Re: [PATCH net 1/1] net: tcp: Permit user set TCP_MAXSEG to default value

[Eric Dumazet]

On Tue, 2017-03-21 at 05:30 +0800, f...@ikuai8.com wrote:
> From: Gao Feng <f...@ikuai8.com>
> 
> When user_mss is zero, it means use the default value. But the current
> codes don't permit user set TCP_MAXSEG to the default value.
> It would return the -EINVAL when val is zero.
> 
> Signed-off-by: Gao Feng <f...@ikuai8.com>
> ---
>  net/ipv4/tcp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 1e319a5..dd5e8e2 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2470,7 +2470,8 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
>               /* 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 < TCP_MIN_MSS || val > MAX_TCP_WINDOW) {
> +             if (val < 0 || (val > 0 && val < TCP_MIN_MSS) ||
> +                 val > MAX_TCP_WINDOW) {
>                       err = -EINVAL;
>                       break;
>               }

This is a convoluted way to express that val == 0 is accepted ...

What about :

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 
1e319a525d51b0b603a5ccc5143381c752b9f2c7..7db78d72896ac7c4befba5966704ed18ecbac409
 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2469,8 +2469,9 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
        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 < TCP_MIN_MSS || val > MAX_TCP_WINDOW) {
+                * know which interface is going to be used.
+                */
+               if (val && (val < TCP_MIN_MSS || val > MAX_TCP_WINDOW)) {
                        err = -EINVAL;
                        break;
                }