netdev - Re: [PATCHv2] ipv4: Do not cache routing failures due to disabled forwarding.

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Date:	Tue, 23 Sep 2014 10:34:08 +0200
From:	Nicolas Cavallari <Nicolas.Cavallari@...en-communications.fr>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, kuznet@....inr.ac.ru, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net
Subject: Re: [PATCHv2] ipv4: Do not cache routing failures due to disabled
 forwarding.

On 16/09/2014 21:52, Nicolas Cavallari wrote:
> On 16/09/2014 20:54, David Miller wrote:
>> From: Nicolas Cavallari <nicolas.cavallari@...en-communications.fr>
>> Date: Mon, 15 Sep 2014 12:28:13 +0200
>>
>>> If we cache them, the kernel will reuse them, independently of
>>> whether forwarding is enabled or not.  Which means that if forwarding is
>>> disabled on the input interface where the first routing request comes
>>> from, then that unreachable result will be cached and reused for
>>> other interfaces, even if forwarding is enabled on them.
>>>
>>> This can be verified with two interfaces A and B and an output interface
>>> C, where B has forwarding enabled, but not A and trying
>>> ip route get $dst iif A from $src && ip route get $dst iif B from $src
>>>
>>> Signed-off-by: Nicolas Cavallari <nicolas.cavallari@...en-communications.fr>
>>> ---
>>> v2: simplify patch using julian anastasov's suggestion.
>>
>> This also disables caching for the cases of a simple fib lookup failure.
> 
> Caching is already not done on a fib lookup failure. On which fib_info
> would you cache anyway ? res.fi is already NULL in this case.
> 
>> Handle cached route invalidation the way it's meant to be, by bumping
>> the rt_genid.
>>
>> diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
>> index 214882e..aa4e63c 100644
>> --- a/net/ipv4/devinet.c
>> +++ b/net/ipv4/devinet.c
>> @@ -1965,6 +1965,8 @@ static void inet_forward_change(struct net *net)
>>  		}
>>  		rcu_read_unlock();
>>  	}
>> +
>> +	rt_cache_flush(net);
>>  }
>>  
>>  static int devinet_conf_ifindex(struct net *net, struct ipv4_devconf *cnf)
> 
> This doesn't solve the problem. Flushing the cache only changes how the
> bug manifests:
> 
> ip route flush cache
> ip route get 10.0.0.1 from 10.0.0.2 iif A
> # unreachable (because forwarding on A is disabled). This is cached.
> ip route get 10.0.0.1 from 10.0.0.2 iif B
> # unreachable (at first fib_lookup finds it reachable and forwarding on
> B is enabled. And then there is a valid cache entry, which is reused
> blindly even if it says "unreachable").
> 
> ip route flush cache
> ip route get 10.0.0.1 from 10.0.0.2 iif B
> # reachable (because forwarding on B is enabled).  This is cached
> ip route get 10.0.0.1 from 10.0.0.2 iif A
> # reachable (forwarding on A is disabled, so it is unreachable
> initially.  But there is a valid cache entry, which says "reachable",
> which is reused)

Any news on this patch ? Is there anything I can do so that this bug
is finally fixed ?
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