From fcf04b14334641f4b0b8647824480935e9416d52 Mon Sep 17 00:00:00 2001 From: Gang Yan Date: Mon, 20 Apr 2026 18:19:23 +0200 Subject: [PATCH 1/2] mptcp: sync the msk->sndbuf at accept() time 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 Acked-by: Paolo Abeni Reviewed-by: Matthieu Baerts (NGI0) Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260420-net-mptcp-sync-sndbuf-accept-v1-1-e3523e3aeb44@kernel.org Signed-off-by: Paolo Abeni --- 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. From d0576eb8508e68b950ee9d2820116a6fc205fd07 Mon Sep 17 00:00:00 2001 From: Gang Yan Date: Mon, 20 Apr 2026 18:19:24 +0200 Subject: [PATCH 2/2] selftests: mptcp: add a check for sndbuf of S/C Add a new chk_sndbuf() helper to diag.sh that extracts the sndbuf (the 'tb' field from 'ss -m' skmem output) for both server and client MPTCP sockets, and verifies they are equal. Without the previous patch, it will fail: ''' 07 ....chk sndbuf server/client [FAIL] sndbuf S=20480 != C=2630656 ''' Signed-off-by: Gang Yan Reviewed-by: Matthieu Baerts (NGI0) Signed-off-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260420-net-mptcp-sync-sndbuf-accept-v1-2-e3523e3aeb44@kernel.org Signed-off-by: Paolo Abeni --- tools/testing/selftests/net/mptcp/diag.sh | 28 +++++++++++++++++++++++ 1 file changed, 28 insertions(+) diff --git a/tools/testing/selftests/net/mptcp/diag.sh b/tools/testing/selftests/net/mptcp/diag.sh index d847ff1737c3..27cbda68144e 100755 --- a/tools/testing/selftests/net/mptcp/diag.sh +++ b/tools/testing/selftests/net/mptcp/diag.sh @@ -322,6 +322,33 @@ wait_connected() done } +chk_sndbuf() +{ + local server_sndbuf client_sndbuf msg + local port=${1} + + msg="....chk sndbuf server/client" + server_sndbuf=$(ss -N "${ns}" -inmHM "sport" "${port}" | \ + sed -n 's/.*tb\([0-9]\+\).*/\1/p') + client_sndbuf=$(ss -N "${ns}" -inmHM "dport" "${port}" | \ + sed -n 's/.*tb\([0-9]\+\).*/\1/p') + + mptcp_lib_print_title "${msg}" + if [ -z "${server_sndbuf}" ] || [ -z "${client_sndbuf}" ]; then + mptcp_lib_pr_fail "sndbuf S=${server_sndbuf} C=${client_sndbuf}" + mptcp_lib_result_fail "${msg}" + ret=${KSFT_FAIL} + elif [ "${server_sndbuf}" != "${client_sndbuf}" ]; then + mptcp_lib_pr_fail "sndbuf S=${server_sndbuf} != C=${client_sndbuf}" + mptcp_lib_result_fail "${msg}" + ret=${KSFT_FAIL} + else + mptcp_lib_pr_ok + mptcp_lib_result_pass "${msg}" + fi +} + + trap cleanup EXIT mptcp_lib_ns_init ns @@ -341,6 +368,7 @@ echo "b" | \ 127.0.0.1 >/dev/null & wait_connected $ns 10000 chk_msk_nr 2 "after MPC handshake" +chk_sndbuf 10000 chk_last_time_info 10000 chk_msk_remote_key_nr 2 "....chk remote_key" chk_msk_fallback_nr 0 "....chk no fallback"