linux-kernel - Re: [PATCH mptcp] mptcp: sockopt: make sync_socket_options propagate SOCK

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Message-ID: <ab6ff5d8-2ef1-44de-b6db-8174795028a1@kernel.org>
Date: Sat, 6 Sep 2025 15:26:20 +0200
From: Matthieu Baerts <matttbe@...nel.org>
To: Krister Johansen <kjlx@...pleofstupid.com>,
 Mat Martineau <martineau@...nel.org>
Cc: 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>, Florian Westphal <fw@...len.de>,
 netdev@...r.kernel.org, mptcp@...ts.linux.dev, linux-kernel@...r.kernel.org,
 David Reaver <me@...idreaver.com>
Subject: Re: [PATCH mptcp] mptcp: sockopt: make sync_socket_options propagate
 SOCK_KEEPOPEN

Hi Krister,

On 06/09/2025 02:43, Krister Johansen wrote:
> Users reported a scenario where MPTCP connections that were configured
> with SO_KEEPALIVE prior to connect would fail to enable their keepalives
> if MTPCP fell back to TCP mode.
> 
> After investigating, this affects keepalives for any connection where
> sync_socket_options is called on a socket that is in the closed or
> listening state.  Joins are handled properly. For connects,
> sync_socket_options is called when the socket is still in the closed
> state.  The tcp_set_keepalive() function does not act on sockets that
> are closed or listening, hence keepalive is not immediately enabled.
> Since the SO_KEEPOPEN flag is absent, it is not enabled later in the
> connect sequence via tcp_finish_connect.  Setting the keepalive via
> sockopt after connect does work, but would not address any subsequently
> created flows.
> 
> Fortunately, the fix here is straight-forward: set SOCK_KEEPOPEN on the
> subflow when calling sync_socket_options.
> 
> The fix was valdidated both by using tcpdump to observe keeplaive
> packets not being sent before the fix, and being sent after the fix.  It
> was also possible to observe via ss that the keepalive timer was not
> enabled on these sockets before the fix, but was enabled afterwards.

Thank you for the fix! Indeed, the SOCK_KEEPOPEN flag was missing! This
patch looks good to me as well:

Reviewed-by: Matthieu Baerts (NGI0) <matttbe@...nel.org>

@Netdev Maintainers: please apply this patch in 'net' directly. But I
can always re-send it later if preferred.

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