tcp_skb_shift(skb, next_skb, 1, next_skb_size))
                 return false;
 
         tcp_highest_sack_replace(sk, next_skb, skb);
 
         /* Update sequence range on original skb. */
         TCP_SKB_CB(skb)->end_seq = TCP_SKB_CB(next_skb)->end_seq;
 
         /* Merge over control information. This moves PSH/FIN etc. over */
         TCP_SKB_CB(skb)->tcp_flags |= TCP_SKB_CB(next_skb)->tcp_flags;
 
         /* All done, get rid of second SKB and account for it so
          * packet counting does not break.
          */
         TCP_SKB_CB(skb)->sacked |= TCP_SKB_CB(next_skb)->sacked & TCPCB_EVER_RETRANS;
         TCP_SKB_CB(skb)->eor = TCP_SKB_CB(next_skb)->eor;
 
         /* changed transmit queue under us so clear hints */
         tcp_clear_retrans_hints_partial(tp);
         if (next_skb == tp->retransmit_skb_hint)
                 tp->retransmit_skb_hint = skb;
 
         tcp_adjust_pcount(sk, next_skb, tcp_skb_pcount(next_skb));
 
         tcp_skb_collapse_tstamp(skb, next_skb);
 
         tcp_rtx_queue_unlink_and_free(next_skb, sk);
         return true;
 }
 
 /* Check if coalescing SKBs is legal. */
 static bool tcp_can_collapse(const struct sock *sk, const struct sk_buff *skb)
 {
         if (tcp_skb_pcount(skb) > 1)
                 return false;
         if (skb_cloned(skb))
                 return false;
         /* Some heuristics for collapsing over SACK'd could be invented */
         if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED)
                 return false;
 
         return true;
 }
 
 /* Collapse packets in the retransmit queue to make to create
  * less packets on the wire. This is only done on retransmission.
  */
 static void tcp_retrans_try_collapse(struct sock *sk, struct sk_buff *to,
                                      int space)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb = to, *tmp;
         bool first = true;
 
         if (!READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_retrans_collapse))
                 return;
         if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
                 return;
 
         skb_rbtree_walk_from_safe(skb, tmp) {
                 if (!tcp_can_collapse(sk, skb))
                         break;
 
                 if (!tcp_skb_can_collapse(to, skb))
                         break;
 
                 space -= skb->len;
 
                 if (first) {
                         first = false;
                         continue;
                 }
 
                 if (space < 0)
                         break;
 
                 if (after(TCP_SKB_CB(skb)->end_seq, tcp_wnd_end(tp)))
                         break;
 
                 if (!tcp_collapse_retrans(sk, to))
                         break;
         }
 }
 
 /* This retransmits one SKB.  Policy decisions and retransmit queue
  * state updates are done by the caller.  Returns non-zero if an
  * error occurred which prevented the send.
  */
 int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         unsigned int cur_mss;
         int diff, len, err;
         int avail_wnd;
 
         /* Inconclusive MTU probe */
         if (icsk->icsk_mtup.probe_size)
                 icsk->icsk_mtup.probe_size = 0;
 
         if (skb_still_in_host_queue(sk, skb))
                 return -EBUSY;
 
 start:
         if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
                 if (unlikely(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
                         TCP_SKB_CB(skb)->tcp_flags &= ~TCPHDR_SYN;
                         TCP_SKB_CB(skb)->seq++;
                         goto start;
                 }
                 if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
                         WARN_ON_ONCE(1);
                         return -EINVAL;
                 }
                 if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
                         return -ENOMEM;
         }
 
         if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
                 return -EHOSTUNREACH; /* Routing failure or similar. */
 
         cur_mss = tcp_current_mss(sk);
         avail_wnd = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
         /* If receiver has shrunk his window, and skb is out of
          * new window, do not retransmit it. The exception is the
          * case, when window is shrunk to zero. In this case
          * our retransmit of one segment serves as a zero window probe.
          */
         if (avail_wnd <= 0) {
                 if (TCP_SKB_CB(skb)->seq != tp->snd_una)
                         return -EAGAIN;
                 avail_wnd = cur_mss;
         }
 
         len = cur_mss * segs;
         if (len > avail_wnd) {
                 len = rounddown(avail_wnd, cur_mss);
                 if (!len)
                         len = avail_wnd;
         }
         if (skb->len > len) {
                 if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
                                  cur_mss, GFP_ATOMIC))
                         return -ENOMEM; /* We'll try again later. */
         } else {
                 if (skb_unclone_keeptruesize(skb, GFP_ATOMIC))
                         return -ENOMEM;
 
                 diff = tcp_skb_pcount(skb);
                 tcp_set_skb_tso_segs(skb, cur_mss);
                 diff -= tcp_skb_pcount(skb);
                 if (diff)
                         tcp_adjust_pcount(sk, skb, diff);
                 avail_wnd = min_t(int, avail_wnd, cur_mss);
                 if (skb->len < avail_wnd)
                         tcp_retrans_try_collapse(sk, skb, avail_wnd);
         }
 
         /* RFC3168, section 6.1.1.1. ECN fallback */
         if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
                 tcp_ecn_clear_syn(sk, skb);
 
         /* Update global and local TCP statistics. */
         segs = tcp_skb_pcount(skb);
         TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
         if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
                 __NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
         tp->total_retrans += segs;
         tp->bytes_retrans += skb->len;
 
