netdev - Re: Extensible hashing and RCU

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Message-ID: <f2b55d220702200749l4fa2b14foa861cf37288cfb8d@mail.gmail.com>
Date:	Tue, 20 Feb 2007 07:49:11 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Evgeniy Polyakov" <johnpol@....mipt.ru>
Cc:	"David Miller" <davem@...emloft.net>, dada1@...mosbay.com,
	akepner@....com, linux@...izon.com, netdev@...r.kernel.org,
	bcrl@...ck.org
Subject: Re: Extensible hashing and RCU

On 2/20/07, Evgeniy Polyakov <johnpol@....mipt.ru> wrote:
> Jenkins _does_ have them, I showed tests half a year ago and in this
> thread too. Actually _any_ hash has them it is just a matter of time
> to find one.

I think you misunderstood me.  If you are trying to DoS me from
outside with a hash collision attack, you are trying to feed me
packets that fall into the same hash bucket.  The Jenkins hash does
not have to be artifact-free, and does not have to be
cryptographically strong.  It just has to do a passable job of mixing
a random salt into the tuple, so you don't know which string of
packets to feed me in order to fill one (or a few) of my buckets.
XORing salt into a folded tuple doesn't help; it just permutes the
buckets.

Cheers,
- Michael
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