Re: ipv4: Simplify ARP hash function.

From: David Miller <davem@davemloft.net>
Date: Fri, 08 Jul 2011 10:47:39 -0700 (PDT)

From: Martin Mares <mj@ucw.cz>
Date: Fri, 8 Jul 2011 19:40:55 +0200

quoted

The hash function is linear, so it can be reduced to:

	a = key ^ dev->ifindex
	return (a >> 8) ^ (a >> 16) ^ (a >> 24)				// (1)
	     ^ (hash_rnd >> 8) ^ (hash_rnd >> 16) ^ (hash_rnd >> 24)	// (2)

Is this really the same?  The inclusion of a full 32-bit xor
with hash_rnd before folding was intentional, so that the
final folding occurs on a completely "random" value.

For example, try out this test program.  Run as "./x ${RANDOM_VALUE}",
it shows that the attacker cannot simply just iterate by the number of
hash table slots to create collisions, assuming a hash table size of
256 slots:

--------------------
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char **argp)
{
	int i, rnd;

	rnd = atoi(argp[1]);
	for (i = 1; i < (64 * 1024); i += 256) {
		int x = (i ^ rnd);

		x ^= (x >> 8) ^ (x << 16) ^ (x >> 24);

		x &= 0xff;

		printf("%d\n", x);
	}
	return 0;
}