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linux/arch/arc
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Kees Cook e8e0637b85 arc: Fix __fls() const-foldability via __builtin_clzl() 
[ Upstream commit a3fecb9160 ]

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).

For arc architecture with CONFIG_ISA_ARCV2=y, the __fls() function
uses __builtin_arc_fls() which lacks GCC's const attribute, preventing
compile-time constant folding, and KUnit testing of ffs/fls fails on
arc[3]. A patch[2] to GCC to solve this has been sent.

Add a fix for this by handling compile-time constants with the standard
__builtin_clzl() builtin (which has const attribute) while preserving
the optimized arc-specific builtin for runtime cases. This has the added
benefit of skipping runtime calculation of compile-time constant values.
Even with the GCC bug fixed (which is about "attribute const") this is a
good change to avoid needless runtime costs, and should be done
regardless of the state of GCC's bug.

Build tested ARCH=arc allyesconfig with GCC arc-linux 15.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Link: https://gcc.gnu.org/pipermail/gcc-patches/2025-August/693273.html
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508031025.doWxtzzc-lkp@intel.com/ [3]
Signed-off-by: Kees Cook <kees@kernel.org>
Acked-by: Vineet Gupta <vgupta@kernel.org>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-11-13 15:34:03 -05:00
..
boot Kbuild updates for v6.10 2024-05-18 12:39:20 -07:00
configs ARC: configs: enable I2C_DESIGNWARE_CORE with I2C_DESIGNWARE_PLATFORM 2024-09-10 00:36:50 +02:00
include arc: Fix __fls() const-foldability via __builtin_clzl() 2025-11-13 15:34:03 -05:00
kernel arch: Add the macro COMPILE_OFFSETS to all the asm-offsets.c 2025-11-02 22:15:22 +09:00
lib ARCv2: memset: don't prefetch for len == 0 which happens a alot 2023-08-17 12:40:26 -07:00
mm mm: make arch_get_unmapped_area() take vm_flags by default 2024-09-09 16:39:13 -07:00
net ARC: bpf: Correct conditional check in 'check_jmp_32' 2025-01-09 13:33:45 +01:00
plat-axs10x arc: Explicitly include correct DT includes 2023-08-23 22:28:53 -07:00
plat-hsdk arc: Replace lkml.org links with lore 2021-12-28 19:48:49 -08:00
plat-sim ARC: merge HAPS-HS with nSIM-HS configs 2019-10-28 12:12:31 -07:00
plat-tb10x treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 333 2019-06-05 17:37:06 +02:00
Kbuild ARC: Add eBPF JIT support 2024-05-12 16:51:36 -07:00
Kconfig ARC: build: disallow invalid PAE40 + 4K page config 2025-01-09 13:33:45 +01:00
Kconfig.debug Kconfig: consolidate the "Kernel hacking" menu 2018-08-02 08:06:48 +09:00
Makefile ARC: build: Try to guess GCC variant of cross compiler 2025-01-09 13:33:47 +01:00