netdev - Re: [PATCH net 1/2] ipv6: reset dst.expires value when clearing expire flag

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Date:	Thu, 24 Oct 2013 09:52:09 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, sgunderson@...foot.com, valentyn@...b.net,
	yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH net 1/2] ipv6: reset dst.expires value when clearing expire flag

On Wed, Oct 23, 2013 at 11:36:53PM -0700, Eric Dumazet wrote:
> On Thu, 2013-10-24 at 07:48 +0200, Hannes Frederic Sowa wrote:
> > On receiving a packet too big icmp error we update the expire value by
> > calling rt6_update_expires. This function uses dst_set_expires which is
> > implemented that it can only reduce the expiration value of the dst entry.
> > 
> > If we insert new routing non-expiry information into the ipv6 fib where
> > we already have a matching rt6_info we only clear the RTF_EXPIRES flag
> > in rt6i_flags and leave the dst.expires value as is.
> > 
> > When new mtu information arrives for that cached dst_entry we again
> > call dst_set_expires. This time it won't update the dst.expire value
> > because we left the dst.expire value intact from the last update. So
> > dst_set_expires won't touch dst.expires.
> > 
> > Fix this by resetting dst.expires when clearing the RTF_EXPIRE flag.
> > dst_set_expires checks for a zero expiration and updates the
> > dst.expires.
> > 
> > In the past this (not updating dst.expires) was necessary because
> > dst.expire was placed in a union with the dst_entry *from reference. So
> > an update on the value would have caused page faults. This split happend
> > in ecd9883724b78cc72ed92c98bcb1a46c764fff21 ("ipv6: fix race condition
> > regarding dst->expires and dst->from").
> > 
> 
> Well, this patch removed the :
> 
> rt->dst.from = NULL;
> 
> from rt6_clean_expires(), right ?

Exactly.

Greetings,

  Hannes

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