FAILED: patch "[PATCH] mptcp: propagate shutdown to subflows when possible" failed to apply to 5.15-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x f755be0b1ff429a2ecf709beeb1bcd7abc111c2b
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2025092143-detonator-snowcap-57d7@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f755be0b1ff429a2ecf709beeb1bcd7abc111c2b Mon Sep 17 00:00:00 2001
From: "Matthieu Baerts (NGI0)" <matttbe@kernel.org>
Date: Fri, 12 Sep 2025 14:25:50 +0200
Subject: [PATCH] mptcp: propagate shutdown to subflows when possible

When the MPTCP DATA FIN have been ACKed, there is no more MPTCP related
metadata to exchange, and all subflows can be safely shutdown.

Before this patch, the subflows were actually terminated at 'close()'
time. That's certainly fine most of the time, but not when the userspace
'shutdown()' a connection, without close()ing it. When doing so, the
subflows were staying in LAST_ACK state on one side -- and consequently
in FIN_WAIT2 on the other side -- until the 'close()' of the MPTCP
socket.

Now, when the DATA FIN have been ACKed, all subflows are shutdown. A
consequence of this is that the TCP 'FIN' flag can be set earlier now,
but the end result is the same. This affects the packetdrill tests
looking at the end of the MPTCP connections, but for a good reason.

Note that tcp_shutdown() will check the subflow state, so no need to do
that again before calling it.

Fixes: 3721b9b64676 ("mptcp: Track received DATA_FIN sequence number and add related helpers")
Cc: stable@vger.kernel.org
Fixes: 16a9a9da1723 ("mptcp: Add helper to process acks of DATA_FIN")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-fix-sft-connect-v1-1-d40e77cbbf02@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index e6fd97b21e9e..5e497a83e967 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -371,6 +371,20 @@ static void mptcp_close_wake_up(struct sock *sk)
 		sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
 }
 
+static void mptcp_shutdown_subflows(struct mptcp_sock *msk)
+{
+	struct mptcp_subflow_context *subflow;
+
+	mptcp_for_each_subflow(msk, subflow) {
+		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
+		bool slow;
+
+		slow = lock_sock_fast(ssk);
+		tcp_shutdown(ssk, SEND_SHUTDOWN);
+		unlock_sock_fast(ssk, slow);
+	}
+}
+
 /* called under the msk socket lock */
 static bool mptcp_pending_data_fin_ack(struct sock *sk)
 {
@@ -395,6 +409,7 @@ static void mptcp_check_data_fin_ack(struct sock *sk)
 			break;
 		case TCP_CLOSING:
 		case TCP_LAST_ACK:
+			mptcp_shutdown_subflows(msk);
 			mptcp_set_state(sk, TCP_CLOSE);
 			break;
 		}
@@ -563,6 +578,7 @@ static bool mptcp_check_data_fin(struct sock *sk)
 			mptcp_set_state(sk, TCP_CLOSING);
 			break;
 		case TCP_FIN_WAIT2:
+			mptcp_shutdown_subflows(msk);
 			mptcp_set_state(sk, TCP_CLOSE);
 			break;
 		default: