[PATCH export v3 4/4] mptcp: fallback to TCP on MP_FAIL with a single subflow

[Chenguang Zhao <chenguang.zhao@linux.dev>]

From: Chenguang Zhao <zhaochenguang@kylinos.cn>

Fall back immediately via mptcp_try_fallback() after accepting MP_FAIL
on a single contiguous subflow, as required by RFC8684 §3.7.

Fixes: 1e39e5a32ad7 ("mptcp: infinite mapping sending")
Signed-off-by: Chenguang Zhao <zhaochenguang@kylinos.cn>
---
 net/mptcp/pm.c       | 9 ++++++---
 net/mptcp/protocol.c | 4 +++-
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/mptcp/pm.c b/net/mptcp/pm.c
index 8c263084db7b..cb85caf1df43 100644
--- a/net/mptcp/pm.c
+++ b/net/mptcp/pm.c
@@ -876,7 +876,6 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
 
 	pr_debug("fail_seq=%llu\n", fail_seq);
 
-	/* After accepting the fail, we can't create any other subflows */
 	spin_lock_bh(&msk->fallback_lock);
 	if (!msk->allow_infinite_fallback) {
 		spin_unlock_bh(&msk->fallback_lock);
@@ -891,8 +890,6 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
 		mptcp_subflow_reset(sk);
 		return;
 	}
-
-	msk->allow_subflows = false;
 	spin_unlock_bh(&msk->fallback_lock);
 
 	if (!subflow->fail_tout) {
@@ -901,6 +898,12 @@ void mptcp_pm_mp_fail_received(struct sock *sk, u64 fail_seq)
 		subflow->send_mp_fail = 1;
 		subflow->send_infinite_map = 1;
 		tcp_send_ack(sk);
+
+		/* RFC8684 §3.7: fallback with a single subflow */
+		if (!mptcp_try_fallback(sk, MPTCP_MIB_MPFAILFALLBACK)) {
+			MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_FALLBACKFAILED);
+			mptcp_subflow_reset(sk);
+		}
 	} else {
 		pr_debug("MP_FAIL response received\n");
 		WRITE_ONCE(subflow->fail_tout, 0);
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index f879b1061f2d..519e8d9c3164 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1418,7 +1418,9 @@ static void mptcp_update_infinite_map(struct mptcp_sock *msk,
 	mpext->infinite_map = 1;
 	mpext->data_len = 0;
 
-	if (!mptcp_try_fallback(ssk, MPTCP_MIB_INFINITEMAPTX)) {
+	if (__mptcp_check_fallback(msk)) {
+		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_INFINITEMAPTX);
+	} else if (!mptcp_try_fallback(ssk, MPTCP_MIB_INFINITEMAPTX)) {
 		MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_FALLBACKFAILED);
 		mptcp_subflow_reset(ssk);
 		return;
-- 
2.25.1