netdev - Re: [PATCH for 2.6.33] conntrack: restrict runtime hashsize modifications

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Message-ID: <4B6AFE22.20304@trash.net>
Date:	Thu, 04 Feb 2010 18:04:34 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Alexey Dobriyan <adobriyan@...il.com>
CC:	davem@...emloft.net, jonathan@...masters.org,
	eric.dumazet@...il.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [PATCH for 2.6.33] conntrack: restrict runtime hashsize modifications

Patrick McHardy wrote:
> Alexey Dobriyan wrote:
>> Jon Masters correctly points out that conntrack hash sizes
>> (nf_conntrack_htable_size) are global (not per-netns) and
>> modifiable at runtime via /sys/module/nf_conntrack/hashsize .
>>
>> Steps to reproduce:
>> 	clone(CLONE_NEWNET)
>> 	[grow /sys/module/nf_conntrack/hashsize]
>> 	exit()
>>
>> At netns exit we are going to scan random memory for conntracks to be killed.
>>
>> Apparently there is a code which deals with hashtable resize for
>> init_net (and it was there befode netns conntrack code), so prohibit
>> hashsize modification if there is more than one netns exists.
>>
>> To change hashtable sizes, you need to reload module.
>>
>> Expectation hashtable size was simply glued to a variable with no code
>> to rehash expectations, so it was a bug to allow writing to it.
>> Make "expect_hashsize" readonly.
>>
>> This is temporarily until we figure out what to do.
> 
> How about alternatively moving nf_conntrack_hsize into the
> per-namespace struct? It doesn't look more complicated or
> intrusive and would allow to still change the init_net
> hashsize. Also seems less hackish :)

How about this (so far untested) patch? The htable_size is moved into
the per-namespace struct and initialized from the current (global)
value of nf_conntrack_htable_size. Changes through sysfs are still
permitted, but only affect the init namespace and newly created ones.

Additionally I removed reinitializing the hash random value when
changing the hash size since that also requires to rehash in all
namespaces.

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