Re: [PATCH bpf-next] bpf: Allow bpf_tcp_sock() helper in XDP

[Martin KaFai Lau]

On 2/4/26 5:04 AM, bot+bpf-ci@xxxxxxxxxx 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.