MPTCP currently generate a dummy data_fin for fallback socket
when the fallback subflow has completed data reception using
the current ack_seq.

We are going to introduce backlog usage for the msk soon, even
for fallback sockets: the ack_seq value will not match the most recent
sequence number seen by the fallback subflow socket, as it will ignore
data_seq sitting in the backlog.

Instead use the last map sequence number to set the data_fin,
as fallback (dummy) map sequences are always in sequence.

Reviewed-by: Geliang Tang <geliang@kernel.org>
Tested-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
---
v2 -> v3:
  - keep the close check in subflow_sched_work_if_closed, fix
    CI failures
---
 net/mptcp/subflow.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index e8325890a3223..b9455c04e8a46 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1285,6 +1285,7 @@ static bool subflow_is_done(const struct sock *sk)
 /* sched mptcp worker for subflow cleanup if no more data is pending */
 static void subflow_sched_work_if_closed(struct mptcp_sock *msk, struct sock *ssk)
 {
+	const struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct sock *sk = (struct sock *)msk;
 
 	if (likely(ssk->sk_state != TCP_CLOSE &&
@@ -1303,7 +1304,8 @@ static void subflow_sched_work_if_closed(struct mptcp_sock *msk, struct sock *ss
 	 */
 	if (__mptcp_check_fallback(msk) && subflow_is_done(ssk) &&
 	    msk->first == ssk &&
-	    mptcp_update_rcv_data_fin(msk, READ_ONCE(msk->ack_seq), true))
+	    mptcp_update_rcv_data_fin(msk, subflow->map_seq +
+				      subflow->map_data_len, true))
 		mptcp_schedule_work(sk);
 }
 
-- 
2.51.0