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

From: Nicolas Dichtel <[email-masked]>
Date: Thu, 14 May 2015 20:58:59 +0200

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.

> +		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.