Re: [PATCH net] mptcp: return sk_wait_data() errors from recvmsg()

[Matthieu Baerts <matttbe@kernel.org>]

Hi Mark,

On 13/09/2026 12:30, Mark Amirkan via B4 Relay wrote:
> From: Mark Amirkan <markdamirkan@gmail.com>
> 
> Commit 581302298524 ("mptcp: error out earlier on disconnect") made
> mptcp_recvmsg() stop when sk_wait_data() returns an error.  The error is
> stored in err, but the function then jumps to a path which returns
> copied.  When no data was copied, recvmsg() therefore returns zero and
> reports a false EOF.
> 
> Store the result in copied, which is the value returned by the function.
> This also keeps the usual partial-read result when data was copied before
> the error.

Good catch! Indeed, it looks like it is a typo and "copied" was supposed
to be assigned.

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

@Net maintainers: can you please apply it directly in the net tree? (if
no issues are spot by Clashiko) No need to take a detour via the MPTCP
tree for this fix.

> A recvmsg() blocked in one thread reproduces the issue when another
> thread disconnects the same MPTCP socket with connect(AF_UNSPEC).
> Before this change recvmsg() returns zero; afterwards it returns -EPIPE.

I see that our test suite currently doesn't exercise this error path. By
chance, do you have a reproducer to avoid similar regressions later on?
Ideally, one using Packetdrill, the MPTCP fork in this case [1].

[1] https://github.com/multipath-tcp/packetdrill

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