         /* make sure skb->data is aligned on arches that require it
          * and check if ack-trimming & collapsing extended the headroom
          * beyond what csum_start can cover.
          */
         if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
                      skb_headroom(skb) >= 0xFFFF)) {
                 struct sk_buff *nskb;
 
                 tcp_skb_tsorted_save(skb) {
                         nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
                         if (nskb) {
                                 nskb->dev = NULL;
                                 err = tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC);
                         } else {
                                 err = -ENOBUFS;
                         }
                 } tcp_skb_tsorted_restore(skb);
 
                 if (!err) {
                         tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
                         tcp_rate_skb_sent(sk, skb);
                 }
         } else {
                 err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
         }
 
         if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
                 tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
                                   TCP_SKB_CB(skb)->seq, segs, err);
 
         if (likely(!err)) {
                 trace_tcp_retransmit_skb(sk, skb);
         } else if (err != -EBUSY) {
                 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
         }
 
         /* To avoid taking spuriously low RTT samples based on a timestamp
          * for a transmit that never happened, always mark EVER_RETRANS
          */
         TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;
 
         return err;
 }
 
 int tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         int err = __tcp_retransmit_skb(sk, skb, segs);
 
         if (err == 0) {
 #if FASTRETRANS_DEBUG > 0
                 if (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS) {
                         net_dbg_ratelimited("retrans_out leaked\n");
                 }
 #endif
                 TCP_SKB_CB(skb)->sacked |= TCPCB_RETRANS;
                 tp->retrans_out += tcp_skb_pcount(skb);
         }
 
         /* Save stamp of the first (attempted) retransmit. */
         if (!tp->retrans_stamp)
                 tp->retrans_stamp = tcp_skb_timestamp_ts(tp->tcp_usec_ts, skb);
 
         if (tp->undo_retrans < 0)
                 tp->undo_retrans = 0;
         tp->undo_retrans += tcp_skb_pcount(skb);
         return err;
 }
 
 /* This gets called after a retransmit timeout, and the initially
  * retransmitted data is acknowledged.  It tries to continue
  * resending the rest of the retransmit queue, until either
  * we've sent it all or the congestion window limit is reached.
  */
 void tcp_xmit_retransmit_queue(struct sock *sk)
 {
         const struct inet_connection_sock *icsk = inet_csk(sk);
         struct sk_buff *skb, *rtx_head, *hole = NULL;
         struct tcp_sock *tp = tcp_sk(sk);
         bool rearm_timer = false;
         u32 max_segs;
         int mib_idx;
 
         if (!tp->packets_out)
                 return;
 
         rtx_head = tcp_rtx_queue_head(sk);
         skb = tp->retransmit_skb_hint ?: rtx_head;
         max_segs = tcp_tso_segs(sk, tcp_current_mss(sk));
         skb_rbtree_walk_from(skb) {
                 __u8 sacked;
                 int segs;
 
                 if (tcp_pacing_check(sk))
                         break;
 
                 /* we could do better than to assign each time */
                 if (!hole)
                         tp->retransmit_skb_hint = skb;
 
                 segs = tcp_snd_cwnd(tp) - tcp_packets_in_flight(tp);
                 if (segs <= 0)
                         break;
                 sacked = TCP_SKB_CB(skb)->sacked;
                 /* In case tcp_shift_skb_data() have aggregated large skbs,
                  * we need to make sure not sending too bigs TSO packets
                  */
                 segs = min_t(int, segs, max_segs);
 
                 if (tp->retrans_out >= tp->lost_out) {
                         break;
                 } else if (!(sacked & TCPCB_LOST)) {
                         if (!hole && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
                                 hole = skb;
                         continue;
 
                 } else {
                         if (icsk->icsk_ca_state != TCP_CA_Loss)
                                 mib_idx = LINUX_MIB_TCPFASTRETRANS;
                         else
                                 mib_idx = LINUX_MIB_TCPSLOWSTARTRETRANS;
                 }
 
                 if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
                         continue;
 
                 if (tcp_small_queue_check(sk, skb, 1))
                         break;
 
                 if (tcp_retransmit_skb(sk, skb, segs))
                         break;
 
                 NET_ADD_STATS(sock_net(sk), mib_idx, tcp_skb_pcount(skb));
 
                 if (tcp_in_cwnd_reduction(sk))
                         tp->prr_out += tcp_skb_pcount(skb);
 
                 if (skb == rtx_head &&
                     icsk->icsk_pending != ICSK_TIME_REO_TIMEOUT)
                         rearm_timer = true;
 
         }
         if (rearm_timer)
                 tcp_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                      inet_csk(sk)->icsk_rto,
                                      TCP_RTO_MAX);
 }
 
 /* We allow to exceed memory limits for FIN packets to expedite
  * connection tear down and (memory) recovery.
  * Otherwise tcp_send_fin() could be tempted to either delay FIN
  * or even be forced to close flow without any FIN.
  * In general, we want to allow one skb per socket to avoid hangs
  * with edge trigger epoll()
  */
 void sk_forced_mem_schedule(struct sock *sk, int size)
 {
         int delta, amt;
 
         delta = size - sk->sk_forward_alloc;
         if (delta <= 0)
                 return;
         amt = sk_mem_pages(delta);
         sk_forward_alloc_add(sk, amt << PAGE_SHIFT);
         sk_memory_allocated_add(sk, amt);
 
         if (mem_cgroup_sockets_enabled && sk->sk_memcg)
                 mem_cgroup_charge_skmem(sk->sk_memcg, amt,
                                         gfp_memcg_charge() | __GFP_NOFAIL);
 }
 
 /* Send a FIN. The caller locks the socket for us.
  * We should try to send a FIN packet really hard, but eventually give up.
  */
 void tcp_send_fin(struct sock *sk)
 {
         struct sk_buff *skb, *tskb, *tail = tcp_write_queue_tail(sk);
         struct tcp_sock *tp = tcp_sk(sk);
 
