Commit 854587e6 authored by Eric Dumazet's avatar Eric Dumazet Committed by Jakub Kicinski
Browse files

tcp: improve inet6_ehashfn() entropy 



Instead of only using the 32 low order bits of the local address,
use all of them.

Xor net_hash_mix(net) with the 32 high order bits of the local address
so that we can use __jhash_mix() three times.

If we were hashing 4 extra bytes, we would need one __jhash_final()
which is a bit expensive.

Using net_hash_mix() at the beginning allows better register allocation.

We no longer use a cascade of two jhash and inet6_ehash_secret,
this was dubious/weak.

Add a comment explaining why @lport is not part of the jhash computation.

$ scripts/bloat-o-meter -t vmlinux.0 vmlinux.1
add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-24 (-24)
Function                                     old     new   delta
inet6_ehashfn                                330     306     -24
Total: Before=24855958, After=24855934, chg -0.00%

Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
Reviewed-by: default avatarSimon Horman <horms@kernel.org>
Reviewed-by: default avatarKuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260313120346.3378811-1-edumazet@google.com


Signed-off-by: default avatarJakub Kicinski <kuba@kernel.org>
parent 6a33a706

net/ipv6/inet6_hashtables.c

+35 −5
Original line number Diff line number Diff line
@@ -35,13 +35,43 @@ u32 inet6_ehashfn(const struct net *net, 
		  const struct in6_addr *laddr, const u16 lport,

		  const struct in6_addr *faddr, const __be16 fport)

{

	u32 lhash, fhash;

	u32 a, b, c;



	lhash = (__force u32)laddr->s6_addr32[3];

	fhash = __ipv6_addr_jhash(faddr, tcp_ipv6_hash_secret);

	/*

	 * Please look at jhash() implementation for reference.

	 * Hash laddr + faddr + lport/fport + net_hash_mix.

	 * Notes:

	 * We combine laddr[0] (high order 32 bits of local address)

	 * with net_hash_mix() to avoid using __jhash_final(a, b, c).

	 *

	 * We do not include JHASH_INITVAL + 36 contribution

	 * to initial values of a, b, c.

	 */



	a = b = c = tcp_ipv6_hash_secret;



	return lport + __inet6_ehashfn(lhash, 0, fhash, fport,

				       inet6_ehash_secret + net_hash_mix(net));

	a += (__force u32)laddr->s6_addr32[0] ^ net_hash_mix(net);

	b += (__force u32)laddr->s6_addr32[1];

	c += (__force u32)laddr->s6_addr32[2];

	__jhash_mix(a, b, c);



	a += (__force u32)laddr->s6_addr32[3];

	b += (__force u32)faddr->s6_addr32[0];

	c += (__force u32)faddr->s6_addr32[1];

	__jhash_mix(a, b, c);



	a += (__force u32)faddr->s6_addr32[2];

	b += (__force u32)faddr->s6_addr32[3];

	c += (__force u32)fport;

	__jhash_mix(a, b, c);



	/* Note: We need to add @lport instead of fully hashing it.

	 * See commits 9544d60a2605 ("inet: change lport contribution

	 * to inet_ehashfn() and inet6_ehashfn()") and d4438ce68bf1

	 * ("inet: call inet6_ehashfn() once from inet6_hash_connect()")

	 * for references.

	 */

	return lport + c;

}

EXPORT_SYMBOL_GPL(inet6_ehashfn);