mss-bpf.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /*
   3  * Copyright (C) 2021 Felix Fietkau <nbd@nbd.name>
   4  */
   5 #define KBUILD_MODNAME "foo"
   6 #include <uapi/linux/bpf.h>
   7 #include <uapi/linux/if_ether.h>
   8 #include <uapi/linux/if_packet.h>
   9 #include <uapi/linux/ip.h>
  10 #include <uapi/linux/ipv6.h>
  11 #include <uapi/linux/in.h>
  12 #include <uapi/linux/tcp.h>
  13 #include <uapi/linux/filter.h>
  14 #include <uapi/linux/pkt_cls.h>
  15 #include <linux/ip.h>
  16 #include <net/ipv6.h>
  17 #include <net/tcp.h>
  18 #include <bpf/bpf_helpers.h>
  19 #include <bpf/bpf_endian.h>
  20 #include "bpf_skb_utils.h"
  21
  22 const volatile static uint32_t mtu = 1420;
  23
  24 static __always_inline unsigned int
  25 optlen(const u_int8_t *opt)
  26 {
  27         if (opt[0] <= TCPOPT_NOP || opt[1] == 0)
  28                 return 1;
  29
  30         return opt[1];
  31 }
  32
  33 static __always_inline void
  34 fixup_tcp(struct skb_parser_info *info, __u16 mss)
  35 {
  36         struct tcphdr *tcph;
  37         __u32 end, offset = info->offset + sizeof(*tcph);
  38         __u16 oldmss;
  39         __u8 flags;
  40         __u8 *opt;
  41         int i;
  42
  43         tcph = skb_parse_tcp(info);
  44         if (!tcph)
  45                 return;
  46
  47         flags = tcp_flag_byte(tcph);
  48         if (!(flags & TCPHDR_SYN))
  49                 return;
  50
  51         end = info->offset;
  52
  53 #pragma unroll
  54         for (i = 0; i < 5; i++) {
  55                 if (offset + 4 > end)
  56                         return;
  57
  58                 opt = skb_ptr(info->skb, offset, 4);
  59                 if (!opt)
  60                         return;
  61
  62                 offset += optlen(opt);
  63                 if (opt[0] != TCPOPT_MSS || opt[1] != TCPOLEN_MSS)
  64                         continue;
  65
  66                 goto found;
  67         }
  68         return;
  69
  70 found:
  71         oldmss = (opt[2] << 8) | opt[3];
  72         if (oldmss <= mss)
  73                 return;
  74
  75         opt[2] = mss >> 8;
  76         opt[3] = mss & 0xff;
  77         csum_replace2(&tcph->check, bpf_htons(oldmss), bpf_htons(mss));
  78 }
  79
  80 SEC("tc")
  81 int mssfix(struct __sk_buff *skb)
  82 {
  83         struct skb_parser_info info;
  84         __u16 mss;
  85         int type;
  86
  87         skb_parse_init(&info, skb);
  88         if (!skb_parse_ethernet(&info))
  89                 return TC_ACT_UNSPEC;
  90
  91         skb_parse_vlan(&info);
  92         skb_parse_vlan(&info);
  93
  94         if (!skb_parse_ipv4(&info, 60) && !skb_parse_ipv6(&info, 60))
  95                 return TC_ACT_UNSPEC;
  96
  97         if (info.proto != IPPROTO_TCP)
  98                 return TC_ACT_UNSPEC;
  99
 100         mss = mtu;
 101         mss -= info.offset + sizeof(struct tcphdr);
 102         fixup_tcp(&info, mss);
 103
 104         return TC_ACT_UNSPEC;
 105 }