Re: [PATCH net v2] tcp: Return user_mss for TCP_MAXSEG in CLOSE/LISTEN state if user_mss set

[Jason Xing <kerneljasonxing@gmail.com>]

On Wed, May 24, 2023 at 9:14 PM Cambda Zhu <cambda@linux.alibaba.com> wrote:
>
> This patch replaces the tp->mss_cache check in getting TCP_MAXSEG
> with tp->rx_opt.user_mss check for CLOSE/LISTEN sock. Since
> tp->mss_cache is initialized with TCP_MSS_DEFAULT, checking if
> it's zero is probably a bug.
>
> With this change, getting TCP_MAXSEG before connecting will return
> default MSS normally, and return user_mss if user_mss is set.
>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: Jack Yang <mingliang@linux.alibaba.com>
> Suggested-by: Eric Dumazet <edumazet@google.com>
> Link: https://lore.kernel.org/netdev/CANn89i+3kL9pYtkxkwxwNMzvC_w3LNUum_2=3u+UyLBmGmifHA@mail.gmail.com/#t
> Signed-off-by: Cambda Zhu <cambda@linux.alibaba.com>
> Link: https://lore.kernel.org/netdev/14D45862-36EA-4076-974C-EA67513C92F6@linux.alibaba.com/
> Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
> ---
> v2:
> - Update Fixes tag with commit in current tree.
> - Add Jason's Reviewed-by tag.
>
> v1:
> - Return default MSS if user_mss not set for backwards compatibility.
> - Send patch to net instead of net-next, with Fixes tag.
> - Add Eric's tags.
> ---
>  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 4d6392c16b7a..3e01a58724b8 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -4081,7 +4081,8 @@ int do_tcp_getsockopt(struct sock *sk, int level,
>         switch (optname) {
>         case TCP_MAXSEG:
>                 val = tp->mss_cache;
> -               if (!val && ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
> +               if (tp->rx_opt.user_mss &&
> +                   ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN)))
>                         val = tp->rx_opt.user_mss;
>                 if (tp->repair)
>                         val = tp->rx_opt.mss_clamp;
> --
> 2.16.6
>

Ah, I just realised that you didn't CC other maintainers though
reading the patchwork. Please run this command [1] before you submit a
patch next time.

[1]: $./scripts/get_maintainer.pl net/ipv4/tcp.c

+CC kuba, davem, dsahern

Thanks,
Jason