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

Martin KaFai Lau
2026-03-31T03:18:51Z

On 3/30/26 6:35 PM, Kuniyuki Iwashima wrote:
> 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.

While at header reading, does it need a pskb_may_pull() before reading?