From: Geliang Tang <tanggeliang@kylinos.cn>

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

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
---
 include/linux/tcp.h |  1 +
 net/ipv4/tcp.c      | 24 +++++++++++++++---------
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 1669d95bb0f9..0041bc0aaee9 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -623,6 +623,7 @@ 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 4516f93f6cd2..402f4a9b3fd1 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3725,6 +3725,20 @@ 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;
+}
+EXPORT_SYMBOL(tcp_sock_set_maxseg);
+
 /*
  *	Socket option code for TCP.
  */
@@ -3857,15 +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

Hi Geliang,

On 23/04/2025 11:40, Geliang Tang wrote:
> From: Geliang Tang <tanggeliang@kylinos.cn>
> 
> Add a helper tcp_sock_set_maxseg() to directly set the TCP_MAXSEG sockopt
> from kernel space.

Probably clearer to add:

  This new helper will be used in the following patch.

> 
> Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> ---
>  include/linux/tcp.h |  1 +
>  net/ipv4/tcp.c      | 24 +++++++++++++++---------
>  2 files changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 1669d95bb0f9..0041bc0aaee9 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -623,6 +623,7 @@ 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 4516f93f6cd2..402f4a9b3fd1 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -3725,6 +3725,20 @@ 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;
> +}
> +EXPORT_SYMBOL(tcp_sock_set_maxseg);

This export is not needed: it is only needed when used from a module.

The rest looks OK to me:

Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>

> +
>  /*
>   *	Socket option code for TCP.
>   */
> @@ -3857,15 +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:

Cheers,
Matt
-- 
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