Re: [net-next] tcp: use __jhash_final() in inet6_ehashfn()

[Jakub Kicinski <kuba@kernel.org>]

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---
tcp: use __jhash_final() in inet6_ehashfn()

This commit fixes a bug in inet6_ehashfn() where the last round of
jhash incorrectly used __jhash_mix() instead of __jhash_final(),
which left entropy distributed incompletely and didn't provide proper
avalanche properties.

> diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
> index 109fbf620ad7..0a781cba4085 100644
> --- a/net/ipv6/inet6_hashtables.c
> +++ b/net/ipv6/inet6_hashtables.c
> @@ -63,7 +63,7 @@ u32 inet6_ehashfn(const struct net *net,
>  	a += (__force u32)faddr->s6_addr32[2];
>  	b += (__force u32)faddr->s6_addr32[3];
>  	c += (__force u32)fport;
> -	__jhash_mix(a, b, c);
> +	__jhash_final(a, b, c);
>
>  	/* Note: We need to add @lport instead of fully hashing it.
>  	 * See commits 9544d60a2605 ("inet: change lport contribution

This isn't a bug, but the comment earlier in the function needs updating.
The comment currently says:

	/*
	 * 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).
	 *

The line "to avoid using __jhash_final(a, b, c)" is now misleading since
the code was just changed to use __jhash_final() in the last round. Should
this comment be updated to explain that we combine laddr[0] with
net_hash_mix() to avoid an extra __jhash_mix() round, and that the final
round correctly uses __jhash_final() for proper avalanche properties?
-- 
pw-bot: cr