[PATCH 6.1.y] mptcp: fastclose msk when linger time is 0

[Sasha Levin <sashal@kernel.org>]

From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>

[ Upstream commit f14d6e9c3678a067f304abba561e0c5446c7e845 ]

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]
Cc: stable@vger.kernel.org
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]
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260427-net-mptcp-misc-fixes-7-1-rc2-v1-3-7432b7f279fa@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ kept `mptcp_check_readable()` name and explicit `inet_sk_state_store(sk, TCP_CLOSE)` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/mptcp/protocol.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 965819ddc04c9..fec40ce165101 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3154,7 +3154,8 @@ bool __mptcp_close(struct sock *sk, long timeout)
 		goto cleanup;
 	}
 
-	if (mptcp_check_readable(msk)) {
+	if (mptcp_check_readable(msk) ||
+	    (sock_flag(sk, SOCK_LINGER) && !sk->sk_lingertime)) {
 		/* the msk has read data, do the MPTCP equivalent of TCP reset */
 		inet_sk_state_store(sk, TCP_CLOSE);
 		mptcp_do_fastclose(sk);
-- 
2.53.0