netdev - Re: [PATCH v3] netfilter: save the hash of the tuple in the original direction for latter use

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Date:	Fri, 20 Aug 2010 23:22:23 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	akpm@...ux-foundation.org, netfilter-devel@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] netfilter: save the hash of the tuple in the original
 direction for latter use

On Fri, Aug 20, 2010 at 11:10 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le vendredi 20 août 2010 à 22:53 +0800, Changli Gao a écrit :
>> Since we don't change the tuple in the original direction, we can save it
>> in ct->tuplehash[IP_CT_DIR_REPLY].hnode.pprev for __nf_conntrack_confirm()
>> use.
>>
>> __hash_conntrack() is split into two steps: ____hash_conntrack() is used
>> to get the raw hash, and __hash_bucket() is used to get the bucket id.
>>
>> In SYN-flood case, early_drop() doesn't need to recompute the hash again.
>>
>> Signed-off-by: Changli Gao <xiaosuo@...il.com>
>> ---
>
> Hmm... so to accept a few more SYN packets per second in SYNFLOOD
> attack, we slow a bit normal operations ?

SYN-flood case is just a side effect. What I want to do is eliminating
the second call to hash_conntrack() of the original tuple in
__nf_conntrack_confirm().

> (adding one test on each
> packet going through conntrack)

Do you mean the rnd test?

>
> If yes (I dont think we should, hackers are stronger than you anyway,
> just face it)
>
> v4:
>        __read_mostly on nf_conntrack_rnd
>
>
> What would happen if we let the initialization of nf_conntrack_rnd
> only in the insertion case (like currently done) ?
> Only the first packet received on the machine/conntrack might be hashed
> on a wrong slot. Is it a big deal ? If yes, maybe find a way to
> recompute the hash in this case, instead of reusing 'wrong' one ?
>

I should keep the old way, but fix a race.

        if (unlikely(!nf_conntrack_hash_rnd_initted)) {
                get_random_bytes(&nf_conntrack_hash_rnd,
                                sizeof(nf_conntrack_hash_rnd));
                nf_conntrack_hash_rnd_initted = 1;
        }

nf_conntrack_alloc() isn't called with in the nf_conntrack_lock. So
the above code maybe executed more than once on different CPUs. It is
easy to fix with the cmpxchg() trick.

Thanks.

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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