mptcp: pm: ADD_ADDR rtx: skip inactive subflows

When looking at the maximum RTO amongst the subflows, inactive subflows
were taken into account: that includes stale ones, and the initial one
if it has been already been closed.

Unusable subflows are now simply skipped. Stale ones are used as an
alternative: if there are only stale ones, to take their maximum RTO and
avoid to eventually fallback to net.mptcp.add_addr_timeout, which is set
to 2 minutes by default.

Fixes: 30549ee ("mptcp: make ADD_ADDR retransmission timeout adaptive")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260505-net-mptcp-pm-fixes-7-1-rc3-v1-7-fca8091060a4@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

net/mptcp/pm.c

@@ -306,18 +306,28 @@ static unsigned int mptcp_adjust_add_addr_timeout(struct mptcp_sock *msk)
 	const struct net *net = sock_net((struct sock *)msk);
 	unsigned int rto = mptcp_get_add_addr_timeout(net);
 	struct mptcp_subflow_context *subflow;
-	unsigned int max = 0;
+	unsigned int max = 0, max_stale = 0;
 
 	mptcp_for_each_subflow(msk, subflow) {
 		struct sock *ssk = mptcp_subflow_tcp_sock(subflow);
 		struct inet_connection_sock *icsk = inet_csk(ssk);
 
-		if (icsk->icsk_rto > max)
+		if (!__mptcp_subflow_active(subflow))
+			continue;
+
+		if (unlikely(subflow->stale)) {
+			if (icsk->icsk_rto > max_stale)
+				max_stale = icsk->icsk_rto;
+		} else if (icsk->icsk_rto > max) {
 			max = icsk->icsk_rto;
+		}
 	}
 
-	if (max && max < rto)
-		rto = max;
+	if (max)
+		return min(max, rto);
+
+	if (max_stale)
+		return min(max_stale, rto);
 
 	return rto;
 }