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([124.123.89.203]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-1616980c3b7sm14096967c88.16.2026.10.08.05.16.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Oct 2026 05:16:08 -0700 (PDT) From: T S Rameshkumar X-Google-Original-From: T S Rameshkumar To: matttbe@kernel.org, mptcp@lists.linux.dev Cc: petar.sakic@ink.fish, T S Rameshkumar Subject: [PATCH v4] mptcp: push queued data on passive TFO subflows becoming established Date: Thu, 8 Oct 2026 17:46:02 +0530 Message-Id: <20261008121602.206507-1-rameshkumar.t@phytecembedded.in> X-Mailer: git-send-email 2.34.1 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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() and clear is_mptfo. Reported-by: Petar Sakic Closes: https://lore.kernel.org/netdev/CAFPPu1gU2Y-D+d4i3F0MoNkYK+e1U+=3DX3= qf6QycjfKBw+8snPg@mail.gmail.com/ Fixes: fb7084501a61 ("mptcp: add support for TCP_FASTOPEN sockopt") Assisted-by: Gemini 3.7 Signed-off-by: T S Rameshkumar --- net/mptcp/options.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/net/mptcp/options.c b/net/mptcp/options.c index ce0de02f5..b865ee6af 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -1042,6 +1042,15 @@ static bool check_fully_established(struct mptcp_soc= k *msk, struct sock *ssk, =20 mptcp_data_lock((struct sock *)msk); __mptcp_subflow_fully_established(msk, subflow, mp_opt); + /* Passive TFO: the application may have written data while the + * subflow was still in SYN_RECV; __mptcp_subflow_active() refused + * it then and nothing else spools the msk write queue when the + * MPC third ack (no DSS) arrives. Push it now. + */ + if (subflow->is_mptfo) { + subflow->is_mptfo =3D 0; + __mptcp_check_push((struct sock *)msk, ssk); + } mptcp_data_unlock((struct sock *)msk); =20 check_notify: --=20 2.34.1