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_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 struct bpf_map_def SEC("maps") icmpcnt = {
29 	.type = BPF_MAP_TYPE_ARRAY,
30 	.key_size = sizeof(__u32),
31 	.value_size = sizeof(__u64),
32 	.max_entries = 1,
33 };
34 
35 static __always_inline void count_icmp(void)
36 {
37 	u64 key = 0;
38 	u64 *icmp_count;
39 
40 	icmp_count = bpf_map_lookup_elem(&icmpcnt, &key);
41 	if (icmp_count)
42 		*icmp_count += 1;
43 }
44 
45 static __always_inline void swap_mac(void *data, struct ethhdr *orig_eth)
46 {
47 	struct ethhdr *eth;
48 
49 	eth = data;
50 	memcpy(eth->h_source, orig_eth->h_dest, ETH_ALEN);
51 	memcpy(eth->h_dest, orig_eth->h_source, ETH_ALEN);
52 	eth->h_proto = orig_eth->h_proto;
53 }
54 
55 static __always_inline __u16 csum_fold_helper(__u32 csum)
56 {
57 	return ~((csum & 0xffff) + (csum >> 16));
58 }
59 
60 static __always_inline void ipv4_csum(void *data_start, int data_size,
61 				      __u32 *csum)
62 {
63 	*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
64 	*csum = csum_fold_helper(*csum);
65 }
66 
67 static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
68 {
69 	int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);
70 
71 	if (bpf_xdp_adjust_head(xdp, 0 - headroom))
72 		return XDP_DROP;
73 	void *data = (void *)(long)xdp->data;
74 	void *data_end = (void *)(long)xdp->data_end;
75 
76 	if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)
77 		return XDP_DROP;
78 
79 	struct iphdr *iph, *orig_iph;
80 	struct icmphdr *icmp_hdr;
81 	struct ethhdr *orig_eth;
82 	__u32 csum = 0;
83 	__u64 off = 0;
84 
85 	orig_eth = data + headroom;
86 	swap_mac(data, orig_eth);
87 	off += sizeof(struct ethhdr);
88 	iph = data + off;
89 	off += sizeof(struct iphdr);
90 	icmp_hdr = data + off;
91 	off += sizeof(struct icmphdr);
92 	orig_iph = data + off;
93 	icmp_hdr->type = ICMP_DEST_UNREACH;
94 	icmp_hdr->code = ICMP_FRAG_NEEDED;
95 	icmp_hdr->un.frag.mtu = htons(MAX_PCKT_SIZE-sizeof(struct ethhdr));
96 	icmp_hdr->checksum = 0;
97 	ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
98 	icmp_hdr->checksum = csum;
99 	iph->ttl = DEFAULT_TTL;
100 	iph->daddr = orig_iph->saddr;
101 	iph->saddr = orig_iph->daddr;
102 	iph->version = 4;
103 	iph->ihl = 5;
104 	iph->protocol = IPPROTO_ICMP;
105 	iph->tos = 0;
106 	iph->tot_len = htons(
107 		ICMP_TOOBIG_SIZE + headroom - sizeof(struct ethhdr));
108 	iph->check = 0;
109 	csum = 0;
110 	ipv4_csum(iph, sizeof(struct iphdr), &csum);
111 	iph->check = csum;
112 	count_icmp();
113 	return XDP_TX;
114 }
115 
116 
117 static __always_inline int handle_ipv4(struct xdp_md *xdp)
118 {
119 	void *data_end = (void *)(long)xdp->data_end;
120 	void *data = (void *)(long)xdp->data;
121 	int pckt_size = data_end - data;
122 	int offset;
123 
124 	if (pckt_size > MAX_PCKT_SIZE) {
125 		offset = pckt_size - ICMP_TOOBIG_SIZE;
126 		if (bpf_xdp_adjust_tail(xdp, 0 - offset))
127 			return XDP_PASS;
128 		return send_icmp4_too_big(xdp);
129 	}
130 	return XDP_PASS;
131 }
132 
133 SEC("xdp_icmp")
134 int _xdp_icmp(struct xdp_md *xdp)
135 {
136 	void *data_end = (void *)(long)xdp->data_end;
137 	void *data = (void *)(long)xdp->data;
138 	struct ethhdr *eth = data;
139 	__u16 h_proto;
140 
141 	if (eth + 1 > data_end)
142 		return XDP_DROP;
143 
144 	h_proto = eth->h_proto;
145 
146 	if (h_proto == htons(ETH_P_IP))
147 		return handle_ipv4(xdp);
148 	else
149 		return XDP_PASS;
150 }
151 
152 char _license[] SEC("license") = "GPL";
153