TOMOYO Linux Cross Reference
Linux/fs/eventpoll.c

   // SPDX-License-Identifier: GPL-2.0-or-later
   /*
    *  fs/eventpoll.c (Efficient event retrieval implementation)
    *  Copyright (C) 2001,...,2009  Davide Libenzi
    *
    *  Davide Libenzi <davidel@xmailserver.org>
    */
   
   #include <linux/init.h>
  #include <linux/kernel.h>
  #include <linux/sched/signal.h>
  #include <linux/fs.h>
  #include <linux/file.h>
  #include <linux/signal.h>
  #include <linux/errno.h>
  #include <linux/mm.h>
  #include <linux/slab.h>
  #include <linux/poll.h>
  #include <linux/string.h>
  #include <linux/list.h>
  #include <linux/hash.h>
  #include <linux/spinlock.h>
  #include <linux/syscalls.h>
  #include <linux/rbtree.h>
  #include <linux/wait.h>
  #include <linux/eventpoll.h>
  #include <linux/mount.h>
  #include <linux/bitops.h>
  #include <linux/mutex.h>
  #include <linux/anon_inodes.h>
  #include <linux/device.h>
  #include <linux/uaccess.h>
  #include <asm/io.h>
  #include <asm/mman.h>
  #include <linux/atomic.h>
  #include <linux/proc_fs.h>
  #include <linux/seq_file.h>
  #include <linux/compat.h>
  #include <linux/rculist.h>
  #include <linux/capability.h>
  #include <net/busy_poll.h>
  
  /*
   * LOCKING:
   * There are three level of locking required by epoll :
   *
   * 1) epnested_mutex (mutex)
   * 2) ep->mtx (mutex)
   * 3) ep->lock (rwlock)
   *
   * The acquire order is the one listed above, from 1 to 3.
   * We need a rwlock (ep->lock) because we manipulate objects
   * from inside the poll callback, that might be triggered from
   * a wake_up() that in turn might be called from IRQ context.
   * So we can't sleep inside the poll callback and hence we need
   * a spinlock. During the event transfer loop (from kernel to
   * user space) we could end up sleeping due a copy_to_user(), so
   * we need a lock that will allow us to sleep. This lock is a
   * mutex (ep->mtx). It is acquired during the event transfer loop,
   * during epoll_ctl(EPOLL_CTL_DEL) and during eventpoll_release_file().
   * The epnested_mutex is acquired when inserting an epoll fd onto another
   * epoll fd. We do this so that we walk the epoll tree and ensure that this
   * insertion does not create a cycle of epoll file descriptors, which
   * could lead to deadlock. We need a global mutex to prevent two
   * simultaneous inserts (A into B and B into A) from racing and
   * constructing a cycle without either insert observing that it is
   * going to.
   * It is necessary to acquire multiple "ep->mtx"es at once in the
   * case when one epoll fd is added to another. In this case, we
   * always acquire the locks in the order of nesting (i.e. after
   * epoll_ctl(e1, EPOLL_CTL_ADD, e2), e1->mtx will always be acquired
   * before e2->mtx). Since we disallow cycles of epoll file
   * descriptors, this ensures that the mutexes are well-ordered. In
   * order to communicate this nesting to lockdep, when walking a tree
   * of epoll file descriptors, we use the current recursion depth as
   * the lockdep subkey.
   * It is possible to drop the "ep->mtx" and to use the global
   * mutex "epnested_mutex" (together with "ep->lock") to have it working,
   * but having "ep->mtx" will make the interface more scalable.
   * Events that require holding "epnested_mutex" are very rare, while for
   * normal operations the epoll private "ep->mtx" will guarantee
   * a better scalability.
   */
  
  /* Epoll private bits inside the event mask */
  #define EP_PRIVATE_BITS (EPOLLWAKEUP | EPOLLONESHOT | EPOLLET | EPOLLEXCLUSIVE)
  
  #define EPOLLINOUT_BITS (EPOLLIN | EPOLLOUT)
  
  #define EPOLLEXCLUSIVE_OK_BITS (EPOLLINOUT_BITS | EPOLLERR | EPOLLHUP | \
                                  EPOLLWAKEUP | EPOLLET | EPOLLEXCLUSIVE)
  
  /* Maximum number of nesting allowed inside epoll sets */
  #define EP_MAX_NESTS 4
  
  #define EP_MAX_EVENTS (INT_MAX / sizeof(struct epoll_event))
  
  #define EP_UNACTIVE_PTR ((void *) -1L)
  
 #define EP_ITEM_COST (sizeof(struct epitem) + sizeof(struct eppoll_entry))
 
 struct epoll_filefd {
         struct file *file;
         int fd;
 } __packed;
 
 /* Wait structure used by the poll hooks */
 struct eppoll_entry {
         /* List header used to link this structure to the "struct epitem" */
         struct eppoll_entry *next;
 
         /* The "base" pointer is set to the container "struct epitem" */
         struct epitem *base;
 
         /*
          * Wait queue item that will be linked to the target file wait
          * queue head.
          */
         wait_queue_entry_t wait;
 
         /* The wait queue head that linked the "wait" wait queue item */
         wait_queue_head_t *whead;
 };
 
 /*
  * Each file descriptor added to the eventpoll interface will
  * have an entry of this type linked to the "rbr" RB tree.
  * Avoid increasing the size of this struct, there can be many thousands
  * of these on a server and we do not want this to take another cache line.
  */
 struct epitem {
         union {
                 /* RB tree node links this structure to the eventpoll RB tree */
                 struct rb_node rbn;
                 /* Used to free the struct epitem */
                 struct rcu_head rcu;
         };
 
         /* List header used to link this structure to the eventpoll ready list */
         struct list_head rdllink;
 
         /*
          * Works together "struct eventpoll"->ovflist in keeping the
          * single linked chain of items.
          */
         struct epitem *next;
 
         /* The file descriptor information this item refers to */
         struct epoll_filefd ffd;
 
         /*
          * Protected by file->f_lock, true for to-be-released epitem already
          * removed from the "struct file" items list; together with
          * eventpoll->refcount orchestrates "struct eventpoll" disposal
          */
         bool dying;
 
         /* List containing poll wait queues */
         struct eppoll_entry *pwqlist;
 
         /* The "container" of this item */
         struct eventpoll *ep;
 
         /* List header used to link this item to the "struct file" items list */
         struct hlist_node fllink;
 
         /* wakeup_source used when EPOLLWAKEUP is set */
         struct wakeup_source __rcu *ws;
 
         /* The structure that describe the interested events and the source fd */
         struct epoll_event event;
 };
 
 /*
  * This structure is stored inside the "private_data" member of the file
  * structure and represents the main data structure for the eventpoll
  * interface.
  */
 struct eventpoll {
         /*
          * This mutex is used to ensure that files are not removed
          * while epoll is using them. This is held during the event
          * collection loop, the file cleanup path, the epoll file exit
          * code and the ctl operations.
          */
         struct mutex mtx;
 
         /* Wait queue used by sys_epoll_wait() */
         wait_queue_head_t wq;
 
         /* Wait queue used by file->poll() */
         wait_queue_head_t poll_wait;
 
         /* List of ready file descriptors */
         struct list_head rdllist;
 
         /* Lock which protects rdllist and ovflist */
         rwlock_t lock;
 
         /* RB tree root used to store monitored fd structs */
         struct rb_root_cached rbr;
 
         /*
          * This is a single linked list that chains all the "struct epitem" that
          * happened while transferring ready events to userspace w/out
          * holding ->lock.
          */
         struct epitem *ovflist;
 
         /* wakeup_source used when ep_send_events or __ep_eventpoll_poll is running */
         struct wakeup_source *ws;
 
         /* The user that created the eventpoll descriptor */
         struct user_struct *user;
 
         struct file *file;
 
         /* used to optimize loop detection check */
         u64 gen;
         struct hlist_head refs;
 
         /*
          * usage count, used together with epitem->dying to
          * orchestrate the disposal of this struct
          */
         refcount_t refcount;
 
 #ifdef CONFIG_NET_RX_BUSY_POLL
         /* used to track busy poll napi_id */
         unsigned int napi_id;
         /* busy poll timeout */
         u32 busy_poll_usecs;
         /* busy poll packet budget */
         u16 busy_poll_budget;
         bool prefer_busy_poll;
 #endif
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
         /* tracks wakeup nests for lockdep validation */
         u8 nests;
 #endif
 };
 
 /* Wrapper struct used by poll queueing */
 struct ep_pqueue {
         poll_table pt;
         struct epitem *epi;
 };
 
 /*
  * Configuration options available inside /proc/sys/fs/epoll/
  */
 /* Maximum number of epoll watched descriptors, per user */
 static long max_user_watches __read_mostly;
 
 /* Used for cycles detection */
 static DEFINE_MUTEX(epnested_mutex);
 
 static u64 loop_check_gen = 0;
 
 /* Used to check for epoll file descriptor inclusion loops */
 static struct eventpoll *inserting_into;
 
 /* Slab cache used to allocate "struct epitem" */
 static struct kmem_cache *epi_cache __ro_after_init;
 
 /* Slab cache used to allocate "struct eppoll_entry" */
 static struct kmem_cache *pwq_cache __ro_after_init;
 
 /*
  * List of files with newly added links, where we may need to limit the number
  * of emanating paths. Protected by the epnested_mutex.
  */
 struct epitems_head {
         struct hlist_head epitems;
         struct epitems_head *next;
 };
 static struct epitems_head *tfile_check_list = EP_UNACTIVE_PTR;
 
 static struct kmem_cache *ephead_cache __ro_after_init;
 
 static inline void free_ephead(struct epitems_head *head)
 {
         if (head)
                 kmem_cache_free(ephead_cache, head);
 }
 
 static void list_file(struct file *file)
 {
         struct epitems_head *head;
 
         head = container_of(file->f_ep, struct epitems_head, epitems);
         if (!head->next) {
                 head->next = tfile_check_list;
                 tfile_check_list = head;
         }
 }
 
 static void unlist_file(struct epitems_head *head)
 {
         struct epitems_head *to_free = head;
         struct hlist_node *p = rcu_dereference(hlist_first_rcu(&head->epitems));
         if (p) {
                 struct epitem *epi= container_of(p, struct epitem, fllink);
                 spin_lock(&epi->ffd.file->f_lock);
                 if (!hlist_empty(&head->epitems))
                         to_free = NULL;
                 head->next = NULL;
                 spin_unlock(&epi->ffd.file->f_lock);
         }
         free_ephead(to_free);
 }
 
