Re: [PATCH bpf v4 1/2] bpf: tcp: Reject non-TCP skb in bpf_sk_assign_tcp_reqsk()

Kuniyuki Iwashima
2026-03-31T01:36:07Z

On Mon, Mar 30, 2026 at 2:26 AM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>
>
> On 3/30/26 5:00 PM, bot+bpf-ci@kernel.org wrote:
> >> #if IS_BUILTIN(CONFIG_IPV6)
> >> case htons(ETH_P_IPV6):
> >> + if (ipv6_hdr(skb)->nexthdr != IPPROTO_TCP)
> >> + return -EINVAL;
> >> +
> >> ops = &tcp6_request_sock_ops;
> > For IPv6, ipv6_hdr(skb)->nexthdr gives the type of the header
> > immediately following the base IPv6 header. If extension headers
> > are present (hop-by-hop options, routing, etc.), nexthdr would be
> > the extension header type rather than IPPROTO_TCP, even when the
> > packet is a valid TCP segment.
> >
> > Would it be worth using ipv6_find_hdr() here, similar to
> > bpf_update_srh_state() in the same file, to walk past any extension
> > headers? The IPv4 check above is fine since ip_hdr(skb)->protocol
> > always identifies the transport protocol directly.
> >
> > In practice this is very unlikely to matter since TCP SYN packets
> > with IPv6 extension headers are essentially non-existent, but the
> > check as written would reject them.
> >
> >
> > ---
> > 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/23735111188
>
>
> Thanks for the analysis.
>
> There are many places in the kernel that check nexthdr directly without
> walking extension headers.
>
> I'd prefer to keep it simple for now and leave ipv6_find_hdr() as a
> potential future improvement if needed.

+1.

Given this feature is to create a reqsk to process on this running
host, it does not make sense to support such ext options.