With TCP Fast Open on an MPTCP listener, if the server application
writes data while the passive subflow is still in SYN_RECV (after
consuming the client's SYN data but before the MP_CAPABLE third ACK
arrives), __mptcp_subflow_active() refuses transmission and the data
is queued into the msk write queue.

When the MPC third ACK arrives, the subflow transitions to
TCP_ESTABLISHED and is marked fully established in
check_fully_established(), but because the third ACK carries no
DSS data, the queued bytes remain stranded until the peer sends
more data.

Fix this in check_fully_established() by checking if the subflow
was doing passive TFO (subflow->is_mptfo). Under mptcp_data_lock(),
flush queued bytes via __mptcp_check_push().

Suggested-by: Petar Sakic <petar.sakic@ink.fish>
Reported-by: Petar Sakic <petar.sakic@ink.fish>
Closes: https://lore.kernel.org/netdev/CAFPPu1gU2Y-D+d4i3F0MoNkYK+e1U+=X3qf6QycjfKBw+8snPg@mail.gmail.com/
Fixes: fb7084501a61 ("mptcp: add support for TCP_FASTOPEN sockopt")
Signed-off-by: T S Rameshkumar <rameshkumar.t@phytecembedded.in>
---
v2:
 - Reverted moving/clearing is_mptfo since check_fully_established() is
   only called once and is_mptfo is read-only at this stage (Matthieu Baerts).
 - Added Suggested-by tag for Petar Sakic.
 - Rebased cleanly on top of mptcp_net-next export branch.
---
 net/mptcp/options.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/mptcp/options.c b/net/mptcp/options.c
index 2bf368670..f5a258c7d 100644
--- a/net/mptcp/options.c
+++ b/net/mptcp/options.c
@@ -1043,6 +1043,8 @@ static bool check_fully_established(struct mptcp_sock *msk, struct sock *ssk,
 
 	mptcp_data_lock((struct sock *)msk);
 	__mptcp_subflow_fully_established(msk, subflow, mp_opt);
+	if (subflow->is_mptfo)
+		__mptcp_check_push((struct sock *)msk, ssk);
 	mptcp_data_unlock((struct sock *)msk);
 
 check_notify:
-- 
2.34.1

Hi T,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal (except selftest_mptcp_join): Success! ✅
- KVM Validation: normal (only selftest_mptcp_join): Success! ✅
- KVM Validation: debug (except selftest_mptcp_join): Success! ✅
- KVM Validation: debug (only selftest_mptcp_join): Success! ✅
- KVM Validation: btf-normal (only bpftest_all): Success! ✅
- KVM Validation: btf-debug (only bpftest_all): Success! ✅
- Perf: Unstable ⚠️
- Task: https://github.com/multipath-tcp/mptcp_net-next/actions/runs/38243594367

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/7716baaa6447
Patchwork: https://patchwork.kernel.org/project/mptcp/list/?series=1183801


If there are some issues, you can reproduce them using the same environment as
the one used by the CI thanks to a docker image, e.g.:

    $ cd [kernel source code]
    $ docker run -v "${PWD}:${PWD}:rw" -w "${PWD}" --privileged --rm -it \
        --pull always mptcp/mptcp-upstream-virtme-docker:latest \
        auto-normal

For more details:

    https://github.com/multipath-tcp/mptcp-upstream-virtme-docker


Please note that despite all the efforts that have been already done to have a
stable tests suite when executed on a public CI like here, it is possible some
reported issues are not due to your modifications. Still, do not hesitate to
help us improve that ;-)

Cheers,
MPTCP GH Action bot
Bot operated by Matthieu Baerts (NGI0 Core)