netdev - Re: [PATCH bpf-next] bpf: Allow bpf_tcp

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Message-ID: <917eadc0-9341-48d5-a795-8a7d5318886c@linux.dev>
Date: Wed, 4 Feb 2026 14:27:17 -0800
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: afabre@...udflare.com
Cc: daniel@...earbox.net, john.fastabend@...il.com, sdf@...ichev.me,
 ast@...nel.org, andrii@...nel.org, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, haoluo@...gle.com,
 jolsa@...nel.org, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
 pabeni@...hat.com, horms@...nel.org, bpf@...r.kernel.org,
 netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
 kernel-team@...udflare.com, wferguson@...udflare.com, martin.lau@...nel.org,
 clm@...a.com, ihor.solodrai@...ux.dev, bot+bpf-ci@...nel.org
Subject: Re: [PATCH bpf-next] bpf: Allow bpf_tcp_sock() helper in XDP



On 2/4/26 5:04 AM, bot+bpf-ci@...nel.org wrote:
>>      But the TCP TIME-WAIT state needs special handling for this: packets
>>      received for a TIME-WAIT socket can either belong to the current
>>      incarnation of the connection, or a new incarnation. The only way to
>>      determine this is to compare the sequence numbers.
>>
>>      Add the bpf_tcp_sock() helper in XDP to allow the socket sequence
>>      numbers to be read.
> 
> Can this approach work for TIME-WAIT sockets? Looking at bpf_tcp_sock() in
> net/core/filter.c:
> 
>      if (sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP)
>          return (unsigned long)sk;
>      return (unsigned long)NULL;
> 
> And sk_fullsock() in include/net/sock.h:
> 
>      static inline bool sk_fullsock(const struct sock *sk)
>      {
>          return (1 << sk->sk_state) & ~(TCPF_TIME_WAIT | TCPF_NEW_SYN_RECV);
>      }
> 
> TIME-WAIT sockets are explicitly excluded by sk_fullsock(), so bpf_tcp_sock()
> returns NULL for them. The sequence numbers stored in TIME-WAIT sockets
> (skc_tw_rcv_nxt and skc_tw_snd_nxt in sock_common) would not be accessible
> through this helper.
> 
> The change itself is useful for accessing tcp_sock fields from established
> connections in XDP, but the commit message implies this solves the TIME-WAIT
> sequence number problem described, which doesn't appear to be the case.

or use "tp = bpf_core_cast(sk, struct tcp_sock /* or tcp_timewait_sock 
*/);". The bpf prog can also read all members of the kernel struct 
instead of a subset defined in bpf_tcp_sock.

pw-bot: cr

> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21671977729
> 
> AI-authorship-score: low
> AI-authorship-explanation: The commit message style is consistent with human-written kernel patches, with natural technical language and standard formatting; the technical error is more likely human oversight than AI hallucination.
> issues-found: 1
> issue-severity-score: low
> issue-severity-explanation: The issue is a misleading commit message that claims the change enables TIME-WAIT socket sequence number reading, but the bpf_tcp_sock() helper explicitly returns NULL for TIME-WAIT sockets.