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: <CAHk-=wig1E4B-e1_6=it1LfVQ64DJsVgO6f6Ytnbzm2qChbAdw@mail.gmail.com>
Date: Tue, 29 Apr 2025 11:05:18 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>, 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 Tue, 29 Apr 2025 at 07:38, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>
> I tried that.  (The thread started as a question around
> __builtin_constant_p() but did grow to cover __builtin_ffs().)

Maybe we could do something like

   #define ffs(x) \
        (statically_true((x) != 0) ? __ffs(x)+1 : __builtin_ffs(x))

which uses our "statically_true()" helper that is actually fairly good
at the whole "let the compiler tell us that it knows that value cannot
be zero"

I didn't check what code that generated, but I've seen gcc do well on
that statically_true() thing in the past.

Then we can just remove our current variable_ffs() thing entirely,
because we now depend on our (good) __ffs() and the builtin being
"good enough" for the bad case.

(And do the same thing for fls() too, of course)

               Linus