         /* Optimization, tack on the FIN if we have one skb in write queue and
          * this skb was not yet sent, or we are under memory pressure.
          * Note: in the latter case, FIN packet will be sent after a timeout,
          * as TCP stack thinks it has already been transmitted.
          */
         tskb = tail;
         if (!tskb && tcp_under_memory_pressure(sk))
                 tskb = skb_rb_last(&sk->tcp_rtx_queue);
 
         if (tskb) {
                 TCP_SKB_CB(tskb)->tcp_flags |= TCPHDR_FIN;
                 TCP_SKB_CB(tskb)->end_seq++;
                 tp->write_seq++;
                 if (!tail) {
                         /* This means tskb was already sent.
                          * Pretend we included the FIN on previous transmit.
                          * We need to set tp->snd_nxt to the value it would have
                          * if FIN had been sent. This is because retransmit path
                          * does not change tp->snd_nxt.
                          */
                         WRITE_ONCE(tp->snd_nxt, tp->snd_nxt + 1);
                         return;
                 }
         } else {
                 skb = alloc_skb_fclone(MAX_TCP_HEADER,
                                        sk_gfp_mask(sk, GFP_ATOMIC |
                                                        __GFP_NOWARN));
                 if (unlikely(!skb))
                         return;
 
                 INIT_LIST_HEAD(&skb->tcp_tsorted_anchor);
                 skb_reserve(skb, MAX_TCP_HEADER);
                 sk_forced_mem_schedule(sk, skb->truesize);
                 /* FIN eats a sequence byte, write_seq advanced by tcp_queue_skb(). */
                 tcp_init_nondata_skb(skb, tp->write_seq,
                                      TCPHDR_ACK | TCPHDR_FIN);
                 tcp_queue_skb(sk, skb);
         }
         __tcp_push_pending_frames(sk, tcp_current_mss(sk), TCP_NAGLE_OFF);
 }
 
 /* We get here when a process closes a file descriptor (either due to
  * an explicit close() or as a byproduct of exit()'ing) and there
  * was unread data in the receive queue.  This behavior is recommended
  * by RFC 2525, section 2.17.  -DaveM
  */
 void tcp_send_active_reset(struct sock *sk, gfp_t priority,
                            enum sk_rst_reason reason)
 {
         struct sk_buff *skb;
 
         TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTRSTS);
 
         /* NOTE: No TCP options attached and we never retransmit this. */
         skb = alloc_skb(MAX_TCP_HEADER, priority);
         if (!skb) {
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
                 return;
         }
 
         /* Reserve space for headers and prepare control bits. */
         skb_reserve(skb, MAX_TCP_HEADER);
         tcp_init_nondata_skb(skb, tcp_acceptable_seq(sk),
                              TCPHDR_ACK | TCPHDR_RST);
         tcp_mstamp_refresh(tcp_sk(sk));
         /* Send it off. */
         if (tcp_transmit_skb(sk, skb, 0, priority))
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPABORTFAILED);
 
         /* skb of trace_tcp_send_reset() keeps the skb that caused RST,
          * skb here is different to the troublesome skb, so use NULL
          */
         trace_tcp_send_reset(sk, NULL, SK_RST_REASON_NOT_SPECIFIED);
 }
 
 /* Send a crossed SYN-ACK during socket establishment.
  * WARNING: This routine must only be called when we have already sent
  * a SYN packet that crossed the incoming SYN that caused this routine
  * to get called. If this assumption fails then the initial rcv_wnd
  * and rcv_wscale values will not be correct.
  */
 int tcp_send_synack(struct sock *sk)
 {
         struct sk_buff *skb;
 
         skb = tcp_rtx_queue_head(sk);
         if (!skb || !(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)) {
                 pr_err("%s: wrong queue state\n", __func__);
                 return -EFAULT;
         }
         if (!(TCP_SKB_CB(skb)->tcp_flags & TCPHDR_ACK)) {
                 if (skb_cloned(skb)) {
                         struct sk_buff *nskb;
 
                         tcp_skb_tsorted_save(skb) {
                                 nskb = skb_copy(skb, GFP_ATOMIC);
                         } tcp_skb_tsorted_restore(skb);
                         if (!nskb)
                                 return -ENOMEM;
                         INIT_LIST_HEAD(&nskb->tcp_tsorted_anchor);
                         tcp_highest_sack_replace(sk, skb, nskb);
                         tcp_rtx_queue_unlink_and_free(skb, sk);
                         __skb_header_release(nskb);
                         tcp_rbtree_insert(&sk->tcp_rtx_queue, nskb);
                         sk_wmem_queued_add(sk, nskb->truesize);
                         sk_mem_charge(sk, nskb->truesize);
                         skb = nskb;
                 }
 
                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_ACK;
                 tcp_ecn_send_synack(sk, skb);
         }
         return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
 }
 
 /**
  * tcp_make_synack - Allocate one skb and build a SYNACK packet.
  * @sk: listener socket
  * @dst: dst entry attached to the SYNACK. It is consumed and caller
  *       should not use it again.
  * @req: request_sock pointer
  * @foc: cookie for tcp fast open
  * @synack_type: Type of synack to prepare
  * @syn_skb: SYN packet just received.  It could be NULL for rtx case.
  */
 struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
                                 struct request_sock *req,
                                 struct tcp_fastopen_cookie *foc,
                                 enum tcp_synack_type synack_type,
                                 struct sk_buff *syn_skb)
 {
         struct inet_request_sock *ireq = inet_rsk(req);
         const struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_out_options opts;
         struct tcp_key key = {};
         struct sk_buff *skb;
         int tcp_header_size;
         struct tcphdr *th;
         int mss;
         u64 now;
 
         skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
         if (unlikely(!skb)) {
                 dst_release(dst);
                 return NULL;
         }
         /* Reserve space for headers. */
         skb_reserve(skb, MAX_TCP_HEADER);
 
         switch (synack_type) {
         case TCP_SYNACK_NORMAL:
                 skb_set_owner_w(skb, req_to_sk(req));
                 break;
         case TCP_SYNACK_COOKIE:
                 /* Under synflood, we do not attach skb to a socket,
                  * to avoid false sharing.
                  */
                 break;
         case TCP_SYNACK_FASTOPEN:
                 /* sk is a const pointer, because we want to express multiple
                  * cpu might call us concurrently.
                  * sk->sk_wmem_alloc in an atomic, we can promote to rw.
                  */
                 skb_set_owner_w(skb, (struct sock *)sk);
                 break;
         }
         skb_dst_set(skb, dst);
 
         mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
         memset(&opts, 0, sizeof(opts));
         now = tcp_clock_ns();
 #ifdef CONFIG_SYN_COOKIES
         if (unlikely(synack_type == TCP_SYNACK_COOKIE && ireq->tstamp_ok))
                 skb_set_delivery_time(skb, cookie_init_timestamp(req, now),
                                       SKB_CLOCK_MONOTONIC);
         else
 #endif
         {
                 skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
                 if (!tcp_rsk(req)->snt_synack) /* Timestamp first SYNACK */
                         tcp_rsk(req)->snt_synack = tcp_skb_timestamp_us(skb);
         }
 
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
         rcu_read_lock();
 #endif
         if (tcp_rsk_used_ao(req)) {
 #ifdef CONFIG_TCP_AO
                 struct tcp_ao_key *ao_key = NULL;
                 u8 keyid = tcp_rsk(req)->ao_keyid;
                 u8 rnext = tcp_rsk(req)->ao_rcv_next;
 
                 ao_key = tcp_sk(sk)->af_specific->ao_lookup(sk, req_to_sk(req),
                                                             keyid, -1);
                 /* If there is no matching key - avoid sending anything,
                  * especially usigned segments. It could try harder and lookup
                  * for another peer-matching key, but the peer has requested
                  * ao_keyid (RFC5925 RNextKeyID), so let's keep it simple here.
                  */
                 if (unlikely(!ao_key)) {
                         trace_tcp_ao_synack_no_key(sk, keyid, rnext);
                         rcu_read_unlock();
                         kfree_skb(skb);
                         net_warn_ratelimited("TCP-AO: the keyid %u from SYN packet is not present - not sending SYNACK\n",
                                              keyid);
                         return NULL;
                 }
                 key.ao_key = ao_key;
                 key.type = TCP_KEY_AO;
 #endif
         } else {
 #ifdef CONFIG_TCP_MD5SIG
                 key.md5_key = tcp_rsk(req)->af_specific->req_md5_lookup(sk,
                                         req_to_sk(req));
                 if (key.md5_key)
                         key.type = TCP_KEY_MD5;
 #endif
         }
         skb_set_hash(skb, READ_ONCE(tcp_rsk(req)->txhash), PKT_HASH_TYPE_L4);
         /* bpf program will be interested in the tcp_flags */
         TCP_SKB_CB(skb)->tcp_flags = TCPHDR_SYN | TCPHDR_ACK;
         tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts,
                                              &key, foc, synack_type, syn_skb)
                                         + sizeof(*th);
 
         skb_push(skb, tcp_header_size);
         skb_reset_transport_header(skb);
 
         th = (struct tcphdr *)skb->data;
         memset(th, 0, sizeof(struct tcphdr));
         th->syn = 1;
         th->ack = 1;
         tcp_ecn_make_synack(req, th);
         th->source = htons(ireq->ir_num);
         th->dest = ireq->ir_rmt_port;
         skb->mark = ireq->ir_mark;
         skb->ip_summed = CHECKSUM_PARTIAL;
         th->seq = htonl(tcp_rsk(req)->snt_isn);
         /* XXX data is queued and acked as is. No buffer/window check */
         th->ack_seq = htonl(tcp_rsk(req)->rcv_nxt);
 
         /* RFC1323: The window in SYN & SYN/ACK segments is never scaled. */
         th->window = htons(min(req->rsk_rcv_wnd, 65535U));
         tcp_options_write(th, NULL, tcp_rsk(req), &opts, &key);
         th->doff = (tcp_header_size >> 2);
         TCP_INC_STATS(sock_net(sk), TCP_MIB_OUTSEGS);
 
         /* Okay, we have all we need - do the md5 hash if needed */
         if (tcp_key_is_md5(&key)) {
 #ifdef CONFIG_TCP_MD5SIG
                 tcp_rsk(req)->af_specific->calc_md5_hash(opts.hash_location,
                                         key.md5_key, req_to_sk(req), skb);
 #endif
         } else if (tcp_key_is_ao(&key)) {
 #ifdef CONFIG_TCP_AO
                 tcp_rsk(req)->af_specific->ao_synack_hash(opts.hash_location,
                                         key.ao_key, req, skb,
                                         opts.hash_location - (u8 *)th, 0);
 #endif
         }
 #if defined(CONFIG_TCP_MD5SIG) || defined(CONFIG_TCP_AO)
         rcu_read_unlock();
 #endif
 
         bpf_skops_write_hdr_opt((struct sock *)sk, skb, req, syn_skb,
                                 synack_type, &opts);
 
         skb_set_delivery_time(skb, now, SKB_CLOCK_MONOTONIC);
         tcp_add_tx_delay(skb, tp);
 
         return skb;
 }
 EXPORT_SYMBOL(tcp_make_synack);
 
 static void tcp_ca_dst_init(struct sock *sk, const struct dst_entry *dst)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         const struct tcp_congestion_ops *ca;
         u32 ca_key = dst_metric(dst, RTAX_CC_ALGO);
 
         if (ca_key == TCP_CA_UNSPEC)
                 return;
 
         rcu_read_lock();
         ca = tcp_ca_find_key(ca_key);
         if (likely(ca && bpf_try_module_get(ca, ca->owner))) {
                 bpf_module_put(icsk->icsk_ca_ops, icsk->icsk_ca_ops->owner);
                 icsk->icsk_ca_dst_locked = tcp_ca_dst_locked(dst);
                 icsk->icsk_ca_ops = ca;
         }
         rcu_read_unlock();
 }
 
