mptcp: add subflow_v(4,6)_send_synack()

The send_synack() needs to be overridden for MPTCP to support TFO for
two reasons:

- There is not be enough space in the TCP options if the TFO cookie has
  to be added in the SYN+ACK with other options: MSS (4), SACK OK (2),
  Timestamps (10), Window Scale (3+1), TFO (10+2), MP_CAPABLE (12).
  MPTCPv1 specs -- RFC 8684, section B.1 [1] -- suggest to drop the TCP
  timestamps option in this case.

- The data received in the SYN has to be handled: the SKB can be
  dequeued from the subflow sk and transferred to the MPTCP sk. Counters
  need to be updated accordingly and the application can be notified at
  the end because some bytes have been received.

[1] https://www.rfc-editor.org/rfc/rfc8684.html#section-b.1

Co-developed-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index cbeb7718f092..29904303f5c2 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -307,7 +307,48 @@ static struct dst_entry *subflow_v4_route_req(const struct sock *sk,
 	return NULL;
 }
 
+static void subflow_prep_synack(const struct sock *sk, struct request_sock *req,
+				struct tcp_fastopen_cookie *foc,
+				enum tcp_synack_type synack_type)
+{
+	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
+	struct inet_request_sock *ireq = inet_rsk(req);
+
+	/* clear tstamp_ok, as needed depending on cookie */
+	if (foc && foc->len > -1)
+		ireq->tstamp_ok = 0;
+
+	if (synack_type == TCP_SYNACK_FASTOPEN)
+		mptcp_fastopen_subflow_synack_set_params(subflow, req);
+}
+
+static int subflow_v4_send_synack(const struct sock *sk, struct dst_entry *dst,
+				  struct flowi *fl,
+				  struct request_sock *req,
+				  struct tcp_fastopen_cookie *foc,
+				  enum tcp_synack_type synack_type,
+				  struct sk_buff *syn_skb)
+{
+	subflow_prep_synack(sk, req, foc, synack_type);
+
+	return tcp_request_sock_ipv4_ops.send_synack(sk, dst, fl, req, foc,
+						     synack_type, syn_skb);
+}
+
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
+static int subflow_v6_send_synack(const struct sock *sk, struct dst_entry *dst,
+				  struct flowi *fl,
+				  struct request_sock *req,
+				  struct tcp_fastopen_cookie *foc,
+				  enum tcp_synack_type synack_type,
+				  struct sk_buff *syn_skb)
+{
+	subflow_prep_synack(sk, req, foc, synack_type);
+
+	return tcp_request_sock_ipv6_ops.send_synack(sk, dst, fl, req, foc,
+						     synack_type, syn_skb);
+}
+
 static struct dst_entry *subflow_v6_route_req(const struct sock *sk,
 					      struct sk_buff *skb,
 					      struct flowi *fl,
@@ -1945,6 +1986,7 @@ void __init mptcp_subflow_init(void)
 
 	subflow_request_sock_ipv4_ops = tcp_request_sock_ipv4_ops;
 	subflow_request_sock_ipv4_ops.route_req = subflow_v4_route_req;
+	subflow_request_sock_ipv4_ops.send_synack = subflow_v4_send_synack;
 
 	subflow_specific = ipv4_specific;
 	subflow_specific.conn_request = subflow_v4_conn_request;
@@ -1958,6 +2000,7 @@ void __init mptcp_subflow_init(void)
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
 	subflow_request_sock_ipv6_ops = tcp_request_sock_ipv6_ops;
 	subflow_request_sock_ipv6_ops.route_req = subflow_v6_route_req;
+	subflow_request_sock_ipv6_ops.send_synack = subflow_v6_send_synack;
 
 	subflow_v6_specific = ipv6_specific;
 	subflow_v6_specific.conn_request = subflow_v6_conn_request;