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 */
 29static volatile __u32 max_pcktsz = MAX_PCKT_SIZE;
 30
 31struct {
 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
 38static __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
 48static __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
 58static __always_inline __u16 csum_fold_helper(__u32 csum)
 59{
 60	csum = (csum & 0xffff) + (csum >> 16);
 61	return ~((csum & 0xffff) + (csum >> 16));
 62}
 63
 64static __always_inline void ipv4_csum(void *data_start, int data_size,
 65				      __u32 *csum)
 66{
 67	*csum = bpf_csum_diff(0, 0, data_start, data_size, *csum);
 68	*csum = csum_fold_helper(*csum);
 69}
 70
 71static __always_inline int send_icmp4_too_big(struct xdp_md *xdp)
 72{
 73	int headroom = (int)sizeof(struct iphdr) + (int)sizeof(struct icmphdr);
 74
 75	if (bpf_xdp_adjust_head(xdp, 0 - headroom))
 76		return XDP_DROP;
 77	void *data = (void *)(long)xdp->data;
 78	void *data_end = (void *)(long)xdp->data_end;
 79
 80	if (data + (ICMP_TOOBIG_SIZE + headroom) > data_end)
 81		return XDP_DROP;
 82
 83	struct iphdr *iph, *orig_iph;
 84	struct icmphdr *icmp_hdr;
 85	struct ethhdr *orig_eth;
 86	__u32 csum = 0;
 87	__u64 off = 0;
 88
 89	orig_eth = data + headroom;
 90	swap_mac(data, orig_eth);
 91	off += sizeof(struct ethhdr);
 92	iph = data + off;
 93	off += sizeof(struct iphdr);
 94	icmp_hdr = data + off;
 95	off += sizeof(struct icmphdr);
 96	orig_iph = data + off;
 97	icmp_hdr->type = ICMP_DEST_UNREACH;
 98	icmp_hdr->code = ICMP_FRAG_NEEDED;
 99	icmp_hdr->un.frag.mtu = htons(max_pcktsz - sizeof(struct ethhdr));
100	icmp_hdr->checksum = 0;
101	ipv4_csum(icmp_hdr, ICMP_TOOBIG_PAYLOAD_SIZE, &csum);
102	icmp_hdr->checksum = csum;
103	iph->ttl = DEFAULT_TTL;
104	iph->daddr = orig_iph->saddr;
105	iph->saddr = orig_iph->daddr;
106	iph->version = 4;
107	iph->ihl = 5;
108	iph->protocol = IPPROTO_ICMP;
109	iph->tos = 0;
110	iph->tot_len = htons(
111		ICMP_TOOBIG_SIZE + headroom - sizeof(struct ethhdr));
112	iph->check = 0;
113	csum = 0;
114	ipv4_csum(iph, sizeof(struct iphdr), &csum);
115	iph->check = csum;
116	count_icmp();
117	return XDP_TX;
118}
119
120
121static __always_inline int handle_ipv4(struct xdp_md *xdp)
122{
123	void *data_end = (void *)(long)xdp->data_end;
124	void *data = (void *)(long)xdp->data;
125	int pckt_size = data_end - data;
126	int offset;
127
128	if (pckt_size > max(max_pcktsz, ICMP_TOOBIG_SIZE)) {
129		offset = pckt_size - ICMP_TOOBIG_SIZE;
130		if (bpf_xdp_adjust_tail(xdp, 0 - offset))
131			return XDP_PASS;
132		return send_icmp4_too_big(xdp);
133	}
134	return XDP_PASS;
135}
136
137SEC("xdp_icmp")
138int _xdp_icmp(struct xdp_md *xdp)
139{
140	void *data_end = (void *)(long)xdp->data_end;
141	void *data = (void *)(long)xdp->data;
142	struct ethhdr *eth = data;
143	__u16 h_proto;
144
145	if (eth + 1 > data_end)
146		return XDP_DROP;
147
148	h_proto = eth->h_proto;
149
150	if (h_proto == htons(ETH_P_IP))
151		return handle_ipv4(xdp);
152	else
153		return XDP_PASS;
154}
155
156char _license[] SEC("license") = "GPL";