netdev - Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert "misuses" of reciprocal

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1389842050.31367.396.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 15 Jan 2014 19:14:10 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jakub Zawadzki <darkjames-ws@...kjames.pl>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net-next 1/2] random32: add prandom_u32_lt_N and convert
 "misuses" of reciprocal_divide

On Thu, 2014-01-16 at 00:23 +0100, Daniel Borkmann wrote:

> @@ -1220,7 +1219,7 @@ static unsigned int fanout_demux_hash(struct packet_fanout *f,
>  				      struct sk_buff *skb,
>  				      unsigned int num)
>  {
> -	return reciprocal_divide(skb->rxhash, num);
> +	return (u32)(((u64) skb->rxhash * num) >> 32);
>  }
>  

This is unfortunate.

(This reverts one of your patch : f55d112e529386 )

Please add a helper to explain what's going on here, and on many other
spots we do this computation (as in get_rps_cpu()).
Few people really understand this.

Or keep reciprocal_divide() as is, and introduce a new set of functions
for people really wanting the precise divides.



--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html