net: ipv6: fallback to full lookup if table lookup is unsuitable

On 9/16/16 6:55 AM, Vincent Bernat wrote:
> Commit 8c14586fc320 ("net: ipv6: Use passed in table for nexthop
> lookups") introduced a regression: insertion of an IPv6 route in a table
> not containing the appropriate connected route for the gateway but which
> contained a non-connected route (like a default gateway) fails while it
> was previously working:
> 
>     $ ip link add eth0 type dummy
>     $ ip link set up dev eth0
>     $ ip addr add 2001:db8::1/64 dev eth0
>     $ ip route add ::/0 via 2001:db8::5 dev eth0 table 20
>     $ ip route add 2001:db8:cafe::1/128 via 2001:db8::6 dev eth0 table 20
>     RTNETLINK answers: No route to host
>     $ ip -6 route show table 20
>     default via 2001:db8::5 dev eth0  metric 1024  pref medium

so your table 20 is not complete in that it lacks a connected route to resolve 2001:db8::6 as a nexthop, so you are relying on a fallback to other tables (main in this case).

-----8<-----

> @@ -1991,33 +2015,15 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
>  			if (!(gwa_type & IPV6_ADDR_UNICAST))
>  				goto out;
>  
> +			err = -EHOSTUNREACH;
>  			if (cfg->fc_table)
>  				grt = ip6_nh_lookup_table(net, cfg, gw_addr);

-----8<-----

> -			if (!(grt->rt6i_flags & RTF_GATEWAY))
> -				err = 0;

This is the check that is failing for your use case. ip6_nh_lookup_table is returning the default route and nexthops can not rely on a gateway. Given that a simpler and more direct change is (whitespace mangled on paste):

❦ 16 septembre 2016 20:36 CEST, David Ahern <dsa@cumulusnetworks.com> :

>> contained a non-connected route (like a default gateway) fails while it
>> was previously working:
>> 
>>     $ ip link add eth0 type dummy
>>     $ ip link set up dev eth0
>>     $ ip addr add 2001:db8::1/64 dev eth0
>>     $ ip route add ::/0 via 2001:db8::5 dev eth0 table 20
>>     $ ip route add 2001:db8:cafe::1/128 via 2001:db8::6 dev eth0 table 20
>>     RTNETLINK answers: No route to host
>>     $ ip -6 route show table 20
>>     default via 2001:db8::5 dev eth0  metric 1024  pref medium
>
> so your table 20 is not complete in that it lacks a connected route to
> resolve 2001:db8::6 as a nexthop, so you are relying on a fallback to
> other tables (main in this case).

Yes.

>> @@ -1991,33 +2015,15 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
>>  			if (!(gwa_type & IPV6_ADDR_UNICAST))
>>  				goto out;
>>  
>> +			err = -EHOSTUNREACH;
>>  			if (cfg->fc_table)
>>  				grt = ip6_nh_lookup_table(net, cfg, gw_addr);
>
> -----8<-----
>
>> -			if (!(grt->rt6i_flags & RTF_GATEWAY))
>> -				err = 0;
>
> This is the check that is failing for your use
> case. ip6_nh_lookup_table is returning the default route and nexthops
> can not rely on a gateway. Given that a simpler and more direct change
> is (whitespace mangled on paste):
>
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index ad4a7ff301fc..48bae2ee2e18 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -1991,9 +1991,19 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
>                         if (!(gwa_type & IPV6_ADDR_UNICAST))
>                                 goto out;
>
> -                       if (cfg->fc_table)
> +                       if (cfg->fc_table) {
>                                 grt = ip6_nh_lookup_table(net, cfg, gw_addr);
>
> +                               /* a nexthop lookup can not go through a gw.
> +                                * if this happens on a table based lookup
> +                                * then fallback to a full lookup
> +                                */
> +                               if (grt && grt->rt6i_flags & RTF_GATEWAY) {
> +                                       ip6_rt_put(grt);
> +                                       grt = NULL;
> +                               }
> +                       }
> +
>                         if (!grt)
>                                 grt = rt6_lookup(net, gw_addr, NULL,
>                                                  cfg->fc_ifindex, 1);

OK. Should the dev check be dismissed or do we add "dev && dev !=
grt->dst.dev" just as a safety net (this would be a convulated setup,
but the correct direct route could be in an ip rule with higher priority
while the one in this table is incorrect)?

On 9/16/16 1:15 PM, Vincent Bernat wrote:
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index ad4a7ff301fc..48bae2ee2e18 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -1991,9 +1991,19 @@ static struct rt6_info *ip6_route_info_create(struct fib6_config *cfg)
>>                         if (!(gwa_type & IPV6_ADDR_UNICAST))
>>                                 goto out;
>>
>> -                       if (cfg->fc_table)
>> +                       if (cfg->fc_table) {
>>                                 grt = ip6_nh_lookup_table(net, cfg, gw_addr);
>>
>> +                               /* a nexthop lookup can not go through a gw.
>> +                                * if this happens on a table based lookup
>> +                                * then fallback to a full lookup
>> +                                */
>> +                               if (grt && grt->rt6i_flags & RTF_GATEWAY) {
>> +                                       ip6_rt_put(grt);
>> +                                       grt = NULL;
>> +                               }
>> +                       }
>> +
>>                         if (!grt)
>>                                 grt = rt6_lookup(net, gw_addr, NULL,
>>                                                  cfg->fc_ifindex, 1);
> 
> OK. Should the dev check be dismissed or do we add "dev && dev !=
> grt->dst.dev" just as a safety net (this would be a convulated setup,
> but the correct direct route could be in an ip rule with higher priority
> while the one in this table is incorrect)?
> 

yes. So the validity check becomes:

	grt = ip6_nh_lookup_table(net, cfg, gw_addr);
	if (grt) {
		if (grt->rt6i_flags & RTF_GATEWAY ||
		    dev && dev != grt->dst.dev) {
			ip6_rt_put(grt);
			grt = NULL;            <---- causes the full rt6_lookup
		}
	}