Subject: Re: PMTU discovery is broken on kernel 3.7.1 for UDP sockets

From: Steffen Klassert <[email-masked]>
Date: Thu, 20 Dec 2012 13:35:35 +0100

On Thu, Dec 20, 2012 at 03:22:13PM +0400, Yurij M. Plotnikov wrote:
> On 12/20/12 11:34, Steffen Klassert wrote:
> >
> >diff --git a/net/ipv4/ip_sockglue.c b/net/ipv4/ip_sockglue.c
> >index 3c9d208..1049ce0 100644
> >--- a/net/ipv4/ip_sockglue.c
> >+++ b/net/ipv4/ip_sockglue.c
> >@@ -1198,7 +1198,7 @@ static int do_ip_getsockopt(struct sock *sk, int level, int optname,
> >  	{
> >  		struct dst_entry *dst;
> >  		val = 0;
> >-		dst = sk_dst_get(sk);
> >+		dst = sk_dst_check(sk, 0);
> >  		if (dst) {
> >  			val = dst_mtu(dst);
> >  			dst_release(dst);
> With this patch kernel 3.7.1 works perfect. All described problems
> are fixed.

Thanks for testing!

I'm not sure if we can't use this as a fix. I think with this patch it
could happen that we return -ENOTCONN instead of a pmtu value on a
connected socket. Perhaps it is better to update the cached dst_entry in
ipv4_sk_update_pmtu() when we receive the -EMSGSIZE. I'll do some
investigation.

Anyway, it is still odd that reverting my other patch 'fixes'
this issue too.