 #ifdef CONFIG_SYSCTL
 
 #include <linux/sysctl.h>
 
 static long long_zero;
 static long long_max = LONG_MAX;
 
 static struct ctl_table epoll_table[] = {
         {
                 .procname       = "max_user_watches",
                 .data           = &max_user_watches,
                 .maxlen         = sizeof(max_user_watches),
                 .mode           = 0644,
                 .proc_handler   = proc_doulongvec_minmax,
                 .extra1         = &long_zero,
                 .extra2         = &long_max,
         },
 };
 
 static void __init epoll_sysctls_init(void)
 {
         register_sysctl("fs/epoll", epoll_table);
 }
 #else
 #define epoll_sysctls_init() do { } while (0)
 #endif /* CONFIG_SYSCTL */
 
 static const struct file_operations eventpoll_fops;
 
 static inline int is_file_epoll(struct file *f)
 {
         return f->f_op == &eventpoll_fops;
 }
 
 /* Setup the structure that is used as key for the RB tree */
 static inline void ep_set_ffd(struct epoll_filefd *ffd,
                               struct file *file, int fd)
 {
         ffd->file = file;
         ffd->fd = fd;
 }
 
 /* Compare RB tree keys */
 static inline int ep_cmp_ffd(struct epoll_filefd *p1,
                              struct epoll_filefd *p2)
 {
         return (p1->file > p2->file ? +1:
                 (p1->file < p2->file ? -1 : p1->fd - p2->fd));
 }
 
 /* Tells us if the item is currently linked */
 static inline int ep_is_linked(struct epitem *epi)
 {
         return !list_empty(&epi->rdllink);
 }
 
 static inline struct eppoll_entry *ep_pwq_from_wait(wait_queue_entry_t *p)
 {
         return container_of(p, struct eppoll_entry, wait);
 }
 
 /* Get the "struct epitem" from a wait queue pointer */
 static inline struct epitem *ep_item_from_wait(wait_queue_entry_t *p)
 {
         return container_of(p, struct eppoll_entry, wait)->base;
 }
 
 /**
  * ep_events_available - Checks if ready events might be available.
  *
  * @ep: Pointer to the eventpoll context.
  *
  * Return: a value different than %zero if ready events are available,
  *          or %zero otherwise.
  */
 static inline int ep_events_available(struct eventpoll *ep)
 {
         return !list_empty_careful(&ep->rdllist) ||
                 READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR;
 }
 
 #ifdef CONFIG_NET_RX_BUSY_POLL
 /**
  * busy_loop_ep_timeout - check if busy poll has timed out. The timeout value
  * from the epoll instance ep is preferred, but if it is not set fallback to
  * the system-wide global via busy_loop_timeout.
  *
  * @start_time: The start time used to compute the remaining time until timeout.
  * @ep: Pointer to the eventpoll context.
  *
  * Return: true if the timeout has expired, false otherwise.
  */
 static bool busy_loop_ep_timeout(unsigned long start_time,
                                  struct eventpoll *ep)
 {
         unsigned long bp_usec = READ_ONCE(ep->busy_poll_usecs);
 
         if (bp_usec) {
                 unsigned long end_time = start_time + bp_usec;
                 unsigned long now = busy_loop_current_time();
 
                 return time_after(now, end_time);
         } else {
                 return busy_loop_timeout(start_time);
         }
 }
 
 static bool ep_busy_loop_on(struct eventpoll *ep)
 {
         return !!READ_ONCE(ep->busy_poll_usecs) || net_busy_loop_on();
 }
 
 static bool ep_busy_loop_end(void *p, unsigned long start_time)
 {
         struct eventpoll *ep = p;
 
         return ep_events_available(ep) || busy_loop_ep_timeout(start_time, ep);
 }
 
 /*
  * Busy poll if globally on and supporting sockets found && no events,
  * busy loop will return if need_resched or ep_events_available.
  *
  * we must do our busy polling with irqs enabled
  */
 static bool ep_busy_loop(struct eventpoll *ep, int nonblock)
 {
         unsigned int napi_id = READ_ONCE(ep->napi_id);
         u16 budget = READ_ONCE(ep->busy_poll_budget);
         bool prefer_busy_poll = READ_ONCE(ep->prefer_busy_poll);
 
         if (!budget)
                 budget = BUSY_POLL_BUDGET;
 
         if (napi_id >= MIN_NAPI_ID && ep_busy_loop_on(ep)) {
                 napi_busy_loop(napi_id, nonblock ? NULL : ep_busy_loop_end,
                                ep, prefer_busy_poll, budget);
                 if (ep_events_available(ep))
                         return true;
                 /*
                  * Busy poll timed out.  Drop NAPI ID for now, we can add
                  * it back in when we have moved a socket with a valid NAPI
                  * ID onto the ready list.
                  */
                 ep->napi_id = 0;
                 return false;
         }
         return false;
 }
 
 /*
  * Set epoll busy poll NAPI ID from sk.
  */
 static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
 {
         struct eventpoll *ep = epi->ep;
         unsigned int napi_id;
         struct socket *sock;
         struct sock *sk;
 
         if (!ep_busy_loop_on(ep))
                 return;
 
         sock = sock_from_file(epi->ffd.file);
         if (!sock)
                 return;
 
         sk = sock->sk;
         if (!sk)
                 return;
 
         napi_id = READ_ONCE(sk->sk_napi_id);
 
         /* Non-NAPI IDs can be rejected
          *      or
          * Nothing to do if we already have this ID
          */
         if (napi_id < MIN_NAPI_ID || napi_id == ep->napi_id)
                 return;
 
         /* record NAPI ID for use in next busy poll */
         ep->napi_id = napi_id;
 }
 
 static long ep_eventpoll_bp_ioctl(struct file *file, unsigned int cmd,
                                   unsigned long arg)
 {
         struct eventpoll *ep = file->private_data;
         void __user *uarg = (void __user *)arg;
         struct epoll_params epoll_params;
 
         switch (cmd) {
         case EPIOCSPARAMS:
                 if (copy_from_user(&epoll_params, uarg, sizeof(epoll_params)))
                         return -EFAULT;
 
                 /* pad byte must be zero */
                 if (epoll_params.__pad)
                         return -EINVAL;
 
                 if (epoll_params.busy_poll_usecs > S32_MAX)
                         return -EINVAL;
 
                 if (epoll_params.prefer_busy_poll > 1)
                         return -EINVAL;
 
                 if (epoll_params.busy_poll_budget > NAPI_POLL_WEIGHT &&
                     !capable(CAP_NET_ADMIN))
                         return -EPERM;
 
                 WRITE_ONCE(ep->busy_poll_usecs, epoll_params.busy_poll_usecs);
                 WRITE_ONCE(ep->busy_poll_budget, epoll_params.busy_poll_budget);
                 WRITE_ONCE(ep->prefer_busy_poll, epoll_params.prefer_busy_poll);
                 return 0;
         case EPIOCGPARAMS:
                 memset(&epoll_params, 0, sizeof(epoll_params));
                 epoll_params.busy_poll_usecs = READ_ONCE(ep->busy_poll_usecs);
                 epoll_params.busy_poll_budget = READ_ONCE(ep->busy_poll_budget);
                 epoll_params.prefer_busy_poll = READ_ONCE(ep->prefer_busy_poll);
                 if (copy_to_user(uarg, &epoll_params, sizeof(epoll_params)))
                         return -EFAULT;
                 return 0;
         default:
                 return -ENOIOCTLCMD;
         }
 }
 
 #else
 
 static inline bool ep_busy_loop(struct eventpoll *ep, int nonblock)
 {
         return false;
 }
 
 static inline void ep_set_busy_poll_napi_id(struct epitem *epi)
 {
 }
 
 static long ep_eventpoll_bp_ioctl(struct file *file, unsigned int cmd,
                                   unsigned long arg)
 {
         return -EOPNOTSUPP;
 }
 
 #endif /* CONFIG_NET_RX_BUSY_POLL */
 
 /*
  * As described in commit 0ccf831cb lockdep: annotate epoll
  * the use of wait queues used by epoll is done in a very controlled
  * manner. Wake ups can nest inside each other, but are never done
  * with the same locking. For example:
  *
  *   dfd = socket(...);
  *   efd1 = epoll_create();
  *   efd2 = epoll_create();
  *   epoll_ctl(efd1, EPOLL_CTL_ADD, dfd, ...);
  *   epoll_ctl(efd2, EPOLL_CTL_ADD, efd1, ...);
  *
  * When a packet arrives to the device underneath "dfd", the net code will
  * issue a wake_up() on its poll wake list. Epoll (efd1) has installed a
  * callback wakeup entry on that queue, and the wake_up() performed by the
  * "dfd" net code will end up in ep_poll_callback(). At this point epoll
  * (efd1) notices that it may have some event ready, so it needs to wake up
  * the waiters on its poll wait list (efd2). So it calls ep_poll_safewake()
  * that ends up in another wake_up(), after having checked about the
  * recursion constraints. That are, no more than EP_MAX_NESTS, to avoid
  * stack blasting.
  *
  * When CONFIG_DEBUG_LOCK_ALLOC is enabled, make sure lockdep can handle
  * this special case of epoll.
  */
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 
 static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
                              unsigned pollflags)
 {
         struct eventpoll *ep_src;
         unsigned long flags;
         u8 nests = 0;
 
         /*
          * To set the subclass or nesting level for spin_lock_irqsave_nested()
          * it might be natural to create a per-cpu nest count. However, since
          * we can recurse on ep->poll_wait.lock, and a non-raw spinlock can
          * schedule() in the -rt kernel, the per-cpu variable are no longer
          * protected. Thus, we are introducing a per eventpoll nest field.
          * If we are not being call from ep_poll_callback(), epi is NULL and
          * we are at the first level of nesting, 0. Otherwise, we are being
          * called from ep_poll_callback() and if a previous wakeup source is
          * not an epoll file itself, we are at depth 1 since the wakeup source
          * is depth 0. If the wakeup source is a previous epoll file in the
          * wakeup chain then we use its nests value and record ours as
          * nests + 1. The previous epoll file nests value is stable since its
          * already holding its own poll_wait.lock.
          */
         if (epi) {
                 if ((is_file_epoll(epi->ffd.file))) {
                         ep_src = epi->ffd.file->private_data;
                         nests = ep_src->nests;
                 } else {
                         nests = 1;
                 }
         }
         spin_lock_irqsave_nested(&ep->poll_wait.lock, flags, nests);
         ep->nests = nests + 1;
         wake_up_locked_poll(&ep->poll_wait, EPOLLIN | pollflags);
         ep->nests = 0;
         spin_unlock_irqrestore(&ep->poll_wait.lock, flags);
 }
 
 #else
 
 static void ep_poll_safewake(struct eventpoll *ep, struct epitem *epi,
                              __poll_t pollflags)
 {
         wake_up_poll(&ep->poll_wait, EPOLLIN | pollflags);
 }
 
 #endif
 
 static void ep_remove_wait_queue(struct eppoll_entry *pwq)
 {
         wait_queue_head_t *whead;
 
         rcu_read_lock();
         /*
          * If it is cleared by POLLFREE, it should be rcu-safe.
          * If we read NULL we need a barrier paired with
          * smp_store_release() in ep_poll_callback(), otherwise
          * we rely on whead->lock.
          */
         whead = smp_load_acquire(&pwq->whead);
         if (whead)
                 remove_wait_queue(whead, &pwq->wait);
         rcu_read_unlock();
 }
 
 /*
  * This function unregisters poll callbacks from the associated file
  * descriptor.  Must be called with "mtx" held.
  */
 static void ep_unregister_pollwait(struct eventpoll *ep, struct epitem *epi)
 {
         struct eppoll_entry **p = &epi->pwqlist;
         struct eppoll_entry *pwq;
 
         while ((pwq = *p) != NULL) {
                 *p = pwq->next;
                 ep_remove_wait_queue(pwq);
                 kmem_cache_free(pwq_cache, pwq);
         }
 }
 
 /* call only when ep->mtx is held */
 static inline struct wakeup_source *ep_wakeup_source(struct epitem *epi)
 {
         return rcu_dereference_check(epi->ws, lockdep_is_held(&epi->ep->mtx));
 }
 
 /* call only when ep->mtx is held */
 static inline void ep_pm_stay_awake(struct epitem *epi)
 {
         struct wakeup_source *ws = ep_wakeup_source(epi);
 
         if (ws)
                 __pm_stay_awake(ws);
 }
 
 static inline bool ep_has_wakeup_source(struct epitem *epi)
 {
         return rcu_access_pointer(epi->ws) ? true : false;
 }
 
 /* call when ep->mtx cannot be held (ep_poll_callback) */
 static inline void ep_pm_stay_awake_rcu(struct epitem *epi)
 {
         struct wakeup_source *ws;
 
         rcu_read_lock();
         ws = rcu_dereference(epi->ws);
         if (ws)
                 __pm_stay_awake(ws);
         rcu_read_unlock();
 }
 
 
 /*
  * ep->mutex needs to be held because we could be hit by
  * eventpoll_release_file() and epoll_ctl().
  */
 static void ep_start_scan(struct eventpoll *ep, struct list_head *txlist)
 {
         /*
          * Steal the ready list, and re-init the original one to the
          * empty list. Also, set ep->ovflist to NULL so that events
          * happening while looping w/out locks, are not lost. We cannot
          * have the poll callback to queue directly on ep->rdllist,
          * because we want the "sproc" callback to be able to do it
          * in a lockless way.
          */
         lockdep_assert_irqs_enabled();
         write_lock_irq(&ep->lock);
         list_splice_init(&ep->rdllist, txlist);
         WRITE_ONCE(ep->ovflist, NULL);
         write_unlock_irq(&ep->lock);
 }
 
 static void ep_done_scan(struct eventpoll *ep,
                          struct list_head *txlist)
 {
         struct epitem *epi, *nepi;
 
         write_lock_irq(&ep->lock);
         /*
          * During the time we spent inside the "sproc" callback, some
          * other events might have been queued by the poll callback.
          * We re-insert them inside the main ready-list here.
          */
         for (nepi = READ_ONCE(ep->ovflist); (epi = nepi) != NULL;
              nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {
                 /*
                  * We need to check if the item is already in the list.
                  * During the "sproc" callback execution time, items are
                  * queued into ->ovflist but the "txlist" might already
                  * contain them, and the list_splice() below takes care of them.
                  */
                 if (!ep_is_linked(epi)) {
                         /*
                          * ->ovflist is LIFO, so we have to reverse it in order
                          * to keep in FIFO.
                          */
                         list_add(&epi->rdllink, &ep->rdllist);
                         ep_pm_stay_awake(epi);
                 }
         }
         /*
          * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after
          * releasing the lock, events will be queued in the normal way inside
          * ep->rdllist.
          */
         WRITE_ONCE(ep->ovflist, EP_UNACTIVE_PTR);
 
         /*
          * Quickly re-inject items left on "txlist".
          */
         list_splice(txlist, &ep->rdllist);
         __pm_relax(ep->ws);
 
         if (!list_empty(&ep->rdllist)) {
                 if (waitqueue_active(&ep->wq))
                         wake_up(&ep->wq);
         }
 
         write_unlock_irq(&ep->lock);
 }
 
 static void ep_get(struct eventpoll *ep)
 {
         refcount_inc(&ep->refcount);
 }
 
 /*
  * Returns true if the event poll can be disposed
  */
 static bool ep_refcount_dec_and_test(struct eventpoll *ep)
 {
         if (!refcount_dec_and_test(&ep->refcount))
                 return false;
 
         WARN_ON_ONCE(!RB_EMPTY_ROOT(&ep->rbr.rb_root));
         return true;
 }
 
 static void ep_free(struct eventpoll *ep)
 {
         mutex_destroy(&ep->mtx);
         free_uid(ep->user);
         wakeup_source_unregister(ep->ws);
         kfree(ep);
 }
 
 /*
  * Removes a "struct epitem" from the eventpoll RB tree and deallocates
  * all the associated resources. Must be called with "mtx" held.
  * If the dying flag is set, do the removal only if force is true.
  * This prevents ep_clear_and_put() from dropping all the ep references
  * while running concurrently with eventpoll_release_file().
  * Returns true if the eventpoll can be disposed.
  */
 static bool __ep_remove(struct eventpoll *ep, struct epitem *epi, bool force)
 {
         struct file *file = epi->ffd.file;
         struct epitems_head *to_free;
         struct hlist_head *head;
 
         lockdep_assert_irqs_enabled();
 
         /*
          * Removes poll wait queue hooks.
          */
         ep_unregister_pollwait(ep, epi);
 
         /* Remove the current item from the list of epoll hooks */
         spin_lock(&file->f_lock);
         if (epi->dying && !force) {
                 spin_unlock(&file->f_lock);
                 return false;
         }
 
         to_free = NULL;
         head = file->f_ep;
         if (head->first == &epi->fllink && !epi->fllink.next) {
                 file->f_ep = NULL;
                 if (!is_file_epoll(file)) {
                         struct epitems_head *v;
                         v = container_of(head, struct epitems_head, epitems);
                         if (!smp_load_acquire(&v->next))
                                 to_free = v;
                 }
         }
         hlist_del_rcu(&epi->fllink);
         spin_unlock(&file->f_lock);
         free_ephead(to_free);
 
         rb_erase_cached(&epi->rbn, &ep->rbr);
 
         write_lock_irq(&ep->lock);
         if (ep_is_linked(epi))
                 list_del_init(&epi->rdllink);
         write_unlock_irq(&ep->lock);
 
         wakeup_source_unregister(ep_wakeup_source(epi));
         /*
          * At this point it is safe to free the eventpoll item. Use the union
          * field epi->rcu, since we are trying to minimize the size of
          * 'struct epitem'. The 'rbn' field is no longer in use. Protected by
          * ep->mtx. The rcu read side, reverse_path_check_proc(), does not make
          * use of the rbn field.
          */
         kfree_rcu(epi, rcu);
 
         percpu_counter_dec(&ep->user->epoll_watches);
         return ep_refcount_dec_and_test(ep);
 }
 
 /*
  * ep_remove variant for callers owing an additional reference to the ep
  */
 static void ep_remove_safe(struct eventpoll *ep, struct epitem *epi)
 {
         WARN_ON_ONCE(__ep_remove(ep, epi, false));
 }
 
 static void ep_clear_and_put(struct eventpoll *ep)
 {
         struct rb_node *rbp, *next;
         struct epitem *epi;
         bool dispose;
 
         /* We need to release all tasks waiting for these file */
         if (waitqueue_active(&ep->poll_wait))
                 ep_poll_safewake(ep, NULL, 0);
 
         mutex_lock(&ep->mtx);
 
         /*
          * Walks through the whole tree by unregistering poll callbacks.
          */
         for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                 epi = rb_entry(rbp, struct epitem, rbn);
 
                 ep_unregister_pollwait(ep, epi);
                 cond_resched();
         }
 
         /*
          * Walks through the whole tree and try to free each "struct epitem".
          * Note that ep_remove_safe() will not remove the epitem in case of a
          * racing eventpoll_release_file(); the latter will do the removal.
          * At this point we are sure no poll callbacks will be lingering around.
          * Since we still own a reference to the eventpoll struct, the loop can't
          * dispose it.
          */
         for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = next) {
                 next = rb_next(rbp);
                 epi = rb_entry(rbp, struct epitem, rbn);
                 ep_remove_safe(ep, epi);
                 cond_resched();
         }
 
         dispose = ep_refcount_dec_and_test(ep);
         mutex_unlock(&ep->mtx);
 
         if (dispose)
                 ep_free(ep);
 }
 
 static long ep_eventpoll_ioctl(struct file *file, unsigned int cmd,
                                unsigned long arg)
 {
         int ret;
 
         if (!is_file_epoll(file))
                 return -EINVAL;
 
         switch (cmd) {
         case EPIOCSPARAMS:
         case EPIOCGPARAMS:
                 ret = ep_eventpoll_bp_ioctl(file, cmd, arg);
                 break;
         default:
                 ret = -EINVAL;
                 break;
         }
 
         return ret;
 }
 
 static int ep_eventpoll_release(struct inode *inode, struct file *file)
 {
         struct eventpoll *ep = file->private_data;
 
         if (ep)
                 ep_clear_and_put(ep);
 
         return 0;
 }
 
 static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt, int depth);
 
 static __poll_t __ep_eventpoll_poll(struct file *file, poll_table *wait, int depth)
 {
         struct eventpoll *ep = file->private_data;
         LIST_HEAD(txlist);
         struct epitem *epi, *tmp;
         poll_table pt;
         __poll_t res = 0;
 
         init_poll_funcptr(&pt, NULL);
 
         /* Insert inside our poll wait queue */
         poll_wait(file, &ep->poll_wait, wait);
 
