linux-hardening - Re: [PATCH] arc: Fix __fls() const-foldability via __builtin

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Message-ID: <aK5eNDa5H-PNGHpj@yury>
Date: Tue, 26 Aug 2025 21:24:04 -0400
From: Yury Norov <yury.norov@...il.com>
To: Kees Cook <kees@...nel.org>
Cc: Rasmus Villemoes <ravi@...vas.dk>, kernel test robot <lkp@...el.com>,
	Vineet Gupta <vgupta@...nel.org>,
	linux-snps-arc@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-hardening@...r.kernel.org
Subject: Re: [PATCH] arc: Fix __fls() const-foldability via __builtin_clzl()

On Tue, Aug 26, 2025 at 09:56:06AM -0700, Kees Cook wrote:
> On Tue, Aug 26, 2025 at 03:08:59PM +0200, Rasmus Villemoes wrote:
> > If __builtin_arc_fls() simply doesn't qualify for attr_const for
> > $reason, I think it would be good to have that documented in the commit
> > msg. If it does, I think a gcc ticket and link to that would be in order.
> 
> I already sent the patch to fix it. :)
> 
> https://gcc.gnu.org/pipermail/gcc-patches/2025-August/693273.html

I'm OK taking the patch if it fixes real problem for you, but it looks
more like a GCC problem, right? Is Clang also affected?

If, say, Clang is not affected, and you expect that newer GCC versions
will not be affected too, let's protect the new code with a proper
ifdefery, so that it will be easier to drop the workaround later?

Thanks,
Yury