Re: [Qemu-devel] [PATCH 1/1] Fix wrong mss bug.

[Philippe Mathieu-Daudé]

Hi Tao,

On 04/20/2017 04:36 PM, Tao Wu wrote:
> This bug was introduced by https://github.com/qemu/qemu/commit/98c6305

Nice catch...

In 98c6305 Guillaume probably missed parentheses :S
- mss = min(IF_MTU, IF_MRU) - sizeof(struct tcpiphdr);
+ mss = min(IF_MTU, IF_MRU) - sizeof(struct tcphdr) + sizeof(struct ip);

This would have been ok:
mss = min(IF_MTU, IF_MRU) - (sizeof(struct tcphdr) + sizeof(struct ip));

"The MSS value to be sent in an MSS option should be equal to the 
effective MTU minus the fixed IP and TCP headers." (RFC6691)

> Signed-off-by: Tao Wu <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> ---
>  slirp/tcp_input.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/slirp/tcp_input.c b/slirp/tcp_input.c
> index edb98f06f3..07bcbdb2dd 100644
> --- a/slirp/tcp_input.c
> +++ b/slirp/tcp_input.c
> @@ -1587,11 +1587,11 @@ tcp_mss(struct tcpcb *tp, u_int offer)
>         switch (so->so_ffamily) {
>         case AF_INET:
>              mss = MIN(IF_MTU, IF_MRU) - sizeof(struct tcphdr)
> -                                     + sizeof(struct ip);
> +                                     - sizeof(struct ip);
>             break;
>         case AF_INET6:
>              mss = MIN(IF_MTU, IF_MRU) - sizeof(struct tcphdr)
> -                                     + sizeof(struct ip6);
> +                                     - sizeof(struct ip6);
>             break;
>         default:
>             g_assert_not_reached();