         /*
          * Proceed to find out if wanted events are really available inside
          * the ready list.
          */
         mutex_lock_nested(&ep->mtx, depth);
         ep_start_scan(ep, &txlist);
         list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
                 if (ep_item_poll(epi, &pt, depth + 1)) {
                         res = EPOLLIN | EPOLLRDNORM;
                         break;
                 } else {
                         /*
                          * Item has been dropped into the ready list by the poll
                          * callback, but it's not actually ready, as far as
                          * caller requested events goes. We can remove it here.
                          */
                         __pm_relax(ep_wakeup_source(epi));
                         list_del_init(&epi->rdllink);
                 }
         }
         ep_done_scan(ep, &txlist);
         mutex_unlock(&ep->mtx);
         return res;
 }
 
 /*
  * The ffd.file pointer may be in the process of being torn down due to
  * being closed, but we may not have finished eventpoll_release() yet.
  *
  * Normally, even with the atomic_long_inc_not_zero, the file may have
  * been free'd and then gotten re-allocated to something else (since
  * files are not RCU-delayed, they are SLAB_TYPESAFE_BY_RCU).
  *
  * But for epoll, users hold the ep->mtx mutex, and as such any file in
  * the process of being free'd will block in eventpoll_release_file()
  * and thus the underlying file allocation will not be free'd, and the
  * file re-use cannot happen.
  *
  * For the same reason we can avoid a rcu_read_lock() around the
  * operation - 'ffd.file' cannot go away even if the refcount has
  * reached zero (but we must still not call out to ->poll() functions
  * etc).
  */
 static struct file *epi_fget(const struct epitem *epi)
 {
         struct file *file;
 
         file = epi->ffd.file;
         if (!atomic_long_inc_not_zero(&file->f_count))
                 file = NULL;
         return file;
 }
 
 /*
  * Differs from ep_eventpoll_poll() in that internal callers already have
  * the ep->mtx so we need to start from depth=1, such that mutex_lock_nested()
  * is correctly annotated.
  */
 static __poll_t ep_item_poll(const struct epitem *epi, poll_table *pt,
                                  int depth)
 {
         struct file *file = epi_fget(epi);
         __poll_t res;
 
         /*
          * We could return EPOLLERR | EPOLLHUP or something, but let's
          * treat this more as "file doesn't exist, poll didn't happen".
          */
         if (!file)
                 return 0;
 
         pt->_key = epi->event.events;
         if (!is_file_epoll(file))
                 res = vfs_poll(file, pt);
         else
                 res = __ep_eventpoll_poll(file, pt, depth);
         fput(file);
         return res & epi->event.events;
 }
 
 static __poll_t ep_eventpoll_poll(struct file *file, poll_table *wait)
 {
         return __ep_eventpoll_poll(file, wait, 0);
 }
 
 #ifdef CONFIG_PROC_FS
 static void ep_show_fdinfo(struct seq_file *m, struct file *f)
 {
         struct eventpoll *ep = f->private_data;
         struct rb_node *rbp;
 
         mutex_lock(&ep->mtx);
         for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                 struct epitem *epi = rb_entry(rbp, struct epitem, rbn);
                 struct inode *inode = file_inode(epi->ffd.file);
 
                 seq_printf(m, "tfd: %8d events: %8x data: %16llx "
                            " pos:%lli ino:%lx sdev:%x\n",
                            epi->ffd.fd, epi->event.events,
                            (long long)epi->event.data,
                            (long long)epi->ffd.file->f_pos,
                            inode->i_ino, inode->i_sb->s_dev);
                 if (seq_has_overflowed(m))
                         break;
         }
         mutex_unlock(&ep->mtx);
 }
 #endif
 
 /* File callbacks that implement the eventpoll file behaviour */
 static const struct file_operations eventpoll_fops = {
 #ifdef CONFIG_PROC_FS
         .show_fdinfo    = ep_show_fdinfo,
 #endif
         .release        = ep_eventpoll_release,
         .poll           = ep_eventpoll_poll,
         .llseek         = noop_llseek,
         .unlocked_ioctl = ep_eventpoll_ioctl,
         .compat_ioctl   = compat_ptr_ioctl,
 };
 
 /*
  * This is called from eventpoll_release() to unlink files from the eventpoll
  * interface. We need to have this facility to cleanup correctly files that are
  * closed without being removed from the eventpoll interface.
  */
 void eventpoll_release_file(struct file *file)
 {
         struct eventpoll *ep;
         struct epitem *epi;
         bool dispose;
 
         /*
          * Use the 'dying' flag to prevent a concurrent ep_clear_and_put() from
          * touching the epitems list before eventpoll_release_file() can access
          * the ep->mtx.
          */
 again:
         spin_lock(&file->f_lock);
         if (file->f_ep && file->f_ep->first) {
                 epi = hlist_entry(file->f_ep->first, struct epitem, fllink);
                 epi->dying = true;
                 spin_unlock(&file->f_lock);
 
                 /*
                  * ep access is safe as we still own a reference to the ep
                  * struct
                  */
                 ep = epi->ep;
                 mutex_lock(&ep->mtx);
                 dispose = __ep_remove(ep, epi, true);
                 mutex_unlock(&ep->mtx);
 
                 if (dispose)
                         ep_free(ep);
                 goto again;
         }
         spin_unlock(&file->f_lock);
 }
 
 static int ep_alloc(struct eventpoll **pep)
 {
         struct eventpoll *ep;
 
         ep = kzalloc(sizeof(*ep), GFP_KERNEL);
         if (unlikely(!ep))
                 return -ENOMEM;
 
         mutex_init(&ep->mtx);
         rwlock_init(&ep->lock);
         init_waitqueue_head(&ep->wq);
         init_waitqueue_head(&ep->poll_wait);
         INIT_LIST_HEAD(&ep->rdllist);
         ep->rbr = RB_ROOT_CACHED;
         ep->ovflist = EP_UNACTIVE_PTR;
         ep->user = get_current_user();
         refcount_set(&ep->refcount, 1);
 
         *pep = ep;
 
         return 0;
 }
 
 /*
  * Search the file inside the eventpoll tree. The RB tree operations
  * are protected by the "mtx" mutex, and ep_find() must be called with
  * "mtx" held.
  */
 static struct epitem *ep_find(struct eventpoll *ep, struct file *file, int fd)
 {
         int kcmp;
         struct rb_node *rbp;
         struct epitem *epi, *epir = NULL;
         struct epoll_filefd ffd;
 
         ep_set_ffd(&ffd, file, fd);
         for (rbp = ep->rbr.rb_root.rb_node; rbp; ) {
                 epi = rb_entry(rbp, struct epitem, rbn);
                 kcmp = ep_cmp_ffd(&ffd, &epi->ffd);
                 if (kcmp > 0)
                         rbp = rbp->rb_right;
                 else if (kcmp < 0)
                         rbp = rbp->rb_left;
                 else {
                         epir = epi;
                         break;
                 }
         }
 
         return epir;
 }
 
 #ifdef CONFIG_KCMP
 static struct epitem *ep_find_tfd(struct eventpoll *ep, int tfd, unsigned long toff)
 {
         struct rb_node *rbp;
         struct epitem *epi;
 
         for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                 epi = rb_entry(rbp, struct epitem, rbn);
                 if (epi->ffd.fd == tfd) {
                         if (toff == 0)
                                 return epi;
                         else
                                 toff--;
                 }
                 cond_resched();
         }
 
         return NULL;
 }
 
 struct file *get_epoll_tfile_raw_ptr(struct file *file, int tfd,
                                      unsigned long toff)
 {
         struct file *file_raw;
         struct eventpoll *ep;
         struct epitem *epi;
 
         if (!is_file_epoll(file))
                 return ERR_PTR(-EINVAL);
 
         ep = file->private_data;
 
         mutex_lock(&ep->mtx);
         epi = ep_find_tfd(ep, tfd, toff);
         if (epi)
                 file_raw = epi->ffd.file;
         else
                 file_raw = ERR_PTR(-ENOENT);
         mutex_unlock(&ep->mtx);
 
         return file_raw;
 }
 #endif /* CONFIG_KCMP */
 
 /*
  * Adds a new entry to the tail of the list in a lockless way, i.e.
  * multiple CPUs are allowed to call this function concurrently.
  *
  * Beware: it is necessary to prevent any other modifications of the
  *         existing list until all changes are completed, in other words
  *         concurrent list_add_tail_lockless() calls should be protected
  *         with a read lock, where write lock acts as a barrier which
  *         makes sure all list_add_tail_lockless() calls are fully
  *         completed.
  *
  *        Also an element can be locklessly added to the list only in one
  *        direction i.e. either to the tail or to the head, otherwise
  *        concurrent access will corrupt the list.
  *
  * Return: %false if element has been already added to the list, %true
  * otherwise.
  */
 static inline bool list_add_tail_lockless(struct list_head *new,
                                           struct list_head *head)
 {
         struct list_head *prev;
 
         /*
          * This is simple 'new->next = head' operation, but cmpxchg()
          * is used in order to detect that same element has been just
          * added to the list from another CPU: the winner observes
          * new->next == new.
          */
         if (!try_cmpxchg(&new->next, &new, head))
                 return false;
 
         /*
          * Initially ->next of a new element must be updated with the head
          * (we are inserting to the tail) and only then pointers are atomically
          * exchanged.  XCHG guarantees memory ordering, thus ->next should be
          * updated before pointers are actually swapped and pointers are
          * swapped before prev->next is updated.
          */
 
         prev = xchg(&head->prev, new);
 
         /*
          * It is safe to modify prev->next and new->prev, because a new element
          * is added only to the tail and new->next is updated before XCHG.
          */
 
         prev->next = new;
         new->prev = prev;
 
         return true;
 }
 
 /*
  * Chains a new epi entry to the tail of the ep->ovflist in a lockless way,
  * i.e. multiple CPUs are allowed to call this function concurrently.
  *
  * Return: %false if epi element has been already chained, %true otherwise.
  */
 static inline bool chain_epi_lockless(struct epitem *epi)
 {
         struct eventpoll *ep = epi->ep;
 
         /* Fast preliminary check */
         if (epi->next != EP_UNACTIVE_PTR)
                 return false;
 
         /* Check that the same epi has not been just chained from another CPU */
         if (cmpxchg(&epi->next, EP_UNACTIVE_PTR, NULL) != EP_UNACTIVE_PTR)
                 return false;
 
