Some subflow socket errors need to be reported to the MPTCP socket: the
initial subflow connect (MP_CAPABLE), and the ones from the fallback
sockets. The others are not propagated.

The issue is that sock_error() was used to retrieve the error, which was
also resetting the sk_err field. Because of that, when notifying the
userspace about subflow close events later on from the MPTCP worker, the
ssk->sk_err field was always 0.

Now, the error (sk_err) is only reset when propagating it to the msk.

Fixes: 15cc10453398 ("mptcp: deliver ssk errors to msk")
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Note: I guess we could also duplicate the error in mptcp_subflow_context
  struct before scheduling the worker, and use this field in
  mptcp_event_put_token_and_ssk(), but I don't see why we should always
  reset ssk->sk_err in __mptcp_subflow_error_report() in all cases.
v2: keep 'err' variable (Geliang)
---
 net/mptcp/protocol.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 335a1723c1dc..1b87b3b0d089 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -817,11 +817,8 @@ static bool __mptcp_ofo_queue(struct mptcp_sock *msk)
 
 static bool __mptcp_subflow_error_report(struct sock *sk, struct sock *ssk)
 {
-	int err = sock_error(ssk);
 	int ssk_state;
-
-	if (!err)
-		return false;
+	int err;
 
 	/* only propagate errors on fallen-back sockets or
 	 * on MPC connect
@@ -829,6 +826,10 @@ static bool __mptcp_subflow_error_report(struct sock *sk, struct sock *ssk)
 	if (sk->sk_state != TCP_SYN_SENT && !__mptcp_check_fallback(mptcp_sk(sk)))
 		return false;
 
+	err = sock_error(ssk);
+	if (!err)
+		return false;
+
 	/* We need to propagate only transition to CLOSE state.
 	 * Orphaned socket will see such state change via
 	 * subflow_sched_work_if_closed() and that path will properly

-- 
2.51.0