[PATCH 00/17] Add __attribute_const__ to ffs()-family implementations

[Peter Zijlstra]

On Mon, Aug 04, 2025 at 09:43:56AM -0700, Kees Cook wrote:
> Hi,
> 
> 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 KUnit tests for the family of functions and then add __attribute_const__
> to all architecture implementations and wrappers.
> 
> -Kees
> 
> [1] https://github.com/KSPP/linux/issues/364
> 
> Kees Cook (17):
>   KUnit: Introduce ffs()-family tests
>   bitops: Add __attribute_const__ to generic ffs()-family
>     implementations
>   csky: Add __attribute_const__ to ffs()-family implementations
>   x86: Add __attribute_const__ to ffs()-family implementations
>   powerpc: Add __attribute_const__ to ffs()-family implementations
>   sh: Add __attribute_const__ to ffs()-family implementations
>   alpha: Add __attribute_const__ to ffs()-family implementations
>   hexagon: Add __attribute_const__ to ffs()-family implementations
>   riscv: Add __attribute_const__ to ffs()-family implementations
>   openrisc: Add __attribute_const__ to ffs()-family implementations
>   m68k: Add __attribute_const__ to ffs()-family implementations
>   mips: Add __attribute_const__ to ffs()-family implementations
>   parisc: Add __attribute_const__ to ffs()-family implementations
>   s390: Add __attribute_const__ to ffs()-family implementations
>   xtensa: Add __attribute_const__ to ffs()-family implementations
>   sparc: Add __attribute_const__ to ffs()-family implementations
>   KUnit: ffs: Validate all the __attribute_const__ annotations

Acked-by: Peter Zijlstra (Intel) <peterz at infradead.org>