root/samples/bpf/xdp_fwd_kern.c

DEFINITIONS

1. ip_decrease_ttl
2. xdp_fwd_flags
3. SEC
4. SEC

   1 // SPDX-License-Identifier: GPL-2.0
   2 /* Copyright (c) 2017-18 David Ahern <dsahern@gmail.com>
   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 is distributed in the hope that it will be useful, but
   9  * WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  11  * General Public License for more details.
  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/ipv6.h>
  21 
  22 #include "bpf_helpers.h"
  23 
  24 #define IPV6_FLOWINFO_MASK              cpu_to_be32(0x0FFFFFFF)
  25 
  26 struct {
  27         __uint(type, BPF_MAP_TYPE_DEVMAP);
  28         __uint(key_size, sizeof(int));
  29         __uint(value_size, sizeof(int));
  30         __uint(max_entries, 64);
  31 } xdp_tx_ports SEC(".maps");
  32 
  33 /* from include/net/ip.h */
  34 static __always_inline int ip_decrease_ttl(struct iphdr *iph)
  35 {
  36         u32 check = (__force u32)iph->check;
  37 
  38         check += (__force u32)htons(0x0100);
  39         iph->check = (__force __sum16)(check + (check >= 0xFFFF));
  40         return --iph->ttl;
  41 }
  42 
  43 static __always_inline int xdp_fwd_flags(struct xdp_md *ctx, u32 flags)
  44 {
  45         void *data_end = (void *)(long)ctx->data_end;
  46         void *data = (void *)(long)ctx->data;
  47         struct bpf_fib_lookup fib_params;
  48         struct ethhdr *eth = data;
  49         struct ipv6hdr *ip6h;
  50         struct iphdr *iph;
  51         u16 h_proto;
  52         u64 nh_off;
  53         int rc;
  54 
  55         nh_off = sizeof(*eth);
  56         if (data + nh_off > data_end)
  57                 return XDP_DROP;
  58 
  59         __builtin_memset(&fib_params, 0, sizeof(fib_params));
  60 
  61         h_proto = eth->h_proto;
  62         if (h_proto == htons(ETH_P_IP)) {
  63                 iph = data + nh_off;
  64 
  65                 if (iph + 1 > data_end)
  66                         return XDP_DROP;
  67 
  68                 if (iph->ttl <= 1)
  69                         return XDP_PASS;
  70 
  71                 fib_params.family       = AF_INET;
  72                 fib_params.tos          = iph->tos;
  73                 fib_params.l4_protocol  = iph->protocol;
  74                 fib_params.sport        = 0;
  75                 fib_params.dport        = 0;
  76                 fib_params.tot_len      = ntohs(iph->tot_len);
  77                 fib_params.ipv4_src     = iph->saddr;
  78                 fib_params.ipv4_dst     = iph->daddr;
  79         } else if (h_proto == htons(ETH_P_IPV6)) {
  80                 struct in6_addr *src = (struct in6_addr *) fib_params.ipv6_src;
  81                 struct in6_addr *dst = (struct in6_addr *) fib_params.ipv6_dst;
  82 
  83                 ip6h = data + nh_off;
  84                 if (ip6h + 1 > data_end)
  85                         return XDP_DROP;
  86 
  87                 if (ip6h->hop_limit <= 1)
  88                         return XDP_PASS;
  89 
  90                 fib_params.family       = AF_INET6;
  91                 fib_params.flowinfo     = *(__be32 *)ip6h & IPV6_FLOWINFO_MASK;
  92                 fib_params.l4_protocol  = ip6h->nexthdr;
  93                 fib_params.sport        = 0;
  94                 fib_params.dport        = 0;
  95                 fib_params.tot_len      = ntohs(ip6h->payload_len);
  96                 *src                    = ip6h->saddr;
  97                 *dst                    = ip6h->daddr;
  98         } else {
  99                 return XDP_PASS;
 100         }
 101 
 102         fib_params.ifindex = ctx->ingress_ifindex;
 103 
 104         rc = bpf_fib_lookup(ctx, &fib_params, sizeof(fib_params), flags);
 105         /*
 106          * Some rc (return codes) from bpf_fib_lookup() are important,
 107          * to understand how this XDP-prog interacts with network stack.
 108          *
 109          * BPF_FIB_LKUP_RET_NO_NEIGH:
 110          *  Even if route lookup was a success, then the MAC-addresses are also
 111          *  needed.  This is obtained from arp/neighbour table, but if table is
 112          *  (still) empty then BPF_FIB_LKUP_RET_NO_NEIGH is returned.  To avoid
 113          *  doing ARP lookup directly from XDP, then send packet to normal
 114          *  network stack via XDP_PASS and expect it will do ARP resolution.
 115          *
 116          * BPF_FIB_LKUP_RET_FWD_DISABLED:
 117          *  The bpf_fib_lookup respect sysctl net.ipv{4,6}.conf.all.forwarding
 118          *  setting, and will return BPF_FIB_LKUP_RET_FWD_DISABLED if not
 119          *  enabled this on ingress device.
 120          */
 121         if (rc == BPF_FIB_LKUP_RET_SUCCESS) {
 122                 /* Verify egress index has been configured as TX-port.
 123                  * (Note: User can still have inserted an egress ifindex that
 124                  * doesn't support XDP xmit, which will result in packet drops).
 125                  *
 126                  * Note: lookup in devmap supported since 0cdbb4b09a0.
 127                  * If not supported will fail with:
 128                  *  cannot pass map_type 14 into func bpf_map_lookup_elem#1:
 129                  */
 130                 if (!bpf_map_lookup_elem(&xdp_tx_ports, &fib_params.ifindex))
 131                         return XDP_PASS;
 132 
 133                 if (h_proto == htons(ETH_P_IP))
 134                         ip_decrease_ttl(iph);
 135                 else if (h_proto == htons(ETH_P_IPV6))
 136                         ip6h->hop_limit--;
 137 
 138                 memcpy(eth->h_dest, fib_params.dmac, ETH_ALEN);
 139                 memcpy(eth->h_source, fib_params.smac, ETH_ALEN);
 140                 return bpf_redirect_map(&xdp_tx_ports, fib_params.ifindex, 0);
 141         }
 142 
 143         return XDP_PASS;
 144 }
 145 
 146 SEC("xdp_fwd")
 147 int xdp_fwd_prog(struct xdp_md *ctx)
 148 {
 149         return xdp_fwd_flags(ctx, 0);
 150 }
 151 
 152 SEC("xdp_fwd_direct")
 153 int xdp_fwd_direct_prog(struct xdp_md *ctx)
 154 {
 155         return xdp_fwd_flags(ctx, BPF_FIB_LOOKUP_DIRECT);
 156 }
 157 
 158 char _license[] SEC("license") = "GPL";