Re: [PATCH mptcp-net] mptcp: fastclose msk when linger time is 0

[Matthieu Baerts <matttbe@kernel.org>]

Hi Mat,

On 22/04/2026 17:35, Mat Martineau wrote:
> On Tue, 21 Apr 2026, Matthieu Baerts (NGI0) wrote:
> 
>> The SO_LINGER socket option has been supported for a while with MPTCP
>> sockets [1], but it didn't cause the equivalent of a TCP reset as
>> expected when enabled and its time was set to 0. This was causing some
>> behavioural differences with TCP where some connections were not
>> promptly stopped as expected.
>>
>> To fix that, an extra condition is checked at close() time before
>> sending an MP_FASTCLOSE, the MPTCP equivalent of a TCP reset.
>>
>> Note that backporting up to [1] will be difficult as more changes are
>> needed to be able to send MP_FASTCLOSE. It seems better to stop at [2],
>> which was supposed to already imitate TCP.
>>
>> Validated with MPTCP packetdrill tests [3].
>>
>> Fixes: 268b12387460 ("mptcp: setsockopt: support SO_LINGER") [1]
>> Fixes: d21f83485518 ("mptcp: use fastclose on more edge scenarios") [2]
>> Reported-by: Lance Tuller <lance@lance0.com>
>> Closes: https://github.com/lance0/xfr/pull/67
>> Link: https://github.com/multipath-tcp/packetdrill/pull/196 [3]
>> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
>> ---
>> net/mptcp/protocol.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
> 
> LGTM, thanks Matthieu!
Thank you for the review!

Now in our tree:

New patches for t/upstream-net and t/upstream:
- 0f10cdb3e082: mptcp: fastclose msk when linger time is 0
- Results: 292dd6570cfd..102349628b51 (export-net)
- Results: b3fa12c3ddab..67117a5e3e86 (export)

Tests are now in progress:

- export-net:
https://github.com/multipath-tcp/mptcp_net-next/commit/1acee4b422ad7ee72705775b633d8ae8fa75d508/checks
- export:
https://github.com/multipath-tcp/mptcp_net-next/commit/e676a2d67d616900b5d4be83471f12c5d7a1433e/checks

And the packetdrill PR has been merged.

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