netdev - Re: [PATCH net v4 0/3] mptcp: Fix conflicts between MPTCP and sockmap

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Message-ID: <cc0592aa-a360-437b-bbd1-9a1dda14c132@kernel.org>
Date: Wed, 5 Nov 2025 15:37:01 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Jiayuan Chen <jiayuan.chen@...ux.dev>, mptcp@...ts.linux.dev
Cc: Mat Martineau <martineau@...nel.org>, Geliang Tang <geliang@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, 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 v4 0/3] mptcp: Fix conflicts between MPTCP and sockmap

Hi Jiayuan,

On 05/11/2025 12:36, Jiayuan Chen wrote:
> Overall, we encountered a warning [1] that can be triggered by running the
> selftest I provided.

Thank you for the v4!

> sockmap works by replacing sk_data_ready, recvmsg, sendmsg operations and
> implementing fast socket-level forwarding logic:
> 1. Users can obtain file descriptors through userspace socket()/accept()
>    interfaces, then call BPF syscall to perform these replacements.
> 2. Users can also use the bpf_sock_hash_update helper (in sockops programs)
>    to replace handlers when TCP connections enter ESTABLISHED state
>   (BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB/BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB)
> 
> However, when combined with MPTCP, an issue arises: MPTCP creates subflow
> sk's and performs TCP handshakes, so the BPF program obtains subflow sk's
> and may incorrectly replace their sk_prot. We need to reject such
> operations. In patch 1, we set psock_update_sk_prot to NULL in the
> subflow's custom sk_prot.

This new version looks good to me. I have some small comments on patches
1 and 2 that can only be addressed if a v5 is needed I think.

I have some questions for the 3rd patch. It would be good if someone
else with more experience with the BPF selftests can also look at it.

> Additionally, if the server's listening socket has MPTCP enabled and the
> client's TCP also uses MPTCP, we should allow the combination of subflow
> and sockmap. This is because the latest Golang programs have enabled MPTCP
> for listening sockets by default [2]. For programs already using sockmap,
> upgrading Golang should not cause sockmap functionality to fail.

Note: even if these patches here are needed to avoid stream corruption
and other issues, in your specific case with sockmap, I think it would
be better to set this env var until MPTCP support is added to sockmap:

  GODEBUG=multipathtcp=0

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.