         /* Atomically exchange tail */
         epi->next = xchg(&ep->ovflist, epi);
 
         return true;
 }
 
 /*
  * This is the callback that is passed to the wait queue wakeup
  * mechanism. It is called by the stored file descriptors when they
  * have events to report.
  *
  * This callback takes a read lock in order not to contend with concurrent
  * events from another file descriptor, thus all modifications to ->rdllist
  * or ->ovflist are lockless.  Read lock is paired with the write lock from
  * ep_start/done_scan(), which stops all list modifications and guarantees
  * that lists state is seen correctly.
  *
  * Another thing worth to mention is that ep_poll_callback() can be called
  * concurrently for the same @epi from different CPUs if poll table was inited
  * with several wait queues entries.  Plural wakeup from different CPUs of a
  * single wait queue is serialized by wq.lock, but the case when multiple wait
  * queues are used should be detected accordingly.  This is detected using
  * cmpxchg() operation.
  */
 static int ep_poll_callback(wait_queue_entry_t *wait, unsigned mode, int sync, void *key)
 {
         int pwake = 0;
         struct epitem *epi = ep_item_from_wait(wait);
         struct eventpoll *ep = epi->ep;
         __poll_t pollflags = key_to_poll(key);
         unsigned long flags;
         int ewake = 0;
 
         read_lock_irqsave(&ep->lock, flags);
 
         ep_set_busy_poll_napi_id(epi);
 
         /*
          * If the event mask does not contain any poll(2) event, we consider the
          * descriptor to be disabled. This condition is likely the effect of the
          * EPOLLONESHOT bit that disables the descriptor when an event is received,
          * until the next EPOLL_CTL_MOD will be issued.
          */
         if (!(epi->event.events & ~EP_PRIVATE_BITS))
                 goto out_unlock;
 
         /*
          * Check the events coming with the callback. At this stage, not
          * every device reports the events in the "key" parameter of the
          * callback. We need to be able to handle both cases here, hence the
          * test for "key" != NULL before the event match test.
          */
         if (pollflags && !(pollflags & epi->event.events))
                 goto out_unlock;
 
         /*
          * If we are transferring events to userspace, we can hold no locks
          * (because we're accessing user memory, and because of linux f_op->poll()
          * semantics). All the events that happen during that period of time are
          * chained in ep->ovflist and requeued later on.
          */
         if (READ_ONCE(ep->ovflist) != EP_UNACTIVE_PTR) {
                 if (chain_epi_lockless(epi))
                         ep_pm_stay_awake_rcu(epi);
         } else if (!ep_is_linked(epi)) {
                 /* In the usual case, add event to ready list. */
                 if (list_add_tail_lockless(&epi->rdllink, &ep->rdllist))
                         ep_pm_stay_awake_rcu(epi);
         }
 
         /*
          * Wake up ( if active ) both the eventpoll wait list and the ->poll()
          * wait list.
          */
         if (waitqueue_active(&ep->wq)) {
                 if ((epi->event.events & EPOLLEXCLUSIVE) &&
                                         !(pollflags & POLLFREE)) {
                         switch (pollflags & EPOLLINOUT_BITS) {
                         case EPOLLIN:
                                 if (epi->event.events & EPOLLIN)
                                         ewake = 1;
                                 break;
                         case EPOLLOUT:
                                 if (epi->event.events & EPOLLOUT)
                                         ewake = 1;
                                 break;
                         case 0:
                                 ewake = 1;
                                 break;
                         }
                 }
                 wake_up(&ep->wq);
         }
         if (waitqueue_active(&ep->poll_wait))
                 pwake++;
 
 out_unlock:
         read_unlock_irqrestore(&ep->lock, flags);
 
         /* We have to call this outside the lock */
         if (pwake)
                 ep_poll_safewake(ep, epi, pollflags & EPOLL_URING_WAKE);
 
         if (!(epi->event.events & EPOLLEXCLUSIVE))
                 ewake = 1;
 
         if (pollflags & POLLFREE) {
                 /*
                  * If we race with ep_remove_wait_queue() it can miss
                  * ->whead = NULL and do another remove_wait_queue() after
                  * us, so we can't use __remove_wait_queue().
                  */
                 list_del_init(&wait->entry);
                 /*
                  * ->whead != NULL protects us from the race with
                  * ep_clear_and_put() or ep_remove(), ep_remove_wait_queue()
                  * takes whead->lock held by the caller. Once we nullify it,
                  * nothing protects ep/epi or even wait.
                  */
                 smp_store_release(&ep_pwq_from_wait(wait)->whead, NULL);
         }
 
         return ewake;
 }
 
 /*
  * This is the callback that is used to add our wait queue to the
  * target file wakeup lists.
  */
 static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
                                  poll_table *pt)
 {
         struct ep_pqueue *epq = container_of(pt, struct ep_pqueue, pt);
         struct epitem *epi = epq->epi;
         struct eppoll_entry *pwq;
 
         if (unlikely(!epi))     // an earlier allocation has failed
                 return;
 
         pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL);
         if (unlikely(!pwq)) {
                 epq->epi = NULL;
                 return;
         }
 
         init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
         pwq->whead = whead;
         pwq->base = epi;
         if (epi->event.events & EPOLLEXCLUSIVE)
                 add_wait_queue_exclusive(whead, &pwq->wait);
         else
                 add_wait_queue(whead, &pwq->wait);
         pwq->next = epi->pwqlist;
         epi->pwqlist = pwq;
 }
 
 static void ep_rbtree_insert(struct eventpoll *ep, struct epitem *epi)
 {
         int kcmp;
         struct rb_node **p = &ep->rbr.rb_root.rb_node, *parent = NULL;
         struct epitem *epic;
         bool leftmost = true;
 
         while (*p) {
                 parent = *p;
                 epic = rb_entry(parent, struct epitem, rbn);
                 kcmp = ep_cmp_ffd(&epi->ffd, &epic->ffd);
                 if (kcmp > 0) {
                         p = &parent->rb_right;
                         leftmost = false;
                 } else
                         p = &parent->rb_left;
         }
         rb_link_node(&epi->rbn, parent, p);
         rb_insert_color_cached(&epi->rbn, &ep->rbr, leftmost);
 }
 
 
 
 #define PATH_ARR_SIZE 5
 /*
  * These are the number paths of length 1 to 5, that we are allowing to emanate
  * from a single file of interest. For example, we allow 1000 paths of length
  * 1, to emanate from each file of interest. This essentially represents the
  * potential wakeup paths, which need to be limited in order to avoid massive
  * uncontrolled wakeup storms. The common use case should be a single ep which
  * is connected to n file sources. In this case each file source has 1 path
  * of length 1. Thus, the numbers below should be more than sufficient. These
  * path limits are enforced during an EPOLL_CTL_ADD operation, since a modify
  * and delete can't add additional paths. Protected by the epnested_mutex.
  */
 static const int path_limits[PATH_ARR_SIZE] = { 1000, 500, 100, 50, 10 };
 static int path_count[PATH_ARR_SIZE];
 
 static int path_count_inc(int nests)
 {
         /* Allow an arbitrary number of depth 1 paths */
         if (nests == 0)
                 return 0;
 
         if (++path_count[nests] > path_limits[nests])
                 return -1;
         return 0;
 }
 
 static void path_count_init(void)
 {
         int i;
 
         for (i = 0; i < PATH_ARR_SIZE; i++)
                 path_count[i] = 0;
 }
 
 static int reverse_path_check_proc(struct hlist_head *refs, int depth)
 {
         int error = 0;
         struct epitem *epi;
 
         if (depth > EP_MAX_NESTS) /* too deep nesting */
                 return -1;
 
         /* CTL_DEL can remove links here, but that can't increase our count */
         hlist_for_each_entry_rcu(epi, refs, fllink) {
                 struct hlist_head *refs = &epi->ep->refs;
                 if (hlist_empty(refs))
                         error = path_count_inc(depth);
                 else
                         error = reverse_path_check_proc(refs, depth + 1);
                 if (error != 0)
                         break;
         }
         return error;
 }
 
 /**
  * reverse_path_check - The tfile_check_list is list of epitem_head, which have
  *                      links that are proposed to be newly added. We need to
  *                      make sure that those added links don't add too many
  *                      paths such that we will spend all our time waking up
  *                      eventpoll objects.
  *
  * Return: %zero if the proposed links don't create too many paths,
  *          %-1 otherwise.
  */
 static int reverse_path_check(void)
 {
         struct epitems_head *p;
 
         for (p = tfile_check_list; p != EP_UNACTIVE_PTR; p = p->next) {
                 int error;
                 path_count_init();
                 rcu_read_lock();
                 error = reverse_path_check_proc(&p->epitems, 0);
                 rcu_read_unlock();
                 if (error)
                         return error;
         }
         return 0;
 }
 
 static int ep_create_wakeup_source(struct epitem *epi)
 {
         struct name_snapshot n;
         struct wakeup_source *ws;
 
         if (!epi->ep->ws) {
                 epi->ep->ws = wakeup_source_register(NULL, "eventpoll");
                 if (!epi->ep->ws)
                         return -ENOMEM;
         }
 
         take_dentry_name_snapshot(&n, epi->ffd.file->f_path.dentry);
         ws = wakeup_source_register(NULL, n.name.name);
         release_dentry_name_snapshot(&n);
 
         if (!ws)
                 return -ENOMEM;
         rcu_assign_pointer(epi->ws, ws);
 
         return 0;
 }
 
 /* rare code path, only used when EPOLL_CTL_MOD removes a wakeup source */
 static noinline void ep_destroy_wakeup_source(struct epitem *epi)
 {
         struct wakeup_source *ws = ep_wakeup_source(epi);
 
         RCU_INIT_POINTER(epi->ws, NULL);
 
         /*
          * wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is
          * used internally by wakeup_source_remove, too (called by
          * wakeup_source_unregister), so we cannot use call_rcu
          */
         synchronize_rcu();
         wakeup_source_unregister(ws);
 }
 
 static int attach_epitem(struct file *file, struct epitem *epi)
 {
         struct epitems_head *to_free = NULL;
         struct hlist_head *head = NULL;
         struct eventpoll *ep = NULL;
 
         if (is_file_epoll(file))
                 ep = file->private_data;
 
         if (ep) {
                 head = &ep->refs;
         } else if (!READ_ONCE(file->f_ep)) {
 allocate:
                 to_free = kmem_cache_zalloc(ephead_cache, GFP_KERNEL);
                 if (!to_free)
                         return -ENOMEM;
                 head = &to_free->epitems;
         }
         spin_lock(&file->f_lock);
         if (!file->f_ep) {
                 if (unlikely(!head)) {
                         spin_unlock(&file->f_lock);
                         goto allocate;
                 }
                 file->f_ep = head;
                 to_free = NULL;
         }
         hlist_add_head_rcu(&epi->fllink, file->f_ep);
         spin_unlock(&file->f_lock);
         free_ephead(to_free);
         return 0;
 }
 
