Merge branch 'mptcp-blackhole-only-if-1st-syn-retrans-w-o-mpc-is-accepted'

Matthieu Baerts says:

====================
mptcp: blackhole only if 1st SYN retrans w/o MPC is accepted

Here are two small fixes for issues introduced in v6.12.

- Patch 1: reset the mpc_drop mark for other SYN retransmits, to only
  consider an MPTCP blackhole when the first SYN retransmitted without
  the MPTCP options is accepted, as initially intended.

- Patch 2: also mention in the doc that the blackhole_timeout sysctl
  knob is per-netns, like all the others.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
====================

Link: https://patch.msgid.link/20250129-net-mptcp-blackhole-fix-v1-0-afe88e5a6d2c@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/mptcp-sysctl.rst

@@ -41,7 +41,7 @@ blackhole_timeout - INTEGER (seconds)
 	MPTCP is re-enabled and will reset to the initial value when the
 	blackhole issue goes away.
 
-	0 to disable the blackhole detection.
+	0 to disable the blackhole detection. This is a per-namespace sysctl.
 
 	Default: 3600
 

net/mptcp/ctrl.c

@@ -418,9 +418,9 @@ void mptcp_active_detect_blackhole(struct sock *ssk, bool expired)
 			MPTCP_INC_STATS(net, MPTCP_MIB_MPCAPABLEACTIVEDROP);
 			subflow->mpc_drop = 1;
 			mptcp_subflow_early_fallback(mptcp_sk(subflow->conn), subflow);
-		} else {
-			subflow->mpc_drop = 0;
 		}
+	} else if (ssk->sk_state == TCP_SYN_SENT) {
+		subflow->mpc_drop = 0;
 	}
 }