From: Geliang Tang <tanggeliang@kylinos.cn>

Add a helper tcp_sock_set_maxseg() to directly set the TCP_MAXSEG sockopt
from kernel space.

This new helper will be used in the following patch.

Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
---
 include/linux/tcp.h |  1 +
 net/ipv4/tcp.c      | 23 ++++++++++++++---------
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index a8af71623ba7..41206c3f289f 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -624,6 +624,7 @@ void tcp_sock_set_nodelay(struct sock *sk);
 void tcp_sock_set_quickack(struct sock *sk, int val);
 int tcp_sock_set_syncnt(struct sock *sk, int val);
 int tcp_sock_set_user_timeout(struct sock *sk, int val);
+int tcp_sock_set_maxseg(struct sock *sk, int val);
 
 static inline bool dst_tcp_usec_ts(const struct dst_entry *dst)
 {
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e1bbb070e734..e1f00faed760 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3726,6 +3726,19 @@ int tcp_set_window_clamp(struct sock *sk, int val)
 	return 0;
 }
 
+int tcp_sock_set_maxseg(struct sock *sk, int val)
+{
+	/* Values greater than interface MTU won't take effect. However
+	 * at the point when this call is done we typically don't yet
+	 * know which interface is going to be used
+	 */
+	if (val && (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW))
+		return -EINVAL;
+
+	tcp_sk(sk)->rx_opt.user_mss = val;
+	return 0;
+}
+
 /*
  *	Socket option code for TCP.
  */
@@ -3858,15 +3871,7 @@ int do_tcp_setsockopt(struct sock *sk, int level, int optname,
 
 	switch (optname) {
 	case TCP_MAXSEG:
-		/* Values greater than interface MTU won't take effect. However
-		 * at the point when this call is done we typically don't yet
-		 * know which interface is going to be used
-		 */
-		if (val && (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW)) {
-			err = -EINVAL;
-			break;
-		}
-		tp->rx_opt.user_mss = val;
+		tcp_sock_set_maxseg(sk, val);
 		break;
 
 	case TCP_NODELAY:
-- 
2.43.0