 /*
  * Must be called with "mtx" held.
  */
 static int ep_insert(struct eventpoll *ep, const struct epoll_event *event,
                      struct file *tfile, int fd, int full_check)
 {
         int error, pwake = 0;
         __poll_t revents;
         struct epitem *epi;
         struct ep_pqueue epq;
         struct eventpoll *tep = NULL;
 
         if (is_file_epoll(tfile))
                 tep = tfile->private_data;
 
         lockdep_assert_irqs_enabled();
 
         if (unlikely(percpu_counter_compare(&ep->user->epoll_watches,
                                             max_user_watches) >= 0))
                 return -ENOSPC;
         percpu_counter_inc(&ep->user->epoll_watches);
 
         if (!(epi = kmem_cache_zalloc(epi_cache, GFP_KERNEL))) {
                 percpu_counter_dec(&ep->user->epoll_watches);
                 return -ENOMEM;
         }
 
         /* Item initialization follow here ... */
         INIT_LIST_HEAD(&epi->rdllink);
         epi->ep = ep;
         ep_set_ffd(&epi->ffd, tfile, fd);
         epi->event = *event;
         epi->next = EP_UNACTIVE_PTR;
 
         if (tep)
                 mutex_lock_nested(&tep->mtx, 1);
         /* Add the current item to the list of active epoll hook for this file */
         if (unlikely(attach_epitem(tfile, epi) < 0)) {
                 if (tep)
                         mutex_unlock(&tep->mtx);
                 kmem_cache_free(epi_cache, epi);
                 percpu_counter_dec(&ep->user->epoll_watches);
                 return -ENOMEM;
         }
 
         if (full_check && !tep)
                 list_file(tfile);
 
         /*
          * Add the current item to the RB tree. All RB tree operations are
          * protected by "mtx", and ep_insert() is called with "mtx" held.
          */
         ep_rbtree_insert(ep, epi);
         if (tep)
                 mutex_unlock(&tep->mtx);
 
         /*
          * ep_remove_safe() calls in the later error paths can't lead to
          * ep_free() as the ep file itself still holds an ep reference.
          */
         ep_get(ep);
 
         /* now check if we've created too many backpaths */
         if (unlikely(full_check && reverse_path_check())) {
                 ep_remove_safe(ep, epi);
                 return -EINVAL;
         }
 
         if (epi->event.events & EPOLLWAKEUP) {
                 error = ep_create_wakeup_source(epi);
                 if (error) {
                         ep_remove_safe(ep, epi);
                         return error;
                 }
         }
 
         /* Initialize the poll table using the queue callback */
         epq.epi = epi;
         init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);
 
         /*
          * Attach the item to the poll hooks and get current event bits.
          * We can safely use the file* here because its usage count has
          * been increased by the caller of this function. Note that after
          * this operation completes, the poll callback can start hitting
          * the new item.
          */
         revents = ep_item_poll(epi, &epq.pt, 1);
 
         /*
          * We have to check if something went wrong during the poll wait queue
          * install process. Namely an allocation for a wait queue failed due
          * high memory pressure.
          */
         if (unlikely(!epq.epi)) {
                 ep_remove_safe(ep, epi);
                 return -ENOMEM;
         }
 
         /* We have to drop the new item inside our item list to keep track of it */
         write_lock_irq(&ep->lock);
 
         /* record NAPI ID of new item if present */
         ep_set_busy_poll_napi_id(epi);
 
         /* If the file is already "ready" we drop it inside the ready list */
         if (revents && !ep_is_linked(epi)) {
                 list_add_tail(&epi->rdllink, &ep->rdllist);
                 ep_pm_stay_awake(epi);
 
                 /* Notify waiting tasks that events are available */
                 if (waitqueue_active(&ep->wq))
                         wake_up(&ep->wq);
                 if (waitqueue_active(&ep->poll_wait))
                         pwake++;
         }
 
         write_unlock_irq(&ep->lock);
 
         /* We have to call this outside the lock */
         if (pwake)
                 ep_poll_safewake(ep, NULL, 0);
 
         return 0;
 }
 
 /*
  * Modify the interest event mask by dropping an event if the new mask
  * has a match in the current file status. Must be called with "mtx" held.
  */
 static int ep_modify(struct eventpoll *ep, struct epitem *epi,
                      const struct epoll_event *event)
 {
         int pwake = 0;
         poll_table pt;
 
         lockdep_assert_irqs_enabled();
 
         init_poll_funcptr(&pt, NULL);
 
         /*
          * Set the new event interest mask before calling f_op->poll();
          * otherwise we might miss an event that happens between the
          * f_op->poll() call and the new event set registering.
          */
         epi->event.events = event->events; /* need barrier below */
         epi->event.data = event->data; /* protected by mtx */
         if (epi->event.events & EPOLLWAKEUP) {
                 if (!ep_has_wakeup_source(epi))
                         ep_create_wakeup_source(epi);
         } else if (ep_has_wakeup_source(epi)) {
                 ep_destroy_wakeup_source(epi);
         }
 
         /*
          * The following barrier has two effects:
          *
          * 1) Flush epi changes above to other CPUs.  This ensures
          *    we do not miss events from ep_poll_callback if an
          *    event occurs immediately after we call f_op->poll().
          *    We need this because we did not take ep->lock while
          *    changing epi above (but ep_poll_callback does take
          *    ep->lock).
          *
          * 2) We also need to ensure we do not miss _past_ events
          *    when calling f_op->poll().  This barrier also
          *    pairs with the barrier in wq_has_sleeper (see
          *    comments for wq_has_sleeper).
          *
          * This barrier will now guarantee ep_poll_callback or f_op->poll
          * (or both) will notice the readiness of an item.
          */
         smp_mb();
 
         /*
          * Get current event bits. We can safely use the file* here because
          * its usage count has been increased by the caller of this function.
          * If the item is "hot" and it is not registered inside the ready
          * list, push it inside.
          */
         if (ep_item_poll(epi, &pt, 1)) {
                 write_lock_irq(&ep->lock);
                 if (!ep_is_linked(epi)) {
                         list_add_tail(&epi->rdllink, &ep->rdllist);
                         ep_pm_stay_awake(epi);
 
                         /* Notify waiting tasks that events are available */
                         if (waitqueue_active(&ep->wq))
                                 wake_up(&ep->wq);
                         if (waitqueue_active(&ep->poll_wait))
                                 pwake++;
                 }
                 write_unlock_irq(&ep->lock);
         }
 
         /* We have to call this outside the lock */
         if (pwake)
                 ep_poll_safewake(ep, NULL, 0);
 
         return 0;
 }
 
 static int ep_send_events(struct eventpoll *ep,
                           struct epoll_event __user *events, int maxevents)
 {
         struct epitem *epi, *tmp;
         LIST_HEAD(txlist);
         poll_table pt;
         int res = 0;
 
         /*
          * Always short-circuit for fatal signals to allow threads to make a
          * timely exit without the chance of finding more events available and
          * fetching repeatedly.
          */
         if (fatal_signal_pending(current))
                 return -EINTR;
 
         init_poll_funcptr(&pt, NULL);
 
         mutex_lock(&ep->mtx);
         ep_start_scan(ep, &txlist);
 
         /*
          * We can loop without lock because we are passed a task private list.
          * Items cannot vanish during the loop we are holding ep->mtx.
          */
         list_for_each_entry_safe(epi, tmp, &txlist, rdllink) {
                 struct wakeup_source *ws;
                 __poll_t revents;
 
                 if (res >= maxevents)
                         break;
 
                 /*
                  * Activate ep->ws before deactivating epi->ws to prevent
                  * triggering auto-suspend here (in case we reactive epi->ws
                  * below).
                  *
                  * This could be rearranged to delay the deactivation of epi->ws
                  * instead, but then epi->ws would temporarily be out of sync
                  * with ep_is_linked().
                  */
                 ws = ep_wakeup_source(epi);
                 if (ws) {
                         if (ws->active)
                                 __pm_stay_awake(ep->ws);
                         __pm_relax(ws);
                 }
 
                 list_del_init(&epi->rdllink);
 
                 /*
                  * If the event mask intersect the caller-requested one,
                  * deliver the event to userspace. Again, we are holding ep->mtx,
                  * so no operations coming from userspace can change the item.
                  */
                 revents = ep_item_poll(epi, &pt, 1);
                 if (!revents)
                         continue;
 
                 events = epoll_put_uevent(revents, epi->event.data, events);
                 if (!events) {
                         list_add(&epi->rdllink, &txlist);
                         ep_pm_stay_awake(epi);
                         if (!res)
                                 res = -EFAULT;
                         break;
                 }
                 res++;
                 if (epi->event.events & EPOLLONESHOT)
                         epi->event.events &= EP_PRIVATE_BITS;
                 else if (!(epi->event.events & EPOLLET)) {
                         /*
                          * If this file has been added with Level
                          * Trigger mode, we need to insert back inside
                          * the ready list, so that the next call to
                          * epoll_wait() will check again the events
                          * availability. At this point, no one can insert
                          * into ep->rdllist besides us. The epoll_ctl()
                          * callers are locked out by
                          * ep_send_events() holding "mtx" and the
                          * poll callback will queue them in ep->ovflist.
                          */
                         list_add_tail(&epi->rdllink, &ep->rdllist);
                         ep_pm_stay_awake(epi);
                 }
         }
         ep_done_scan(ep, &txlist);
         mutex_unlock(&ep->mtx);
 
         return res;
 }
 
 static struct timespec64 *ep_timeout_to_timespec(struct timespec64 *to, long ms)
 {
         struct timespec64 now;
 
         if (ms < 0)
                 return NULL;
 
         if (!ms) {
                 to->tv_sec = 0;
                 to->tv_nsec = 0;
                 return to;
         }
 
         to->tv_sec = ms / MSEC_PER_SEC;
         to->tv_nsec = NSEC_PER_MSEC * (ms % MSEC_PER_SEC);
 
         ktime_get_ts64(&now);
         *to = timespec64_add_safe(now, *to);
         return to;
 }
 
 /*
  * autoremove_wake_function, but remove even on failure to wake up, because we
  * know that default_wake_function/ttwu will only fail if the thread is already
  * woken, and in that case the ep_poll loop will remove the entry anyways, not
  * try to reuse it.
  */
 static int ep_autoremove_wake_function(struct wait_queue_entry *wq_entry,
                                        unsigned int mode, int sync, void *key)
 {
         int ret = default_wake_function(wq_entry, mode, sync, key);
 
