Re: arp_hash

[Herbert Xu <herbert@gondor.apana.org.au>]

On Sun, Mar 22, 2015 at 12:54:58PM -0400, David Miller wrote:
> From: Herbert Xu <herbert@gondor.apana.org.au>
> Date: Sun, 22 Mar 2015 22:42:04 +1100
> 
> > If the attacker can somehow get the pointer value of the device
> > then all is lost since they could set any number of the low bits
> > of val to zero.
> 
> This "attacker" with access to your physcial network?

It could be a malicious local user, or it could be any host
if you are a router.
 
> I'm all for shifting the device pointer down a few bits to get more
> entropy, but I don't completely agree with the rest of your
> objections.

Granted for IPv4 you're not going to be in big trouble unless
you have a very large network (e.g. /16) configured.  But with
IPv6 this could seriously hose your box.

> This hash function has to be as fast as possible, even jhash{,2} is
> too expensive.

So what scales we are talking about, twice, three times? Have you
considered more modern hashes such as SipHash or SpookyHash
(successor to jhash/lookup3 and supposedly faster)?

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