linux-kernel - Re: [PATCH net v3 1/3] net,mptcp: fix proto fallback detection with BPF sockmap

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Message-ID: <7dfda5bb-665c-4068-acd4-795972da63e8@redhat.com>
Date: Tue, 28 Oct 2025 12:47:26 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>, mptcp@...ts.linux.dev
Cc: stable@...r.kernel.org, Jakub Sitnicki <jakub@...udflare.com>,
 John Fastabend <john.fastabend@...il.com>, Eric Dumazet
 <edumazet@...gle.com>, Kuniyuki Iwashima <kuniyu@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>, "David S. Miller"
 <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>,
 Simon Horman <horms@...nel.org>, Matthieu Baerts <matttbe@...nel.org>,
 Mat Martineau <martineau@...nel.org>, Geliang Tang <geliang@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Shuah Khan <shuah@...nel.org>,
 Florian Westphal <fw@...len.de>, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net v3 1/3] net,mptcp: fix proto fallback detection with
 BPF sockmap

On 10/28/25 12:30 PM, Paolo Abeni wrote:
> On 10/23/25 2:54 PM, Jiayuan Chen wrote:
>> When the server has MPTCP enabled but receives a non-MP-capable request
>> from a client, it calls mptcp_fallback_tcp_ops().
>>
>> Since non-MPTCP connections are allowed to use sockmap, which replaces
>> sk->sk_prot, using sk->sk_prot to determine the IP version in
>> mptcp_fallback_tcp_ops() becomes unreliable. This can lead to assigning
>> incorrect ops to sk->sk_socket->ops.
> 
> I don't see how sockmap could modify the to-be-accepted socket sk_prot
> before mptcp_fallback_tcp_ops(), as such call happens before the fd is
> installed, and AFAICS sockmap can only fetch sockets via fds.
> 
> Is this patch needed?

Matttbe explained off-list the details of how that could happen. I think
the commit message here must be more verbose to explain clearly the
whys, even to those non proficient in sockmap like me.

Thanks,

Paolo