[PATCH net 0/2] mptcp: sync the msk->sndbuf at accept() time

[PATCH net 0/2] mptcp: sync the msk->sndbuf at accept() time

[Matthieu Baerts (NGI0)]

On passive MPTCP connections, the MPTCP socket send buffer doesn't have
the expected size at accept() time.

Patch 1 fixes the regression introduced in v6.7, while the following one
validates the fix in the selftests.

Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
Gang Yan (2):
      mptcp: sync the msk->sndbuf at accept() time
      selftests: mptcp: add a check for sndbuf of S/C

 net/mptcp/protocol.c                      |  2 +-
 tools/testing/selftests/net/mptcp/diag.sh | 28 ++++++++++++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)
---
base-commit: 0cf004ffb61cd32d140531c3a84afe975f9fc7ea
change-id: 20260420-net-mptcp-sync-sndbuf-accept-5079a6f9c407

Best regards,
--  
Matthieu Baerts (NGI0) <matttbe@kernel.org>

[PATCH net 1/2] mptcp: sync the msk->sndbuf at accept() time

[Matthieu Baerts (NGI0)]

From: Gang Yan <yangang@kylinos.cn>

On passive MPTCP connections, the msk sndbuf is not updated correctly.

The root cause is an order issue in the accept path:

- tcp_check_req() -> subflow_syn_recv_sock() -> mptcp_sk_clone_init()
  calls __mptcp_propagate_sndbuf() to copy the ssk sndbuf into msk

- Later, tcp_child_process() -> tcp_init_transfer() ->
  tcp_sndbuf_expand() grows the ssk sndbuf.

So __mptcp_propagate_sndbuf() runs before the ssk sndbuf has been
expanded and the msk ends up with a much smaller sndbuf than the
subflow:

  MPTCP: msk->sndbuf:20480, msk->first->sndbuf:2626560

Fix this by moving the __mptcp_propagate_sndbuf() call from
mptcp_sk_clone_init() -- the ssk sndbuf is not yet finalized there -- to
__mptcp_propagate_sndbuf() at accept() time, when the ssk sndbuf has
been fully expanded by tcp_sndbuf_expand().

Fixes: 8005184fd1ca ("mptcp: refactor sndbuf auto-tuning")
Cc: stable@vger.kernel.org
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/602
Signed-off-by: Gang Yan <yangang@kylinos.cn>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index fbffd3a43fe8..718e910ff23f 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -3594,7 +3594,6 @@ struct sock *mptcp_sk_clone_init(const struct sock *sk,
 	 * uses the correct data
 	 */
 	mptcp_copy_inaddrs(nsk, ssk);
-	__mptcp_propagate_sndbuf(nsk, ssk);
 
 	mptcp_rcv_space_init(msk, ssk);
 	msk->rcvq_space.time = mptcp_stamp();
@@ -4252,6 +4251,7 @@ static int mptcp_stream_accept(struct socket *sock, struct socket *newsock,
 
 		mptcp_graft_subflows(newsk);
 		mptcp_rps_record_subflows(msk);
+		__mptcp_propagate_sndbuf(newsk, mptcp_subflow_tcp_sock(subflow));
 
 		/* Do late cleanup for the first subflow as necessary. Also
 		 * deal with bad peers not doing a complete shutdown.

-- 
2.53.0

Re: [PATCH net 0/2] mptcp: sync the msk->sndbuf at accept() time

[patchwork-bot+netdevbpf]

Hello:

This series was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Mon, 20 Apr 2026 18:19:22 +0200 you wrote:
> On passive MPTCP connections, the MPTCP socket send buffer doesn't have
> the expected size at accept() time.
> 
> Patch 1 fixes the regression introduced in v6.7, while the following one
> validates the fix in the selftests.
> 
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> 
> [...]

Here is the summary with links:
  - [net,1/2] mptcp: sync the msk->sndbuf at accept() time
    https://git.kernel.org/netdev/net/c/fcf04b143346
  - [net,2/2] selftests: mptcp: add a check for sndbuf of S/C
    https://git.kernel.org/netdev/net/c/d0576eb8508e

You are awesome, thank you!
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