Re: [PATCH 14/17] s390: Add __attribute_const__ to ffs()-family implementations
From: Heiko Carstens <hca@linux.ibm.com>
Date: 2025-08-06 10:33:17
Also in:
linux-alpha, linux-arch, linux-hardening, linux-m68k, linux-mips, linux-riscv, linux-s390, linux-sh, lkml, llvm, sparclinux
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@kernel.org> --- arch/s390/include/asm/bitops.h | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)
Acked-by: Heiko Carstens <hca@linux.ibm.com>