linux-kernel - Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

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: <B364FF6D-DFCC-42A7-ACA1-6A74E27EE57E@zytor.com>
Date: Mon, 28 Apr 2025 14:38:46 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
CC: Andrew Cooper <andrew.cooper3@...rix.com>, Arnd Bergmann <arnd@...db.de>,
        Arnd Bergmann <arnd@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Alexander Usyskin <alexander.usyskin@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mateusz Jończyk <mat.jonczyk@...pl>,
        Mike Rapoport <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH] bitops/32: Convert variable_ffs() and fls() zero-case handling to C

On April 28, 2025 9:14:45 AM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Mon, 28 Apr 2025 at 00:05, Ingo Molnar <mingo@...nel.org> wrote:
>>
>> And once we remove 486, I think we can do the optimization below to
>> just assume the output doesn't get clobbered by BS*L in the zero-case,
>> right?
>
>We probably can't, because who knows what "Pentium" CPU's are out there.
>
>Or even if Pentium really does get it right. I doubt we have any
>developers with an original Pentium around.
>
>So just leave the "we don't know what the CPU result is for zero"
>unless we get some kind of official confirmation.
>
>          Linus

If anyone knows for sure, it is probably Christian Ludloff. However, there was a *huge* tightening of the formal ISA when the i686 was introduced (family=6) and I really believe this was part of it.

I also really don't trust that family=5 really means conforms to undocumented P5 behavior, e.g. for Quark.