         /*
          * Pairs with list_empty_careful in ep_poll, and ensures future loop
          * iterations see the cause of this wakeup.
          */
         list_del_init_careful(&wq_entry->entry);
         return ret;
 }
 
 /**
  * ep_poll - Retrieves ready events, and delivers them to the caller-supplied
  *           event buffer.
  *
  * @ep: Pointer to the eventpoll context.
  * @events: Pointer to the userspace buffer where the ready events should be
  *          stored.
  * @maxevents: Size (in terms of number of events) of the caller event buffer.
  * @timeout: Maximum timeout for the ready events fetch operation, in
  *           timespec. If the timeout is zero, the function will not block,
  *           while if the @timeout ptr is NULL, the function will block
  *           until at least one event has been retrieved (or an error
  *           occurred).
  *
  * Return: the number of ready events which have been fetched, or an
  *          error code, in case of error.
  */
 static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
                    int maxevents, struct timespec64 *timeout)
 {
         int res, eavail, timed_out = 0;
         u64 slack = 0;
         wait_queue_entry_t wait;
         ktime_t expires, *to = NULL;
 
         lockdep_assert_irqs_enabled();
 
         if (timeout && (timeout->tv_sec | timeout->tv_nsec)) {
                 slack = select_estimate_accuracy(timeout);
                 to = &expires;
                 *to = timespec64_to_ktime(*timeout);
         } else if (timeout) {
                 /*
                  * Avoid the unnecessary trip to the wait queue loop, if the
                  * caller specified a non blocking operation.
                  */
                 timed_out = 1;
         }
 
         /*
          * This call is racy: We may or may not see events that are being added
          * to the ready list under the lock (e.g., in IRQ callbacks). For cases
          * with a non-zero timeout, this thread will check the ready list under
          * lock and will add to the wait queue.  For cases with a zero
          * timeout, the user by definition should not care and will have to
          * recheck again.
          */
         eavail = ep_events_available(ep);
 
         while (1) {
                 if (eavail) {
                         /*
                          * Try to transfer events to user space. In case we get
                          * 0 events and there's still timeout left over, we go
                          * trying again in search of more luck.
                          */
                         res = ep_send_events(ep, events, maxevents);
                         if (res)
                                 return res;
                 }
 
                 if (timed_out)
                         return 0;
 
                 eavail = ep_busy_loop(ep, timed_out);
                 if (eavail)
                         continue;
 
                 if (signal_pending(current))
                         return -EINTR;
 
                 /*
                  * Internally init_wait() uses autoremove_wake_function(),
                  * thus wait entry is removed from the wait queue on each
                  * wakeup. Why it is important? In case of several waiters
                  * each new wakeup will hit the next waiter, giving it the
                  * chance to harvest new event. Otherwise wakeup can be
                  * lost. This is also good performance-wise, because on
                  * normal wakeup path no need to call __remove_wait_queue()
                  * explicitly, thus ep->lock is not taken, which halts the
                  * event delivery.
                  *
                  * In fact, we now use an even more aggressive function that
                  * unconditionally removes, because we don't reuse the wait
                  * entry between loop iterations. This lets us also avoid the
                  * performance issue if a process is killed, causing all of its
                  * threads to wake up without being removed normally.
                  */
                 init_wait(&wait);
                 wait.func = ep_autoremove_wake_function;
 
                 write_lock_irq(&ep->lock);
                 /*
                  * Barrierless variant, waitqueue_active() is called under
                  * the same lock on wakeup ep_poll_callback() side, so it
                  * is safe to avoid an explicit barrier.
                  */
                 __set_current_state(TASK_INTERRUPTIBLE);
 
                 /*
                  * Do the final check under the lock. ep_start/done_scan()
                  * plays with two lists (->rdllist and ->ovflist) and there
                  * is always a race when both lists are empty for short
                  * period of time although events are pending, so lock is
                  * important.
                  */
                 eavail = ep_events_available(ep);
                 if (!eavail)
                         __add_wait_queue_exclusive(&ep->wq, &wait);
 
                 write_unlock_irq(&ep->lock);
 
                 if (!eavail)
                         timed_out = !schedule_hrtimeout_range(to, slack,
                                                               HRTIMER_MODE_ABS);
                 __set_current_state(TASK_RUNNING);
 
                 /*
                  * We were woken up, thus go and try to harvest some events.
                  * If timed out and still on the wait queue, recheck eavail
                  * carefully under lock, below.
                  */
                 eavail = 1;
 
                 if (!list_empty_careful(&wait.entry)) {
                         write_lock_irq(&ep->lock);
                         /*
                          * If the thread timed out and is not on the wait queue,
                          * it means that the thread was woken up after its
                          * timeout expired before it could reacquire the lock.
                          * Thus, when wait.entry is empty, it needs to harvest
                          * events.
                          */
                         if (timed_out)
                                 eavail = list_empty(&wait.entry);
                         __remove_wait_queue(&ep->wq, &wait);
                         write_unlock_irq(&ep->lock);
                 }
         }
 }
 
 /**
  * ep_loop_check_proc - verify that adding an epoll file inside another
  *                      epoll structure does not violate the constraints, in
  *                      terms of closed loops, or too deep chains (which can
  *                      result in excessive stack usage).
  *
  * @ep: the &struct eventpoll to be currently checked.
  * @depth: Current depth of the path being checked.
  *
  * Return: %zero if adding the epoll @file inside current epoll
  *          structure @ep does not violate the constraints, or %-1 otherwise.
  */
 static int ep_loop_check_proc(struct eventpoll *ep, int depth)
 {
         int error = 0;
         struct rb_node *rbp;
         struct epitem *epi;
 
         mutex_lock_nested(&ep->mtx, depth + 1);
         ep->gen = loop_check_gen;
         for (rbp = rb_first_cached(&ep->rbr); rbp; rbp = rb_next(rbp)) {
                 epi = rb_entry(rbp, struct epitem, rbn);
                 if (unlikely(is_file_epoll(epi->ffd.file))) {
                         struct eventpoll *ep_tovisit;
                         ep_tovisit = epi->ffd.file->private_data;
                         if (ep_tovisit->gen == loop_check_gen)
                                 continue;
                         if (ep_tovisit == inserting_into || depth > EP_MAX_NESTS)
                                 error = -1;
                         else
                                 error = ep_loop_check_proc(ep_tovisit, depth + 1);
                         if (error != 0)
                                 break;
                 } else {
                         /*
                          * If we've reached a file that is not associated with
                          * an ep, then we need to check if the newly added
                          * links are going to add too many wakeup paths. We do
                          * this by adding it to the tfile_check_list, if it's
                          * not already there, and calling reverse_path_check()
                          * during ep_insert().
                          */
                         list_file(epi->ffd.file);
                 }
         }
         mutex_unlock(&ep->mtx);
 
         return error;
 }
 
 /**
  * ep_loop_check - Performs a check to verify that adding an epoll file (@to)
  *                 into another epoll file (represented by @ep) does not create
  *                 closed loops or too deep chains.
  *
  * @ep: Pointer to the epoll we are inserting into.
  * @to: Pointer to the epoll to be inserted.
  *
  * Return: %zero if adding the epoll @to inside the epoll @from
  * does not violate the constraints, or %-1 otherwise.
  */
 static int ep_loop_check(struct eventpoll *ep, struct eventpoll *to)
 {
         inserting_into = ep;
         return ep_loop_check_proc(to, 0);
 }
 
 static void clear_tfile_check_list(void)
 {
         rcu_read_lock();
         while (tfile_check_list != EP_UNACTIVE_PTR) {
                 struct epitems_head *head = tfile_check_list;
                 tfile_check_list = head->next;
                 unlist_file(head);
         }
         rcu_read_unlock();
 }
 
 /*
  * Open an eventpoll file descriptor.
  */
 static int do_epoll_create(int flags)
 {
         int error, fd;
         struct eventpoll *ep = NULL;
         struct file *file;
 
         /* Check the EPOLL_* constant for consistency.  */
         BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
 
         if (flags & ~EPOLL_CLOEXEC)
                 return -EINVAL;
         /*
          * Create the internal data structure ("struct eventpoll").
          */
         error = ep_alloc(&ep);
         if (error < 0)
                 return error;
         /*
          * Creates all the items needed to setup an eventpoll file. That is,
          * a file structure and a free file descriptor.
          */
         fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
         if (fd < 0) {
                 error = fd;
                 goto out_free_ep;
         }
         file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
                                  O_RDWR | (flags & O_CLOEXEC));
         if (IS_ERR(file)) {
                 error = PTR_ERR(file);
                 goto out_free_fd;
         }
 #ifdef CONFIG_NET_RX_BUSY_POLL
         ep->busy_poll_usecs = 0;
         ep->busy_poll_budget = 0;
         ep->prefer_busy_poll = false;
 #endif
         ep->file = file;
         fd_install(fd, file);
         return fd;
 
 out_free_fd:
         put_unused_fd(fd);
 out_free_ep:
         ep_clear_and_put(ep);
         return error;
 }
 
 SYSCALL_DEFINE1(epoll_create1, int, flags)
 {
         return do_epoll_create(flags);
 }
 
 SYSCALL_DEFINE1(epoll_create, int, size)
 {
         if (size <= 0)
                 return -EINVAL;
 
         return do_epoll_create(0);
 }
 
 #ifdef CONFIG_PM_SLEEP
 static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
 {
         if ((epev->events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
                 epev->events &= ~EPOLLWAKEUP;
 }
 #else
 static inline void ep_take_care_of_epollwakeup(struct epoll_event *epev)
 {
         epev->events &= ~EPOLLWAKEUP;
 }
 #endif
 
 static inline int epoll_mutex_lock(struct mutex *mutex, int depth,
                                    bool nonblock)
 {
         if (!nonblock) {
                 mutex_lock_nested(mutex, depth);
                 return 0;
         }
         if (mutex_trylock(mutex))
                 return 0;
         return -EAGAIN;
 }
 
 int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
                  bool nonblock)
 {
         int error;
         int full_check = 0;
         struct fd f, tf;
         struct eventpoll *ep;
         struct epitem *epi;
         struct eventpoll *tep = NULL;
 
         error = -EBADF;
         f = fdget(epfd);
         if (!f.file)
                 goto error_return;
 
         /* Get the "struct file *" for the target file */
         tf = fdget(fd);
         if (!tf.file)
                 goto error_fput;
 
         /* The target file descriptor must support poll */
         error = -EPERM;
         if (!file_can_poll(tf.file))
                 goto error_tgt_fput;
 
         /* Check if EPOLLWAKEUP is allowed */
         if (ep_op_has_event(op))
                 ep_take_care_of_epollwakeup(epds);
 
