netdev - Re: [PATCH net] ipv6: no need to return rt->dst.error if it is not null entry.

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Date:   Fri, 28 Jul 2017 09:10:38 -0600
From:   David Ahern <dsahern@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Roopa Prabhu <roopa@...ulusnetworks.com>
Cc:     Hangbin Liu <liuhangbin@...il.com>,
        network dev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] ipv6: no need to return rt->dst.error if it is not
 null entry.

On 7/27/17 10:56 PM, Cong Wang wrote:
> On Wed, Jul 26, 2017 at 11:49 AM, David Ahern <dsahern@...il.com> wrote:
>> On 7/26/17 12:27 PM, Roopa Prabhu wrote:
>>> agreed...so looks like the check in v3 should be
>>>
>>>
>>> +       if ( rt == net->ipv6.ip6_null_entry ||
>>> +            (rt->dst.error &&
>>> + #ifdef CONFIG_IPV6_MULTIPLE_TABLES
>>> +              rt != net->ipv6.ip6_prohibit_entry &&
>>> +              rt != net->ipv6.ip6_blk_hole_entry &&
>>> +#endif
>>> +             )) {
>>>                 err = rt->dst.error;
>>>                 ip6_rt_put(rt);
>>>                 goto errout;
>>>
>>
>> I don't think so. If I add a prohibit route and use the fibmatch
>> attribute, I want to see the route from the FIB that was matched.
> 
> But net->ipv6.ip6_prohibit_entry is not the prohibit route you can
> add in user-space, it is only used by rule actions. So do you really
> want to dump it?? My gut feeling is no, but I am definitely not sure.
> 
> When you add a prohibit route, a new rt is allocated dynamically,
> net->ipv6.ip6_prohibit_entry is relatively static, internal and is the
> only one per netns. (Same for net->ipv6.ip6_blk_hole_entry)
> 
> I think Hangbin's example doesn't have ip rules, so this case
> is not shown up.
> 

Understood. The v4 patch returns getroute to the original behavior. The
original behavior returned a route entry not just an error code. The
following is at 5dafc87f40d7 which the commit before roopa's patch set:

# ip -6 ru add to 6000::/120 prohibit
# ip -6 ro get 6000::1
prohibit 6000::1 from :: dev lo proto kernel src 2001:db8::3 metric
4294967295  error -13 pref medium

# ip -6 ro add vrf red prohibit 5000::1/120
# ip -6 ro get vrf red 5000::1
prohibit 5000::1 from :: dev lo table red src 2001:db8::3 metric 1024
error -13 pref medium

Generically, the only time you get just an error response is when the
lookup fails to find a match and returns the null_entry which has
dst.error = -ENETUNREACH.


Now to your point about the new fibmatch option I have gone back and
forth but in the end I think returning the route associated with the FIB
rule is better than just failing with an error code.

Roopa?