netdev - Re: PROBLEM: Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache

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Message-ID: <20130703050007.GA11216@order.stressinduktion.org>
Date:	Wed, 3 Jul 2013 07:00:07 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Pierre Emeriaud <petrus.lt@...il.com>
Cc:	netdev@...r.kernel.org, yoshfuji@...ux-ipv6.org
Subject: Re: PROBLEM: Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache

[Cc YOSHIFUJI Hideaki because of commit
887c95cc1da53f66a5890fdeab13414613010097 ("ipv6: Complete neighbour entry
removal from dst_entry.")]

On Wed, Jul 03, 2013 at 12:04:33AM +0200, Pierre Emeriaud wrote:
> Linux 3.9 more-specific ipv6 route ignored until next-hop is in neighbor cache.
> 
> When adding a route to 2000::/3 with a next-hop that is not in the
> neighbor cache, the route is not preferred over the default.

Thanks for the report!

Well.

We ignore this route because of rt6_score_route returning -1 in this case.
This traces down to rt6_check_neigh returning false.

Before the above mentioned commit we kicked off some logic to create a
neighbour entry in ip6_route_add. Now we end up with neigh == NULL.

This is a hotfix but I need to do more research regarding
CONFIG_IPV6_ROUTER_PREF and further expectations of neigh != NULL (you
can try this at your own risk ;):

--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -547,6 +547,10 @@ static inline bool rt6_check_neigh(struct rt6_info *rt)
                        ret = true;
 #endif
                read_unlock(&neigh->lock);
+       } else {
+#ifdef CONFIG_IPV6_ROUTER_PREF
+               ret = true;
+#endif
        }
        rcu_read_unlock_bh();
 

Greetings,

  Hannes

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