         /*
          * We have to check that the file structure underneath the file descriptor
          * the user passed to us _is_ an eventpoll file. And also we do not permit
          * adding an epoll file descriptor inside itself.
          */
         error = -EINVAL;
         if (f.file == tf.file || !is_file_epoll(f.file))
                 goto error_tgt_fput;
 
         /*
          * epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,
          * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
          * Also, we do not currently supported nested exclusive wakeups.
          */
         if (ep_op_has_event(op) && (epds->events & EPOLLEXCLUSIVE)) {
                 if (op == EPOLL_CTL_MOD)
                         goto error_tgt_fput;
                 if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||
                                 (epds->events & ~EPOLLEXCLUSIVE_OK_BITS)))
                         goto error_tgt_fput;
         }
 
         /*
          * At this point it is safe to assume that the "private_data" contains
          * our own data structure.
          */
         ep = f.file->private_data;
 
         /*
          * When we insert an epoll file descriptor inside another epoll file
          * descriptor, there is the chance of creating closed loops, which are
          * better be handled here, than in more critical paths. While we are
          * checking for loops we also determine the list of files reachable
          * and hang them on the tfile_check_list, so we can check that we
          * haven't created too many possible wakeup paths.
          *
          * We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
          * the epoll file descriptor is attaching directly to a wakeup source,
          * unless the epoll file descriptor is nested. The purpose of taking the
          * 'epnested_mutex' on add is to prevent complex toplogies such as loops and
          * deep wakeup paths from forming in parallel through multiple
          * EPOLL_CTL_ADD operations.
          */
         error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
         if (error)
                 goto error_tgt_fput;
         if (op == EPOLL_CTL_ADD) {
                 if (READ_ONCE(f.file->f_ep) || ep->gen == loop_check_gen ||
                     is_file_epoll(tf.file)) {
                         mutex_unlock(&ep->mtx);
                         error = epoll_mutex_lock(&epnested_mutex, 0, nonblock);
                         if (error)
                                 goto error_tgt_fput;
                         loop_check_gen++;
                         full_check = 1;
                         if (is_file_epoll(tf.file)) {
                                 tep = tf.file->private_data;
                                 error = -ELOOP;
                                 if (ep_loop_check(ep, tep) != 0)
                                         goto error_tgt_fput;
                         }
                         error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
                         if (error)
                                 goto error_tgt_fput;
                 }
         }
 
         /*
          * Try to lookup the file inside our RB tree. Since we grabbed "mtx"
          * above, we can be sure to be able to use the item looked up by
          * ep_find() till we release the mutex.
          */
         epi = ep_find(ep, tf.file, fd);
 
         error = -EINVAL;
         switch (op) {
         case EPOLL_CTL_ADD:
                 if (!epi) {
                         epds->events |= EPOLLERR | EPOLLHUP;
                         error = ep_insert(ep, epds, tf.file, fd, full_check);
                 } else
                         error = -EEXIST;
                 break;
         case EPOLL_CTL_DEL:
                 if (epi) {
                         /*
                          * The eventpoll itself is still alive: the refcount
                          * can't go to zero here.
                          */
                         ep_remove_safe(ep, epi);
                         error = 0;
                 } else {
                         error = -ENOENT;
                 }
                 break;
         case EPOLL_CTL_MOD:
                 if (epi) {
                         if (!(epi->event.events & EPOLLEXCLUSIVE)) {
                                 epds->events |= EPOLLERR | EPOLLHUP;
                                 error = ep_modify(ep, epi, epds);
                         }
                 } else
                         error = -ENOENT;
                 break;
         }
         mutex_unlock(&ep->mtx);
 
 error_tgt_fput:
         if (full_check) {
                 clear_tfile_check_list();
                 loop_check_gen++;
                 mutex_unlock(&epnested_mutex);
         }
 
         fdput(tf);
 error_fput:
         fdput(f);
 error_return:
 
         return error;
 }
 
 /*
  * The following function implements the controller interface for
  * the eventpoll file that enables the insertion/removal/change of
  * file descriptors inside the interest set.
  */
 SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
                 struct epoll_event __user *, event)
 {
         struct epoll_event epds;
 
         if (ep_op_has_event(op) &&
             copy_from_user(&epds, event, sizeof(struct epoll_event)))
                 return -EFAULT;
 
         return do_epoll_ctl(epfd, op, fd, &epds, false);
 }
 
 /*
  * Implement the event wait interface for the eventpoll file. It is the kernel
  * part of the user space epoll_wait(2).
  */
 static int do_epoll_wait(int epfd, struct epoll_event __user *events,
                          int maxevents, struct timespec64 *to)
 {
         int error;
         struct fd f;
         struct eventpoll *ep;
 
         /* The maximum number of event must be greater than zero */
         if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)
                 return -EINVAL;
 
         /* Verify that the area passed by the user is writeable */
         if (!access_ok(events, maxevents * sizeof(struct epoll_event)))
                 return -EFAULT;
 
         /* Get the "struct file *" for the eventpoll file */
         f = fdget(epfd);
         if (!f.file)
                 return -EBADF;
 
         /*
          * We have to check that the file structure underneath the fd
          * the user passed to us _is_ an eventpoll file.
          */
         error = -EINVAL;
         if (!is_file_epoll(f.file))
                 goto error_fput;
 
         /*
          * At this point it is safe to assume that the "private_data" contains
          * our own data structure.
          */
         ep = f.file->private_data;
 
         /* Time to fish for events ... */
         error = ep_poll(ep, events, maxevents, to);
 
 error_fput:
         fdput(f);
         return error;
 }
 
 SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
                 int, maxevents, int, timeout)
 {
         struct timespec64 to;
 
         return do_epoll_wait(epfd, events, maxevents,
                              ep_timeout_to_timespec(&to, timeout));
 }
 
 /*
  * Implement the event wait interface for the eventpoll file. It is the kernel
  * part of the user space epoll_pwait(2).
  */
 static int do_epoll_pwait(int epfd, struct epoll_event __user *events,
                           int maxevents, struct timespec64 *to,
                           const sigset_t __user *sigmask, size_t sigsetsize)
 {
         int error;
 
         /*
          * If the caller wants a certain signal mask to be set during the wait,
          * we apply it here.
          */
         error = set_user_sigmask(sigmask, sigsetsize);
         if (error)
                 return error;
 
         error = do_epoll_wait(epfd, events, maxevents, to);
 
         restore_saved_sigmask_unless(error == -EINTR);
 
         return error;
 }
 
 SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
                 int, maxevents, int, timeout, const sigset_t __user *, sigmask,
                 size_t, sigsetsize)
 {
         struct timespec64 to;
 
         return do_epoll_pwait(epfd, events, maxevents,
                               ep_timeout_to_timespec(&to, timeout),
                               sigmask, sigsetsize);
 }
 
 SYSCALL_DEFINE6(epoll_pwait2, int, epfd, struct epoll_event __user *, events,
                 int, maxevents, const struct __kernel_timespec __user *, timeout,
                 const sigset_t __user *, sigmask, size_t, sigsetsize)
 {
         struct timespec64 ts, *to = NULL;
 
         if (timeout) {
                 if (get_timespec64(&ts, timeout))
                         return -EFAULT;
                 to = &ts;
                 if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
                         return -EINVAL;
         }
 
         return do_epoll_pwait(epfd, events, maxevents, to,
                               sigmask, sigsetsize);
 }
 
 #ifdef CONFIG_COMPAT
 static int do_compat_epoll_pwait(int epfd, struct epoll_event __user *events,
                                  int maxevents, struct timespec64 *timeout,
                                  const compat_sigset_t __user *sigmask,
                                  compat_size_t sigsetsize)
 {
         long err;
 
         /*
          * If the caller wants a certain signal mask to be set during the wait,
          * we apply it here.
          */
         err = set_compat_user_sigmask(sigmask, sigsetsize);
         if (err)
                 return err;
 
         err = do_epoll_wait(epfd, events, maxevents, timeout);
 
         restore_saved_sigmask_unless(err == -EINTR);
 
         return err;
 }
 
 COMPAT_SYSCALL_DEFINE6(epoll_pwait, int, epfd,
                        struct epoll_event __user *, events,
                        int, maxevents, int, timeout,
                        const compat_sigset_t __user *, sigmask,
                        compat_size_t, sigsetsize)
 {
         struct timespec64 to;
 
         return do_compat_epoll_pwait(epfd, events, maxevents,
                                      ep_timeout_to_timespec(&to, timeout),
                                      sigmask, sigsetsize);
 }
 
 COMPAT_SYSCALL_DEFINE6(epoll_pwait2, int, epfd,
                        struct epoll_event __user *, events,
                        int, maxevents,
                        const struct __kernel_timespec __user *, timeout,
                        const compat_sigset_t __user *, sigmask,
                        compat_size_t, sigsetsize)
 {
         struct timespec64 ts, *to = NULL;
 
         if (timeout) {
                 if (get_timespec64(&ts, timeout))
                         return -EFAULT;
                 to = &ts;
                 if (poll_select_set_timeout(to, ts.tv_sec, ts.tv_nsec))
                         return -EINVAL;
         }
 
         return do_compat_epoll_pwait(epfd, events, maxevents, to,
                                      sigmask, sigsetsize);
 }
 
 #endif
 
 static int __init eventpoll_init(void)
 {
         struct sysinfo si;
 
         si_meminfo(&si);
         /*
          * Allows top 4% of lomem to be allocated for epoll watches (per user).
          */
         max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
                 EP_ITEM_COST;
         BUG_ON(max_user_watches < 0);
 
         /*
          * We can have many thousands of epitems, so prevent this from
          * using an extra cache line on 64-bit (and smaller) CPUs
          */
         BUILD_BUG_ON(sizeof(void *) <= 8 && sizeof(struct epitem) > 128);
 
         /* Allocates slab cache used to allocate "struct epitem" items */
         epi_cache = kmem_cache_create("eventpoll_epi", sizeof(struct epitem),
                         0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL);
 
         /* Allocates slab cache used to allocate "struct eppoll_entry" */
         pwq_cache = kmem_cache_create("eventpoll_pwq",
                 sizeof(struct eppoll_entry), 0, SLAB_PANIC|SLAB_ACCOUNT, NULL);
         epoll_sysctls_init();
 
         ephead_cache = kmem_cache_create("ep_head",
                 sizeof(struct epitems_head), 0, SLAB_PANIC|SLAB_ACCOUNT, NULL);
 
         return 0;
 }
 fs_initcall(eventpoll_init);
 

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