 /* Do all connect socket setups that can be done AF independent. */
 static void tcp_connect_init(struct sock *sk)
 {
         const struct dst_entry *dst = __sk_dst_get(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         __u8 rcv_wscale;
         u32 rcv_wnd;
 
         /* We'll fix this up when we get a response from the other end.
          * See tcp_input.c:tcp_rcv_state_process case TCP_SYN_SENT.
          */
         tp->tcp_header_len = sizeof(struct tcphdr);
         if (READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_timestamps))
                 tp->tcp_header_len += TCPOLEN_TSTAMP_ALIGNED;
 
         tcp_ao_connect_init(sk);
 
         /* If user gave his TCP_MAXSEG, record it to clamp */
         if (tp->rx_opt.user_mss)
                 tp->rx_opt.mss_clamp = tp->rx_opt.user_mss;
         tp->max_window = 0;
         tcp_mtup_init(sk);
         tcp_sync_mss(sk, dst_mtu(dst));
 
         tcp_ca_dst_init(sk, dst);
 
         if (!tp->window_clamp)
                 WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
         tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
 
         tcp_initialize_rcv_mss(sk);
 
         /* limit the window selection if the user enforce a smaller rx buffer */
         if (sk->sk_userlocks & SOCK_RCVBUF_LOCK &&
             (tp->window_clamp > tcp_full_space(sk) || tp->window_clamp == 0))
                 WRITE_ONCE(tp->window_clamp, tcp_full_space(sk));
 
         rcv_wnd = tcp_rwnd_init_bpf(sk);
         if (rcv_wnd == 0)
                 rcv_wnd = dst_metric(dst, RTAX_INITRWND);
 
         tcp_select_initial_window(sk, tcp_full_space(sk),
                                   tp->advmss - (tp->rx_opt.ts_recent_stamp ? tp->tcp_header_len - sizeof(struct tcphdr) : 0),
                                   &tp->rcv_wnd,
                                   &tp->window_clamp,
                                   READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_window_scaling),
                                   &rcv_wscale,
                                   rcv_wnd);
 
         tp->rx_opt.rcv_wscale = rcv_wscale;
         tp->rcv_ssthresh = tp->rcv_wnd;
 
         WRITE_ONCE(sk->sk_err, 0);
         sock_reset_flag(sk, SOCK_DONE);
         tp->snd_wnd = 0;
         tcp_init_wl(tp, 0);
         tcp_write_queue_purge(sk);
         tp->snd_una = tp->write_seq;
         tp->snd_sml = tp->write_seq;
         tp->snd_up = tp->write_seq;
         WRITE_ONCE(tp->snd_nxt, tp->write_seq);
 
         if (likely(!tp->repair))
                 tp->rcv_nxt = 0;
         else
                 tp->rcv_tstamp = tcp_jiffies32;
         tp->rcv_wup = tp->rcv_nxt;
         WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
 
         inet_csk(sk)->icsk_rto = tcp_timeout_init(sk);
         inet_csk(sk)->icsk_retransmits = 0;
         tcp_clear_retrans(tp);
 }
 
 static void tcp_connect_queue_skb(struct sock *sk, struct sk_buff *skb)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_skb_cb *tcb = TCP_SKB_CB(skb);
 
         tcb->end_seq += skb->len;
         __skb_header_release(skb);
         sk_wmem_queued_add(sk, skb->truesize);
         sk_mem_charge(sk, skb->truesize);
         WRITE_ONCE(tp->write_seq, tcb->end_seq);
         tp->packets_out += tcp_skb_pcount(skb);
 }
 
 /* Build and send a SYN with data and (cached) Fast Open cookie. However,
  * queue a data-only packet after the regular SYN, such that regular SYNs
  * are retransmitted on timeouts. Also if the remote SYN-ACK acknowledges
  * only the SYN sequence, the data are retransmitted in the first ACK.
  * If cookie is not cached or other error occurs, falls back to send a
  * regular SYN with Fast Open cookie request option.
  */
 static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_fastopen_request *fo = tp->fastopen_req;
         struct page_frag *pfrag = sk_page_frag(sk);
         struct sk_buff *syn_data;
         int space, err = 0;
 
         tp->rx_opt.mss_clamp = tp->advmss;  /* If MSS is not cached */
         if (!tcp_fastopen_cookie_check(sk, &tp->rx_opt.mss_clamp, &fo->cookie))
                 goto fallback;
 
