netdev - Where exactly will arch_fast

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Message-ID: <20141204081147.GA19030@gondor.apana.org.au>
Date:	Thu, 4 Dec 2014 16:11:47 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Thomas Graf <tgraf@...g.ch>, Daniel Borkmann <dborkman@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Theodore Ts'o <tytso@....edu>, netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Where exactly will arch_fast_hash be used

Hi:

While working on rhashtable it came to me that this whole concept
of arch_fast_hash is flawed.  CRCs are linear functions so it's
fairly easy for an attacker to identify collisions or at least
eliminate a large amount of search space (e.g., controlling the
last bit of the hash result is almost trivial, even when you add
a random seed).

So what exactly are we going to use arch_fast_hash for? Presumably
it's places where security is never goint to be an issue, right?

Even if security wasn't an issue, straight CRC32 has really poor
lower-order bit distribution, which makes it a terrible choice for
a hash table that simply uses the lower-order bits.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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