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1/* SPDX-License-Identifier: GPL-2.02 * Copyright (c) 2018 Facebook3 *4 * This program is free software; you can redistribute it and/or5 * modify it under the terms of version 2 of the GNU General Public6 * License as published by the Free Software Foundation.7 *8 * This program shows how to use bpf_xdp_adjust_tail() by9 * generating ICMPv4 "packet to big" (unreachable/ df bit set frag needed10 * to be more preice in case of v4)" where receiving packets bigger then11 * 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 6424#define MAX_PCKT_SIZE 60025#define ICMP_TOOBIG_SIZE 9826#define ICMP_TOOBIG_PAYLOAD_SIZE 9227 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 return ~((csum & 0xffff) + (csum >> 16));61}62 63static __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 70static __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 120static __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 136SEC("xdp_icmp")137int _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 else152 return XDP_PASS;153}154 155char _license[] SEC("license") = "GPL";156