In the following patches we will reuse modified tcp_sendmsg_fastopen().
We call it from mptcp_sendmsg(). 

Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
---
 include/net/tcp.h    |  3 +++
 net/ipv4/tcp.c       | 23 ++++++++++++++++++-----
 net/mptcp/protocol.c | 18 ++++++++++++++++--
 3 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 27e8d378c70a..97cb014ddcab 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1755,6 +1755,9 @@ struct sock *tcp_try_fastopen(struct sock *sk, struct sk_buff *skb,
 			      struct request_sock *req,
 			      struct tcp_fastopen_cookie *foc,
 			      const struct dst_entry *dst);
+int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
+			 int *copied, size_t size,
+			 struct ubuf_info *uarg);
 void tcp_fastopen_init_key_once(struct net *net);
 bool tcp_fastopen_cookie_check(struct sock *sk, u16 *mss,
 			     struct tcp_fastopen_cookie *cookie);
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 5702ca9b952d..e86925b3814a 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -280,6 +280,8 @@
 #include <asm/ioctls.h>
 #include <net/busy_poll.h>
 
+#include "../mptcp/protocol.h"
+
 /* Track pending CMSGs. */
 enum {
 	TCP_CMSG_INQ = 1,
@@ -1162,9 +1164,9 @@ void tcp_free_fastopen_req(struct tcp_sock *tp)
 	}
 }
 
-static int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
-				int *copied, size_t size,
-				struct ubuf_info *uarg)
+int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
+			 int *copied, size_t size,
+			 struct ubuf_info *uarg)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct inet_sock *inet = inet_sk(sk);
@@ -1197,8 +1199,19 @@ static int tcp_sendmsg_fastopen(struct sock *sk, struct msghdr *msg,
 		}
 	}
 	flags = (msg->msg_flags & MSG_DONTWAIT) ? O_NONBLOCK : 0;
-	err = __inet_stream_connect(sk->sk_socket, uaddr,
-				    msg->msg_namelen, flags, 1);
+	if (!sk_is_mptcp(sk)) {
+		err = __inet_stream_connect(sk->sk_socket, uaddr,
+					    msg->msg_namelen, flags, 1);
+	} else {
+		struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(sk);
+
+		release_sock(sk);
+		release_sock(subflow->conn);
+		err = mptcp_stream_connect(sk->sk_socket, uaddr,
+					   msg->msg_namelen, msg->msg_flags);
+		lock_sock(subflow->conn);
+		lock_sock(sk);
+	}
 	/* fastopen_req could already be freed in __inet_stream_connect
 	 * if the connection times out or gets rst
 	 */
diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index c39ed726c1c0..8a5f79de3ab9 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1668,14 +1668,28 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 {
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct page_frag *pfrag;
+	struct socket *ssock;
 	size_t copied = 0;
 	int ret = 0;
 	long timeo;
 
 	/* we don't support FASTOPEN yet */
-	if (msg->msg_flags & MSG_FASTOPEN)
-		return -EOPNOTSUPP;
+	if (msg->msg_flags & MSG_FASTOPEN) {
+		lock_sock(sk);
+
+		ssock = __mptcp_nmpc_socket(msk);
+
+		lock_sock(ssock->sk);
 
+		if (ssock) {
+			int copied_syn_fastopen = 0;
+
+			ret = tcp_sendmsg_fastopen(ssock->sk, msg, &copied_syn_fastopen, len, NULL);
+			copied += copied_syn_fastopen;
+		}
+		release_sock(ssock->sk);
+		release_sock(sk);
+	}
 	/* silently ignore everything else */
 	msg->msg_flags &= MSG_MORE | MSG_DONTWAIT | MSG_NOSIGNAL;
 
-- 
2.25.1

Hi Dmytro,

Thank you for your modifications, that's great!

Our CI did some validations and here is its report:

- KVM Validation: normal:
  - Unstable: 1 failed test(s): selftest_mptcp_join 🔴:
  - Task: https://cirrus-ci.com/task/4837384788377600
  - Summary: https://api.cirrus-ci.com/v1/artifact/task/4837384788377600/summary/summary.txt

- KVM Validation: debug:
  - Unstable: 2 failed test(s): packetdrill_add_addr selftest_mptcp_join 🔴:
  - Task: https://cirrus-ci.com/task/5963284695220224
  - Summary: https://api.cirrus-ci.com/v1/artifact/task/5963284695220224/summary/summary.txt

Initiator: Patchew Applier
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/d6a6496543d4


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-debug

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 (Tessares)

Hi Dmytro,

Thank you for your modifications, that's great!

But sadly, our CI spotted some issues with it when trying to build it.

You can find more details there:

  https://patchwork.kernel.org/project/mptcp/patch/20220923001530.9722-4-dmytro@shytyi.net/
  https://github.com/multipath-tcp/mptcp_net-next/actions/runs/3109657443

Status: failure
Initiator: MPTCPimporter
Commits: https://github.com/multipath-tcp/mptcp_net-next/commits/d6a6496543d4

Feel free to reply to this email if you cannot access logs, if you need
some support to fix the error, if this doesn't seem to be caused by your
modifications or if the error is a false positive one.

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