[PATCH net 1/5] mptcp: pm: in-kernel: C-flag: handle late ADD_ADDR

Matthieu Baerts (NGI0)
2025-10-20T20:53:51Z

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: 4b1ff850e0c1 ("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>
---
net/mptcp/pm_kernel.c | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index e0f44dc232aa..2ae95476dba3 100644
--- a/net/mptcp/pm_kernel.c
+++ b/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);
}

--
2.51.0