netdev - Re: [PATCH net] ipv6: refresh rt6i_genid in ip6_pol

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Date:	Mon, 08 Sep 2014 14:48:00 -0400
From:	Vlad Yasevich <vyasevic@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>
CC:	nicolas.dichtel@...nd.com, therbert@...gle.com,
	alexander.h.duyck@...el.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] ipv6: refresh rt6i_genid in ip6_pol_route()

On 09/08/2014 01:07 AM, Eric Dumazet wrote:
> On Sun, 2014-09-07 at 21:59 -0700, David Miller wrote:
>> From: Eric Dumazet <eric.dumazet@...il.com>
>> Date: Sun, 07 Sep 2014 21:43:54 -0700
>>
>>> On Sun, 2014-09-07 at 21:27 -0700, David Miller wrote:
>>>> From: Eric Dumazet <eric.dumazet@...il.com>
>>>> Date: Sun, 07 Sep 2014 21:18:25 -0700
>>>>
>>>>> On Sun, 2014-09-07 at 15:54 -0700, David Miller wrote:
>>>>>
>>>>>> This might be broken.
>>>>>>
>>>>>> We are dealing here with persistent entries in the ipv6 routine trie.
>>>>>>
>>>>>> If you just bump the genid on the next person to look it up, other
>>>>>> sockets and cached entities might not have validated the route yet,
>>>>>> and now will falsely see the route as valid.  We have to ensure that
>>>>>> they too drop this route and perform a relookup.
>>>>>
>>>>> I am confused, I thought it was the role of the cookie.
>>>>>
>>>>> (Ie socket has to store its own cookie to be able to validate a route)
>>>>>
>>>>> Before 6f3118b571b8 patch, how was this done anyway ?
>>>>>
>>>>> If persistent routes cannot refresh the genid, then they are useless ?
>>>>
>>>> I just speak about the genid aspect.
>>>>
>>>> I understand that cookie (via node->fn_sernum) invalidates the path
>>>> in the fib_trie, but the genid protects against other circumstances
>>>> (matching IPSEC rule, f.e.)
>>>>
>>>> You have to make sure all other sockets did a full route lookup
>>>> (including IPSEC) before you can safely adjust the genid.
>>>>
>>>> I could be wrong, recheck my analysis please :-)
>>>
>>> So this whole genid protection can not work, unless we make sure a
>>> socket cannot share a route with another socket.
>>>
>>> This means we have to clone all routes.
>>
>> I'm willing to revert the change in question if you think that's the
>> sanest way forward.
>>
>> The bug fix for more obscure use cases (IPSEC) if pointless if it
>> breaks more common things (TCP input route caching).
> 
> Lets wait for Nicolas and/or Hannes input, they might have some ideas...
> 
> 

So, looking at the differences between ipv4 and ipv6, it looks like ipv4
genid is only bumped by xfrm, while ipv6 one is bumped every time
__ipv6_ifa_notify() is called which happens in a lot of palaces...

So, my guess is that ipv4 genid doesn't really grow on non-xfrm configured
systems, while ipv6 one can grow very fast.  Probably why we don't see
it with ipv4 and do with ipv6.

-vlad


> 
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