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

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

[Tao Wu]

This bug was introduced by https://github.com/qemu/qemu/commit/98c6305

Signed-off-by: Tao Wu <lepton@google.com>
---
 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();
-- 
2.12.2.816.g2cccc81164-goog

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 <lepton@google.com>

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

> ---
>  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();
>