         /* MSS for SYN-data is based on cached MSS and bounded by PMTU and
          * user-MSS. Reserve maximum option space for middleboxes that add
          * private TCP options. The cost is reduced data space in SYN :(
          */
         tp->rx_opt.mss_clamp = tcp_mss_clamp(tp, tp->rx_opt.mss_clamp);
         /* Sync mss_cache after updating the mss_clamp */
         tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 
         space = __tcp_mtu_to_mss(sk, icsk->icsk_pmtu_cookie) -
                 MAX_TCP_OPTION_SPACE;
 
         space = min_t(size_t, space, fo->size);
 
         if (space &&
             !skb_page_frag_refill(min_t(size_t, space, PAGE_SIZE),
                                   pfrag, sk->sk_allocation))
                 goto fallback;
         syn_data = tcp_stream_alloc_skb(sk, sk->sk_allocation, false);
         if (!syn_data)
                 goto fallback;
         memcpy(syn_data->cb, syn->cb, sizeof(syn->cb));
         if (space) {
                 space = min_t(size_t, space, pfrag->size - pfrag->offset);
                 space = tcp_wmem_schedule(sk, space);
         }
         if (space) {
                 space = copy_page_from_iter(pfrag->page, pfrag->offset,
                                             space, &fo->data->msg_iter);
                 if (unlikely(!space)) {
                         tcp_skb_tsorted_anchor_cleanup(syn_data);
                         kfree_skb(syn_data);
                         goto fallback;
                 }
                 skb_fill_page_desc(syn_data, 0, pfrag->page,
                                    pfrag->offset, space);
                 page_ref_inc(pfrag->page);
                 pfrag->offset += space;
                 skb_len_add(syn_data, space);
                 skb_zcopy_set(syn_data, fo->uarg, NULL);
         }
         /* No more data pending in inet_wait_for_connect() */
         if (space == fo->size)
                 fo->data = NULL;
         fo->copied = space;
 
         tcp_connect_queue_skb(sk, syn_data);
         if (syn_data->len)
                 tcp_chrono_start(sk, TCP_CHRONO_BUSY);
 
         err = tcp_transmit_skb(sk, syn_data, 1, sk->sk_allocation);
 
         skb_set_delivery_time(syn, syn_data->skb_mstamp_ns, SKB_CLOCK_MONOTONIC);
 
         /* Now full SYN+DATA was cloned and sent (or not),
          * remove the SYN from the original skb (syn_data)
          * we keep in write queue in case of a retransmit, as we
          * also have the SYN packet (with no data) in the same queue.
          */
         TCP_SKB_CB(syn_data)->seq++;
         TCP_SKB_CB(syn_data)->tcp_flags = TCPHDR_ACK | TCPHDR_PSH;
         if (!err) {
                 tp->syn_data = (fo->copied > 0);
                 tcp_rbtree_insert(&sk->tcp_rtx_queue, syn_data);
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPORIGDATASENT);
                 goto done;
         }
 
         /* data was not sent, put it in write_queue */
         __skb_queue_tail(&sk->sk_write_queue, syn_data);
         tp->packets_out -= tcp_skb_pcount(syn_data);
 
 fallback:
         /* Send a regular SYN with Fast Open cookie request option */
         if (fo->cookie.len > 0)
                 fo->cookie.len = 0;
         err = tcp_transmit_skb(sk, syn, 1, sk->sk_allocation);
         if (err)
                 tp->syn_fastopen = 0;
 done:
         fo->cookie.len = -1;  /* Exclude Fast Open option for SYN retries */
         return err;
 }
 
 /* Build a SYN and send it off. */
 int tcp_connect(struct sock *sk)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *buff;
         int err;
 
         tcp_call_bpf(sk, BPF_SOCK_OPS_TCP_CONNECT_CB, 0, NULL);
 
 #if defined(CONFIG_TCP_MD5SIG) && defined(CONFIG_TCP_AO)
         /* Has to be checked late, after setting daddr/saddr/ops.
          * Return error if the peer has both a md5 and a tcp-ao key
          * configured as this is ambiguous.
          */
         if (unlikely(rcu_dereference_protected(tp->md5sig_info,
                                                lockdep_sock_is_held(sk)))) {
                 bool needs_ao = !!tp->af_specific->ao_lookup(sk, sk, -1, -1);
                 bool needs_md5 = !!tp->af_specific->md5_lookup(sk, sk);
                 struct tcp_ao_info *ao_info;
 
                 ao_info = rcu_dereference_check(tp->ao_info,
                                                 lockdep_sock_is_held(sk));
                 if (ao_info) {
                         /* This is an extra check: tcp_ao_required() in
                          * tcp_v{4,6}_parse_md5_keys() should prevent adding
                          * md5 keys on ao_required socket.
                          */
                         needs_ao |= ao_info->ao_required;
                         WARN_ON_ONCE(ao_info->ao_required && needs_md5);
                 }
                 if (needs_md5 && needs_ao)
                         return -EKEYREJECTED;
 
                 /* If we have a matching md5 key and no matching tcp-ao key
                  * then free up ao_info if allocated.
                  */
                 if (needs_md5) {
                         tcp_ao_destroy_sock(sk, false);
                 } else if (needs_ao) {
                         tcp_clear_md5_list(sk);
                         kfree(rcu_replace_pointer(tp->md5sig_info, NULL,
                                                   lockdep_sock_is_held(sk)));
                 }
         }
 #endif
 #ifdef CONFIG_TCP_AO
         if (unlikely(rcu_dereference_protected(tp->ao_info,
                                                lockdep_sock_is_held(sk)))) {
                 /* Don't allow connecting if ao is configured but no
                  * matching key is found.
                  */
                 if (!tp->af_specific->ao_lookup(sk, sk, -1, -1))
                         return -EKEYREJECTED;
         }
 #endif
 
         if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
                 return -EHOSTUNREACH; /* Routing failure or similar. */
 
         tcp_connect_init(sk);
 
         if (unlikely(tp->repair)) {
                 tcp_finish_connect(sk, NULL);
                 return 0;
         }
 
         buff = tcp_stream_alloc_skb(sk, sk->sk_allocation, true);
         if (unlikely(!buff))
                 return -ENOBUFS;
 
         tcp_init_nondata_skb(buff, tp->write_seq++, TCPHDR_SYN);
         tcp_mstamp_refresh(tp);
         tp->retrans_stamp = tcp_time_stamp_ts(tp);
         tcp_connect_queue_skb(sk, buff);
         tcp_ecn_send_syn(sk, buff);
         tcp_rbtree_insert(&sk->tcp_rtx_queue, buff);
 
