linux-kernel - Re: [PATCH] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64()

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 for Android: free password hash cracker in your pocket

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

Message-ID: <20201022163739.GU2672@gate.crashing.org>
Date:   Thu, 22 Oct 2020 11:37:39 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, jakub@...hat.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/bitops: Fix possible undefined behaviour with fls() and fls64()

On Thu, Oct 22, 2020 at 02:05:46PM +0000, Christophe Leroy wrote:
> fls() and fls64() are using __builtin_ctz() and _builtin_ctzll().
> On powerpc, those builtins trivially use ctlzw and ctlzd power
> instructions.
> 
> Allthough those instructions provide the expected result with
> input argument 0, __builtin_ctz() and __builtin_ctzll() are
> documented as undefined for value 0.

> When the input of fls(x) is a constant, just check x for nullity and
> return either 0 or __builtin_clz(x). Otherwise, use cntlzw instruction
> directly.

That looks good :-)

Acked-by: Segher Boessenkool <segher@...nel.crashing.org>


Segher