mptcp: pm: in-kernel: C-flag: handle late ADD_ADDR

The special C-flag case expects the ADD_ADDR to be received when
switching to 'fully-established'. But for various reasons, the ADD_ADDR
could be sent after the "4th ACK", and the special case doesn't work.

On NIPA, the new test validating this special case for the C-flag failed
a few times, e.g.

  102 default limits, server deny join id 0
        syn rx                 [FAIL] got 0 JOIN[s] syn rx expected 2

  Server ns stats
  (...)
  MPTcpExtAddAddrTx  1
  MPTcpExtEchoAdd    1

  Client ns stats
  (...)
  MPTcpExtAddAddr    1
  MPTcpExtEchoAddTx  1

        synack rx              [FAIL] got 0 JOIN[s] synack rx expected 2
        ack rx                 [FAIL] got 0 JOIN[s] ack rx expected 2
        join Rx                [FAIL] see above
        syn tx                 [FAIL] got 0 JOIN[s] syn tx expected 2
        join Tx                [FAIL] see above

I had a suspicion about what the issue could be: the ADD_ADDR might have
been received after the switch to the 'fully-established' state. The
issue was not easy to reproduce. The packet capture shown that the
ADD_ADDR can indeed be sent with a delay, and the client would not try
to establish subflows to it as expected.

A simple fix is not to mark the endpoints as 'used' in the C-flag case,
when looking at creating subflows to the remote initial IP address and
port. In this case, there is no need to try.

Note: newly added fullmesh endpoints will still continue to be used as
expected, thanks to the conditions behind mptcp_pm_add_addr_c_flag_case.

Fixes: 4b1ff85 ("mptcp: pm: in-kernel: usable client side with C-flag")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251020-net-mptcp-c-flag-late-add-addr-v1-1-8207030cb0e8@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: matttbe

net/mptcp/pm_kernel.c

@@ -370,6 +370,10 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 	}
 
 subflow:
+	/* No need to try establishing subflows to remote id0 if not allowed */
+	if (mptcp_pm_add_addr_c_flag_case(msk))
+		goto exit;
+
 	/* check if should create a new subflow */
 	while (msk->pm.local_addr_used < endp_subflow_max &&
 	       msk->pm.extra_subflows < limit_extra_subflows) {
@@ -401,6 +405,8 @@ static void mptcp_pm_create_subflow_or_signal_addr(struct mptcp_sock *msk)
 			__mptcp_subflow_connect(sk, &local, &addrs[i]);
 		spin_lock_bh(&msk->pm.lock);
 	}
+
+exit:
 	mptcp_pm_nl_check_work_pending(msk);
 }