         /* Send off SYN; include data in Fast Open. */
         err = tp->fastopen_req ? tcp_send_syn_data(sk, buff) :
               tcp_transmit_skb(sk, buff, 1, sk->sk_allocation);
         if (err == -ECONNREFUSED)
                 return err;
 
         /* We change tp->snd_nxt after the tcp_transmit_skb() call
          * in order to make this packet get counted in tcpOutSegs.
          */
         WRITE_ONCE(tp->snd_nxt, tp->write_seq);
         tp->pushed_seq = tp->write_seq;
         buff = tcp_send_head(sk);
         if (unlikely(buff)) {
                 WRITE_ONCE(tp->snd_nxt, TCP_SKB_CB(buff)->seq);
                 tp->pushed_seq  = TCP_SKB_CB(buff)->seq;
         }
         TCP_INC_STATS(sock_net(sk), TCP_MIB_ACTIVEOPENS);
 
         /* Timer for repeating the SYN until an answer. */
         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
                                   inet_csk(sk)->icsk_rto, TCP_RTO_MAX);
         return 0;
 }
 EXPORT_SYMBOL(tcp_connect);
 
 u32 tcp_delack_max(const struct sock *sk)
 {
         u32 delack_from_rto_min = max(tcp_rto_min(sk), 2) - 1;
 
         return min(inet_csk(sk)->icsk_delack_max, delack_from_rto_min);
 }
 
 /* Send out a delayed ack, the caller does the policy checking
  * to see if we should even be here.  See tcp_input.c:tcp_ack_snd_check()
  * for details.
  */
 void tcp_send_delayed_ack(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         int ato = icsk->icsk_ack.ato;
         unsigned long timeout;
 
         if (ato > TCP_DELACK_MIN) {
                 const struct tcp_sock *tp = tcp_sk(sk);
                 int max_ato = HZ / 2;
 
                 if (inet_csk_in_pingpong_mode(sk) ||
                     (icsk->icsk_ack.pending & ICSK_ACK_PUSHED))
                         max_ato = TCP_DELACK_MAX;
 
                 /* Slow path, intersegment interval is "high". */
 
                 /* If some rtt estimate is known, use it to bound delayed ack.
                  * Do not use inet_csk(sk)->icsk_rto here, use results of rtt measurements
                  * directly.
                  */
                 if (tp->srtt_us) {
                         int rtt = max_t(int, usecs_to_jiffies(tp->srtt_us >> 3),
                                         TCP_DELACK_MIN);
 
                         if (rtt < max_ato)
                                 max_ato = rtt;
                 }
 
                 ato = min(ato, max_ato);
         }
 
         ato = min_t(u32, ato, tcp_delack_max(sk));
 
         /* Stay within the limit we were given */
         timeout = jiffies + ato;
 
         /* Use new timeout only if there wasn't a older one earlier. */
         if (icsk->icsk_ack.pending & ICSK_ACK_TIMER) {
                 /* If delack timer is about to expire, send ACK now. */
                 if (time_before_eq(icsk->icsk_ack.timeout, jiffies + (ato >> 2))) {
                         tcp_send_ack(sk);
                         return;
                 }
 
                 if (!time_before(timeout, icsk->icsk_ack.timeout))
                         timeout = icsk->icsk_ack.timeout;
         }
         icsk->icsk_ack.pending |= ICSK_ACK_SCHED | ICSK_ACK_TIMER;
         icsk->icsk_ack.timeout = timeout;
         sk_reset_timer(sk, &icsk->icsk_delack_timer, timeout);
 }
 
 /* This routine sends an ack and also updates the window. */
 void __tcp_send_ack(struct sock *sk, u32 rcv_nxt)
 {
         struct sk_buff *buff;
 
         /* If we have been reset, we may not send again. */
         if (sk->sk_state == TCP_CLOSE)
                 return;
 
         /* We are not putting this on the write queue, so
          * tcp_transmit_skb() will set the ownership to this
          * sock.
          */
         buff = alloc_skb(MAX_TCP_HEADER,
                          sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
         if (unlikely(!buff)) {
                 struct inet_connection_sock *icsk = inet_csk(sk);
                 unsigned long delay;
 
                 delay = TCP_DELACK_MAX << icsk->icsk_ack.retry;
                 if (delay < TCP_RTO_MAX)
                         icsk->icsk_ack.retry++;
                 inet_csk_schedule_ack(sk);
                 icsk->icsk_ack.ato = TCP_ATO_MIN;
                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK, delay, TCP_RTO_MAX);
                 return;
         }
 
         /* Reserve space for headers and prepare control bits. */
         skb_reserve(buff, MAX_TCP_HEADER);
         tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);
 
         /* We do not want pure acks influencing TCP Small Queues or fq/pacing
          * too much.
          * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784
          */
         skb_set_tcp_pure_ack(buff);
 
         /* Send it off, this clears delayed acks for us. */
         __tcp_transmit_skb(sk, buff, 0, (__force gfp_t)0, rcv_nxt);
 }
 EXPORT_SYMBOL_GPL(__tcp_send_ack);
 
 void tcp_send_ack(struct sock *sk)
 {
         __tcp_send_ack(sk, tcp_sk(sk)->rcv_nxt);
 }
 
