netdev - Re: DDoS attack causing bad effect on conntrack searches

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Date:	Fri, 23 Apr 2010 13:05:17 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Jesper Dangaard Brouer <hawk@...u.dk>, paulmck@...ux.vnet.ibm.com,
	Changli Gao <xiaosuo@...il.com>, hawk@...x.dk,
	Linux Kernel Network Hackers <netdev@...r.kernel.org>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>
Subject: Re: DDoS attack causing bad effect on conntrack searches

Le vendredi 23 avril 2010 à 12:55 +0200, Patrick McHardy a écrit :
> Eric Dumazet wrote:
> > 
> > OK but a lookup last a fraction of a micro second, unless interrupted by
> > hard irq.
> > 
> > Probability of a change during a lookup should be very very small.
> > 
> > Note that the scenario for a restart is :
> > 
> > The lookup go through the chain.
> > While it is examining one object, this object is deleted.
> > The object is re-allocated by another cpu and inserted to a new chain.
> 
> I think another scenario that seems a bit more likely would be
> that a new entry is added to the chain after it was fully searched.
> Perhaps we could continue searching at the last position if the
> last entry is not a nulls entry to improve this.

But the last entry is always a nulls entry, what do you mean exactly ?

When an unsert (of a fresh object, not a reused one) is done, this
doesnt affect lookups in any way, since its done at the head of list.



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