1 /* SPDX-License-Identifier: GPL-2.0
2 * Copyright (c) 2018 Facebook
3 *
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of version 2 of the GNU General Public
6 * License as published by the Free Software Foundation.
7 *
8 * This program shows how to use bpf_xdp_adjust_tail() by
9 * generating ICMPv4 "packet to big" (unreachable/ df bit set frag needed
10 * to be more preice in case of v4)" where receiving packets bigger then
11 * 600 bytes.
12 */
13 #define KBUILD_MODNAME "foo"
14 #include <uapi/linux/bpf.h>
15 #include <linux/in.h>
16 #include <linux/if_ether.h>
17 #include <linux/if_packet.h>
18 #include <linux/if_vlan.h>
19 #include <linux/ip.h>
20 #include <linux/icmp.h>
21 #include <bpf/bpf_helpers.h>
22
23 #define DEFAULT_TTL 64
24 #define MAX_PCKT_SIZE 600
25 #define ICMP_TOOBIG_SIZE 98
26 #define ICMP_TOOBIG_PAYLOAD_SIZE 92
27
28 /* volatile to prevent compiler optimizations */
29 static volatile __u32 max_pcktsz = MAX_PCKT_SIZE;
30
31 struct {
32 __uint(type, BPF_MAP_TYPE_ARRAY);
33 __type(key, __u32);
34 __type(value, __u64);
35 __uint(max_entries, 1);
36 } icmpcnt SEC(".maps");
37
38 static __always_inline void count_icmp(void)
39 {
40 u64 key = 0;
41 u64 *icmp_count;
42
43 icmp_count = bpf_map_lookup_elem(&icmpcnt, &key);
44 if (icmp_count)
45 *icmp_count += 1;
46 }
47
48 static __always_inline void swap_mac(void *data, struct ethhdr *orig_eth)
49 {
50 struct ethhdr *eth;
51
52 eth = data;
53 memcpy(eth->h_source, orig_eth->h_dest, ETH_ALEN);
54 memcpy(eth->h_dest, orig_eth->h_source, ETH_ALEN);
55 eth->h_proto = orig_eth->h_proto;
56 }
57
58 static __always_inline __u16 csum_fold_helper(__u32 csum)
59 {
60 return ~((csum & 0xffff) + (csum >> 16));
61 }
62
63 static __always_inline void ipv4_csum(void *data_start, int data_size,
64 __u32 *csum)
65 {
66 *csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
67 *csum = csum_fold_helper(*csum);
68 }
69
70 static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
71 {
72 int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);
73
74 if (bpf_xdp_adjust_head(xdp, 0 - headroom))
75 return XDP_DROP;
76 void *data = (void *)(long)xdp->data;
77 void *data_end = (void *)(long)xdp->data_end;
78
79 if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)
80 return XDP_DROP;
81
82 struct iphdr *iph, *orig_iph;
83 struct icmphdr *icmp_hdr;
84 struct ethhdr *orig_eth;
85 __u32 csum = 0;
86 __u64 off = 0;
87
88 orig_eth = data + headroom;
89 swap_mac(data, orig_eth);
90 off += sizeof(struct ethhdr);
91 iph = data + off;
92 off += sizeof(struct iphdr);
93 icmp_hdr = data + off;
94 off += sizeof(struct icmphdr);
95 orig_iph = data + off;
96 icmp_hdr->type = ICMP_DEST_UNREACH;
97 icmp_hdr->code = ICMP_FRAG_NEEDED;
98 icmp_hdr->un.frag.mtu = htons(max_pcktsz - sizeof(struct ethhdr));
99 icmp_hdr->checksum = 0;
100 ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
101 icmp_hdr->checksum = csum;
102 iph->ttl = DEFAULT_TTL;
103 iph->daddr = orig_iph->saddr;
104 iph->saddr = orig_iph->daddr;
105 iph->version = 4;
106 iph->ihl = 5;
107 iph->protocol = IPPROTO_ICMP;
108 iph->tos = 0;
109 iph->tot_len = htons(
110 ICMP_TOOBIG_SIZE + headroom - sizeof(struct ethhdr));
111 iph->check = 0;
112 csum = 0;
113 ipv4_csum(iph, sizeof(struct iphdr), &csum);
114 iph->check = csum;
115 count_icmp();
116 return XDP_TX;
117 }
118
119
120 static __always_inline int handle_ipv4(struct xdp_md *xdp)
121 {
122 void *data_end = (void *)(long)xdp->data_end;
123 void *data = (void *)(long)xdp->data;
124 int pckt_size = data_end - data;
125 int offset;
126
127 if (pckt_size > max(max_pcktsz, ICMP_TOOBIG_SIZE)) {
128 offset = pckt_size - ICMP_TOOBIG_SIZE;
129 if (bpf_xdp_adjust_tail(xdp, 0 - offset))
130 return XDP_PASS;
131 return send_icmp4_too_big(xdp);
132 }
133 return XDP_PASS;
134 }
135
136 SEC("xdp_icmp")
137 int _xdp_icmp(struct xdp_md *xdp)
138 {
139 void *data_end = (void *)(long)xdp->data_end;
140 void *data = (void *)(long)xdp->data;
141 struct ethhdr *eth = data;
142 __u16 h_proto;
143
144 if (eth + 1 > data_end)
145 return XDP_DROP;
146
147 h_proto = eth->h_proto;
148
149 if (h_proto == htons(ETH_P_IP))
150 return handle_ipv4(xdp);
151 else
152 return XDP_PASS;
153 }
154
155 char _license[] SEC("license") = "GPL";
156
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