Add set MPTFO socket option for MPTCP.

Signed-off-by: Dmytro Shytyi <dmytro@shytyi.net>
---
 net/mptcp/Makefile   |  2 +-
 net/mptcp/fastopen.c | 32 ++++++++++++++++++++++++++++++++
 net/mptcp/protocol.h |  5 +++++
 net/mptcp/sockopt.c  |  3 +++
 4 files changed, 41 insertions(+), 1 deletion(-)
 create mode 100644 net/mptcp/fastopen.c

diff --git a/net/mptcp/Makefile b/net/mptcp/Makefile
index 8a7f68efa35f..c42ad8609876 100644
--- a/net/mptcp/Makefile
+++ b/net/mptcp/Makefile
@@ -2,7 +2,7 @@
 obj-$(CONFIG_MPTCP) += mptcp.o
 
 mptcp-y := protocol.o subflow.o options.o token.o crypto.o ctrl.o pm.o diag.o \
-	   mib.o pm_netlink.o sockopt.o pm_userspace.o sched.o
+	   mib.o pm_netlink.o sockopt.o pm_userspace.o sched.o fastopen.o
 
 obj-$(CONFIG_SYN_COOKIES) += syncookies.o
 obj-$(CONFIG_INET_MPTCP_DIAG) += mptcp_diag.o
diff --git a/net/mptcp/fastopen.c b/net/mptcp/fastopen.c
new file mode 100644
index 000000000000..9ef49a2d2ea2
--- /dev/null
+++ b/net/mptcp/fastopen.c
@@ -0,0 +1,32 @@
+/* SPDX-License-Identifier: GPL-2.0
+ * MPTCP Fast Open Mechanism. Copyright (c) 2021-2022, Dmytro SHYTYI
+ */
+
+#include "protocol.h"
+
+int mptcp_setsockopt_sol_tcp_fastopen(struct mptcp_sock *msk, sockptr_t optval,
+				      unsigned int optlen)
+{
+	struct sock *sk = (struct sock *)msk;
+	struct net *net = sock_net(sk);
+	int val;
+	int ret;
+
+	ret = 0;
+
+	if (copy_from_sockptr(&val, optval, sizeof(val)))
+		return -EFAULT;
+
+	lock_sock(sk);
+
+	if (val >= 0 && ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))) {
+		tcp_fastopen_init_key_once(net);
+		fastopen_queue_tune(sk, val);
+	} else {
+		ret = -EINVAL;
+	}
+
+	release_sock(sk);
+
+	return ret;
+}
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 632161b13950..57596cdfb1f9 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -839,6 +839,11 @@ void mptcp_event_addr_removed(const struct mptcp_sock *msk, u8 id);
 bool mptcp_userspace_pm_active(const struct mptcp_sock *msk);
 int mptcp_stream_connect(struct socket *sock, struct sockaddr *uaddr, int addr_len, int flags);
 
+// Fast Open Mechanism functions begin
+int mptcp_setsockopt_sol_tcp_fastopen(struct mptcp_sock *msk, sockptr_t optval,
+				      unsigned int optlen);
+// Fast Open Mechanism functions end
+
 static inline bool mptcp_pm_should_add_signal(struct mptcp_sock *msk)
 {
 	return READ_ONCE(msk->pm.addr_signal) &
diff --git a/net/mptcp/sockopt.c b/net/mptcp/sockopt.c
index 423d3826ca1e..f62f0d63b8e6 100644
--- a/net/mptcp/sockopt.c
+++ b/net/mptcp/sockopt.c
@@ -559,6 +559,7 @@ static bool mptcp_supported_sockopt(int level, int optname)
 		case TCP_NOTSENT_LOWAT:
 		case TCP_TX_DELAY:
 		case TCP_INQ:
+		case TCP_FASTOPEN:
 			return true;
 		}
 
@@ -796,6 +797,8 @@ static int mptcp_setsockopt_sol_tcp(struct mptcp_sock *msk, int optname,
 		return mptcp_setsockopt_sol_tcp_nodelay(msk, optval, optlen);
 	case TCP_DEFER_ACCEPT:
 		return mptcp_setsockopt_sol_tcp_defer(msk, optval, optlen);
+	case TCP_FASTOPEN:
+		return mptcp_setsockopt_sol_tcp_fastopen(msk, optval, optlen);
 	}
 
 	return -EOPNOTSUPP;
-- 
2.25.1