linux-kernel - Re: [PATCH net v3 2/3] bpf,sockmap: disallow MPTCP sockets from sockmap

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Message-ID: <c5021188-593c-431c-bf01-6775f5b2b2ed@redhat.com>
Date: Tue, 28 Oct 2025 13:03:58 +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 2/3] bpf,sockmap: disallow MPTCP sockets from
 sockmap

On 10/23/25 2:54 PM, Jiayuan Chen wrote:
> MPTCP creates subflows for data transmission, and these sockets should not
> be added to sockmap because MPTCP sets specialized data_ready handlers
> that would be overridden by sockmap.
> 
> Additionally, for the parent socket of MPTCP subflows (plain TCP socket),
> MPTCP sk requires specific protocol handling that conflicts with sockmap's
> operation(mptcp_prot).
> 
> This patch adds proper checks to reject MPTCP subflows and their parent
> sockets from being added to sockmap, while preserving compatibility with
> reuseport functionality for listening MPTCP sockets.

It's unclear to me why that is safe. sockmap is going to change the
listener msk proto ops.

The listener could disconnect and create an egress connection, still
using the wrong ops.

I think sockmap should always be prevented for mptcp socket, or at least
a solid explanation of why such exception is safe should be included in
the commit message.

Note that the first option allows for solving the issue entirely in the
mptcp code, setting dummy/noop psock_update_sk_prot for mptcp sockets
and mptcp subflows.

/P