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// SPDX-License-Identifier: GPL-2.0-or-later
/*
* Copyright (C) 2021 Felix Fietkau <nbd@nbd.name>
*/
#define KBUILD_MODNAME "foo"
#include <uapi/linux/bpf.h>
#include <uapi/linux/if_ether.h>
#include <uapi/linux/if_packet.h>
#include <uapi/linux/ip.h>
#include <uapi/linux/ipv6.h>
#include <uapi/linux/in.h>
#include <uapi/linux/tcp.h>
#include <uapi/linux/filter.h>
#include <uapi/linux/pkt_cls.h>
#include <linux/ip.h>
#include <net/ipv6.h>
#include <net/tcp.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_endian.h>
#include "bpf_skb_utils.h"
const volatile static uint32_t mtu = 1420;
static __always_inline unsigned int
optlen(const u_int8_t *opt)
{
if (opt[0] <= TCPOPT_NOP || opt[1] == 0)
return 1;
return opt[1];
}
static __always_inline void
fixup_tcp(struct skb_parser_info *info, __u16 mss)
{
struct tcphdr *tcph;
__u32 end, offset = info->offset + sizeof(*tcph);
__u16 oldmss;
__u8 flags;
__u8 *opt;
int i;
tcph = skb_parse_tcp(info);
if (!tcph)
return;
flags = tcp_flag_byte(tcph);
if (!(flags & TCPHDR_SYN))
return;
end = info->offset;
#pragma unroll
for (i = 0; i < 5; i++) {
if (offset + 4 > end)
return;
opt = skb_ptr(info->skb, offset, 4);
if (!opt)
return;
offset += optlen(opt);
if (opt[0] != TCPOPT_MSS || opt[1] != TCPOLEN_MSS)
continue;
goto found;
}
return;
found:
oldmss = (opt[2] << 8) | opt[3];
if (oldmss <= mss)
return;
opt[2] = mss >> 8;
opt[3] = mss & 0xff;
csum_replace2(&tcph->check, bpf_htons(oldmss), bpf_htons(mss));
}
SEC("tc")
int mssfix(struct __sk_buff *skb)
{
struct skb_parser_info info;
__u16 mss;
int type;
skb_parse_init(&info, skb);
if (!skb_parse_ethernet(&info))
return TC_ACT_UNSPEC;
skb_parse_vlan(&info);
skb_parse_vlan(&info);
if (!skb_parse_ipv4(&info, 60) && !skb_parse_ipv6(&info, 60))
return TC_ACT_UNSPEC;
if (info.proto != IPPROTO_TCP)
return TC_ACT_UNSPEC;
mss = mtu;
mss -= info.offset + sizeof(struct tcphdr);
fixup_tcp(&info, mss);
return TC_ACT_UNSPEC;
}
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