Subject: Re: [PATCH net v2 2/2] ipv6: fix ECMP route replacement

From: Michal Kubecek <[email-masked]>
Date: Thu, 14 May 2015 23:49:07 +0200

On Thu, May 14, 2015 at 08:58:59PM +0200, Nicolas Dichtel wrote:
> Le 13/05/2015 21:59, Michal Kubecek a écrit :
> >When replacing an IPv6 multipath route with "ip route replace", i.e.
> >NLM_F_CREATE | NLM_F_REPLACE, fib6_add_rt2node() replaces only first
> >matching route without fixing its siblings, resulting in corrupted
> >siblings linked list; removing one of the siblings can then end in an
> >infinite loop.
> >
> >Replacing the whole set of nexthops does IMHO make more sense than
> >replacing a random one. We also need to remove the NLM_F_REPLACE flag
> >after replacing old nexthops by first new so that each subsequent
> >nexthop does not replace previous one.
> >
> >Fixes: 51ebd3181572 ("ipv6: add support of equal cost multipath (ECMP)")
> >Signed-off-by: Michal Kubecek <[email-masked]>
> >---
> >  net/ipv6/ip6_fib.c | 17 ++++++++++++++---
> >  net/ipv6/route.c   |  8 +++++---
> >  2 files changed, 19 insertions(+), 6 deletions(-)
> >
> >diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> >index 96dbffff5a24..abf4e4e5bdab 100644
> >--- a/net/ipv6/ip6_fib.c
> >+++ b/net/ipv6/ip6_fib.c
> >@@ -815,6 +815,8 @@ add:
> >  		}
> >
> >  	} else {
> >+		struct rt6_info *next;
> >+
> >  		if (!found) {
> >  			if (add)
> >  				goto add;
> >@@ -828,15 +830,24 @@ add:
> >
> >  		*ins = rt;
> >  		rt->rt6i_node = fn;
> >-		rt->dst.rt6_next = iter->dst.rt6_next;
> >+
> >+		/* skip potential siblings */
> >+		next = iter->dst.rt6_next;
> >+		while (next && next->rt6i_metric == rt->rt6i_metric)
> >+			next = next->dst.rt6_next;
> I wonder if we should not loop over the siblings list here
> (rt->rt6i_siblings).  Only routes that match 'rt6_qualify_for_ecmp()'
> are siblings.

Problem with looping over the siblings list is that then we would have
to find each of them in the (unidirectional) list linked by dst.rt6_next
to be able to delete them from this list.  Do we at least know that all
routes in this list with matching metric and rt6_qualify_for_ecmp() are
siblings? If so, we could still do the cleanup on one pass over the
dst.rt6_next list.

                                                         Michal Kubecek

> 
> >+		rt->dst.rt6_next = next;
> >+
> >  		atomic_inc(&rt->rt6i_ref);
> >  		inet6_rt_notify(RTM_NEWROUTE, rt, info);
> >  		if (!(fn->fn_flags & RTN_RTINFO)) {
> >  			info->nl_net->ipv6.rt6_stats->fib_route_nodes++;
> >  			fn->fn_flags |= RTN_RTINFO;
> >  		}
> >-		fib6_purge_rt(iter, fn, info->nl_net);
> >-		rt6_release(iter);
> >+		while (iter != next) {
> >+			fib6_purge_rt(iter, fn, info->nl_net);
> >+			rt6_release(iter);
> >+			iter = iter->dst.rt6_next;
> >+		}
> Same here.