[PATCH] net/socket: clamp negative backlog value to 0 in listen()

[PATCH] net/socket: clamp negative backlog value to 0 in listen()

[Kacper Piwiński]

According to manual: https://man7.org/linux/man-pages/man3/listen.3p.html
If listen() is called with a backlog argument value that is less
than 0, the function behaves as if it had been called with a
backlog argument value of 0.

Signed-off-by: Kacper Piwiński <cosiekvfj@o2.pl>
---
 net/socket.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/net/socket.c b/net/socket.c
index e416920e9..9567223d7 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1873,8 +1873,7 @@ int __sys_listen(int fd, int backlog)
 	sock = sockfd_lookup_light(fd, &err, &fput_needed);
 	if (sock) {
 		somaxconn = READ_ONCE(sock_net(sock->sk)->core.sysctl_somaxconn);
-		if ((unsigned int)backlog > somaxconn)
-			backlog = somaxconn;
+		backlog = clamp(backlog, 0, somaxconn);
 
 		err = security_socket_listen(sock, backlog);
 		if (!err)
-- 
2.34.1

Re: [PATCH] net/socket: clamp negative backlog value to 0 in listen()

[Kuniyuki Iwashima]

From Kacper Piwiński <cosiekvfj@o2.pl>
Date: Fri, 28 Jun 2024 19:28:36 +0200
> According to manual: https://man7.org/linux/man-pages/man3/listen.3p.html
> If listen() is called with a backlog argument value that is less
> than 0, the function behaves as if it had been called with a
> backlog argument value of 0.

This breaks many applications that assume listen(fd, -1) configures
the backlog with the max value allowed in the netns.

The behaviour is useful especially in a container-like env where app
does not have access to procfs.

The man page should be updated instead.


> 
> Signed-off-by: Kacper Piwiński <cosiekvfj@o2.pl>
> ---
>  net/socket.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/net/socket.c b/net/socket.c
> index e416920e9..9567223d7 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -1873,8 +1873,7 @@ int __sys_listen(int fd, int backlog)
>  	sock = sockfd_lookup_light(fd, &err, &fput_needed);
>  	if (sock) {
>  		somaxconn = READ_ONCE(sock_net(sock->sk)->core.sysctl_somaxconn);
> -		if ((unsigned int)backlog > somaxconn)
> -			backlog = somaxconn;
> +		backlog = clamp(backlog, 0, somaxconn);
>  
>  		err = security_socket_listen(sock, backlog);
>  		if (!err)
> --