netdev - Re: [PATCH net-next v5] ipv6: do not overwrite inetpeer metrics prematurely

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Message-Id: <20140327.150928.413158208323640831.davem@davemloft.net>
Date:	Thu, 27 Mar 2014 15:09:28 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	hannes@...essinduktion.org
Cc:	mkubecek@...e.cz, netdev@...r.kernel.org, kuznet@....inr.ac.ru,
	jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net
Subject: Re: [PATCH net-next v5] ipv6: do not overwrite inetpeer metrics
 prematurely

From: Hannes Frederic Sowa <hannes@...essinduktion.org>
Date: Thu, 27 Mar 2014 17:30:46 +0100

> On Thu, Mar 27, 2014 at 01:04:08PM +0100, Michal Kubecek wrote:
>> If an IPv6 host route with metrics exists, an attempt to add a
>> new route for the same target with different metrics fails but
>> rewrites the metrics anyway:
>> 
>> 12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1000
>> 12sp0:~ # ip -6 route show
>> fe80::/64 dev eth0  proto kernel  metric 256
>> fec0::1 dev eth0  metric 1024  rto_min lock 1s
>> 12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1500
>> RTNETLINK answers: File exists
>> 12sp0:~ # ip -6 route show
>> fe80::/64 dev eth0  proto kernel  metric 256
>> fec0::1 dev eth0  metric 1024  rto_min lock 1.5s
>> 
>> This is caused by all IPv6 host routes using the metrics in
>> their inetpeer (or the shared default). This also holds for the
>> new route created in ip6_route_add() which shares the metrics
>> with the already existing route and thus ip6_route_add()
>> rewrites the metrics even if the new route ends up not being
>> used at all.
>> 
>> Another problem is that old metrics in inetpeer can reappear
>> unexpectedly for a new route, e.g.
>> 
>> 12sp0:~ # ip route add fec0::1 dev eth0 rto_min 1000
>> 12sp0:~ # ip route del fec0::1
>> 12sp0:~ # ip route add fec0::1 dev eth0
>> 12sp0:~ # ip route change fec0::1 dev eth0 hoplimit 10
>> 12sp0:~ # ip -6 route show
>> fe80::/64 dev eth0  proto kernel  metric 256
>> fec0::1 dev eth0  metric 1024  hoplimit 10 rto_min lock 1s
>> 
>> Resolve the first problem by moving the setting of metrics down
>> into fib6_add_rt2node() to the point we are sure we are
>> inserting the new route into the tree. Second problem is
>> addressed by introducing new flag DST_METRICS_FORCE_OVERWRITE
>> which is set for a new host route in ip6_route_add() and makes
>> ipv6_cow_metrics() always overwrite the metrics in inetpeer
>> (even if they are not "new"); it is reset after that.
>> 
>> v5: use a flag in _metrics member rather than one in flags
>> 
>> v4: fix a typo making a condition always true (thanks to Hannes
>> Frederic Sowa)
>> 
>> v3: rewritten based on David Miller's idea to move setting the
>> metrics (and allocation in non-host case) down to the point we
>> already know the route is to be inserted. Also rebased to
>> net-next as it is quite late in the cycle.
>> 
>> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
> 
> I like your patch! :)
> 
> Actually I was a bit surprised how easy it turned out.
> 
> Thanks!
> 
> Acked-by: Hannes Frederic Sowa <hannes@...essinduktion.org>

Yeah this looks fantastic, applied, thanks!
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