 /* This routine sends a packet with an out of date sequence
  * number. It assumes the other end will try to ack it.
  *
  * Question: what should we make while urgent mode?
  * 4.4BSD forces sending single byte of data. We cannot send
  * out of window data, because we have SND.NXT==SND.MAX...
  *
  * Current solution: to send TWO zero-length segments in urgent mode:
  * one is with SEG.SEQ=SND.UNA to deliver urgent pointer, another is
  * out-of-date with SND.UNA-1 to probe window.
  */
 static int tcp_xmit_probe_skb(struct sock *sk, int urgent, int mib)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
 
         /* We don't queue it, tcp_transmit_skb() sets ownership. */
         skb = alloc_skb(MAX_TCP_HEADER,
                         sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));
         if (!skb)
                 return -1;
 
         /* Reserve space for headers and set control bits. */
         skb_reserve(skb, MAX_TCP_HEADER);
         /* Use a previous sequence.  This should cause the other
          * end to send an ack.  Don't queue or clone SKB, just
          * send it.
          */
         tcp_init_nondata_skb(skb, tp->snd_una - !urgent, TCPHDR_ACK);
         NET_INC_STATS(sock_net(sk), mib);
         return tcp_transmit_skb(sk, skb, 0, (__force gfp_t)0);
 }
 
 /* Called from setsockopt( ... TCP_REPAIR ) */
 void tcp_send_window_probe(struct sock *sk)
 {
         if (sk->sk_state == TCP_ESTABLISHED) {
                 tcp_sk(sk)->snd_wl1 = tcp_sk(sk)->rcv_nxt - 1;
                 tcp_mstamp_refresh(tcp_sk(sk));
                 tcp_xmit_probe_skb(sk, 0, LINUX_MIB_TCPWINPROBE);
         }
 }
 
 /* Initiate keepalive or window probe from timer. */
 int tcp_write_wakeup(struct sock *sk, int mib)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
 
         if (sk->sk_state == TCP_CLOSE)
                 return -1;
 
         skb = tcp_send_head(sk);
         if (skb && before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp))) {
                 int err;
                 unsigned int mss = tcp_current_mss(sk);
                 unsigned int seg_size = tcp_wnd_end(tp) - TCP_SKB_CB(skb)->seq;
 
                 if (before(tp->pushed_seq, TCP_SKB_CB(skb)->end_seq))
                         tp->pushed_seq = TCP_SKB_CB(skb)->end_seq;
 
                 /* We are probing the opening of a window
                  * but the window size is != 0
                  * must have been a result SWS avoidance ( sender )
                  */
                 if (seg_size < TCP_SKB_CB(skb)->end_seq - TCP_SKB_CB(skb)->seq ||
                     skb->len > mss) {
                         seg_size = min(seg_size, mss);
                         TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
                         if (tcp_fragment(sk, TCP_FRAG_IN_WRITE_QUEUE,
                                          skb, seg_size, mss, GFP_ATOMIC))
                                 return -1;
                 } else if (!tcp_skb_pcount(skb))
                         tcp_set_skb_tso_segs(skb, mss);
 
                 TCP_SKB_CB(skb)->tcp_flags |= TCPHDR_PSH;
                 err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
                 if (!err)
                         tcp_event_new_data_sent(sk, skb);
                 return err;
         } else {
                 if (between(tp->snd_up, tp->snd_una + 1, tp->snd_una + 0xFFFF))
                         tcp_xmit_probe_skb(sk, 1, mib);
                 return tcp_xmit_probe_skb(sk, 0, mib);
         }
 }
 
 /* A window probe timeout has occurred.  If window is not closed send
  * a partial packet else a zero probe.
  */
 void tcp_send_probe0(struct sock *sk)
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct net *net = sock_net(sk);
         unsigned long timeout;
         int err;
 
         err = tcp_write_wakeup(sk, LINUX_MIB_TCPWINPROBE);
 
         if (tp->packets_out || tcp_write_queue_empty(sk)) {
                 /* Cancel probe timer, if it is not required. */
                 icsk->icsk_probes_out = 0;
                 icsk->icsk_backoff = 0;
                 icsk->icsk_probes_tstamp = 0;
                 return;
         }
 
         icsk->icsk_probes_out++;
         if (err <= 0) {
                 if (icsk->icsk_backoff < READ_ONCE(net->ipv4.sysctl_tcp_retries2))
                         icsk->icsk_backoff++;
                 timeout = tcp_probe0_when(sk, TCP_RTO_MAX);
         } else {
                 /* If packet was not sent due to local congestion,
                  * Let senders fight for local resources conservatively.
                  */
                 timeout = TCP_RESOURCE_PROBE_INTERVAL;
         }
 
         timeout = tcp_clamp_probe0_to_user_timeout(sk, timeout);
         tcp_reset_xmit_timer(sk, ICSK_TIME_PROBE0, timeout, TCP_RTO_MAX);
 }
 
 int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 {
         const struct tcp_request_sock_ops *af_ops = tcp_rsk(req)->af_specific;
         struct flowi fl;
         int res;
 
         /* Paired with WRITE_ONCE() in sock_setsockopt() */
         if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
                 WRITE_ONCE(tcp_rsk(req)->txhash, net_tx_rndhash());
         res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
                                   NULL);
         if (!res) {
                 TCP_INC_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS);
                 NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
                 if (unlikely(tcp_passive_fastopen(sk))) {
                         /* sk has const attribute because listeners are lockless.
                          * However in this case, we are dealing with a passive fastopen
                          * socket thus we can change total_retrans value.
                          */
                         tcp_sk_rw(sk)->total_retrans++;
                 }
                 trace_tcp_retransmit_synack(sk, req);
         }
         return res;
 }
 EXPORT_SYMBOL(tcp_rtx_synack);