linux-hardening - Re: [PATCH 14/17] s390: Add __attribute_const_

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Message-ID: <20250806103300.11925Cbe-hca@linux.ibm.com>
Date: Wed, 6 Aug 2025 12:33:00 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Kees Cook <kees@...nel.org>
Cc: linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-alpha@...r.kernel.org, linux-csky@...r.kernel.org,
        linux-hexagon@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-mips@...r.kernel.org, linux-openrisc@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        llvm@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [PATCH 14/17] s390: Add __attribute_const__ to ffs()-family
 implementations

On Mon, Aug 04, 2025 at 09:44:10AM -0700, Kees Cook wrote:
> While tracking down a problem where constant expressions used by
> BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
> initializer was convincing the compiler that it couldn't track the state
> of the prior statically initialized value. Tracing this down found that
> ffs() was used in the initializer macro, but since it wasn't marked with
> __attribute__const__, the compiler had to assume the function might
> change variable states as a side-effect (which is not true for ffs(),
> which provides deterministic math results).
> 
> Add missing __attribute_const__ annotations to S390's implementations of
> ffs(), __ffs(), fls(), and __fls() functions. These are pure mathematical
> functions that always return the same result for the same input with no
> side effects, making them eligible for compiler optimization.
> 
> Build tested ARCH=s390 defconfig with GCC s390x-linux-gnu 14.2.0.
> 
> Link: https://github.com/KSPP/linux/issues/364 [1]
> Signed-off-by: Kees Cook <kees@...nel.org>
> ---
>  arch/s390/include/asm/bitops.h | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)

Acked-by: Heiko Carstens <hca@...ux.ibm.com>