Re: [PATCH 4/4] asm-generic/bitsperlong.h: Add sanity checks for __BITS_PER_LONG

[David Laight <david.laight.linux@gmail.com>]

On Fri, 16 Jan 2026 08:40:27 +0100
Thomas Weißschuh <thomas.weissschuh@linutronix.de> wrote:

> The value of __BITS_PER_LONG from architecture-specific logic should
> always match the generic one if that is available. It should also match
> the actual C type 'long'.
> 
> Mismatches can happen for example when building the compat vDSO. Either
> during the compilation, see commit 9a6d3ff10f7f ("arm64: uapi: Provide
> correct __BITS_PER_LONG for the compat vDSO"), or when running sparse
> when mismatched CHECKFLAGS are inherited from the kernel build.
> 
> Add some consistency checks which detect such issues early and clearly.
> The tests are added to the UAPI header to make sure it is also used when
> building the vDSO as that is not supposed to use regular kernel headers.
> 
> The kernel-interal BITS_PER_LONG is not checked as it is derived from
> CONFIG_64BIT and therefore breaks for the compat vDSO. See the similar,
> deactivated check in include/asm-generic/bitsperlong.h.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> ---
>  include/uapi/asm-generic/bitsperlong.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/include/uapi/asm-generic/bitsperlong.h b/include/uapi/asm-generic/bitsperlong.h
> index fadb3f857f28..9d762097ae0c 100644
> --- a/include/uapi/asm-generic/bitsperlong.h
> +++ b/include/uapi/asm-generic/bitsperlong.h
> @@ -28,4 +28,18 @@
>  #define __BITS_PER_LONG_LONG 64
>  #endif
>  
> +/* Consistency checks */
> +#ifdef __KERNEL__
> +#if defined(__CHAR_BIT__) && defined(__SIZEOF_LONG__)
> +#if __BITS_PER_LONG != (__CHAR_BIT__ * __SIZEOF_LONG__)
> +#error Inconsistent word size. Check uapi/asm/bitsperlong.h
> +#endif
> +#endif
> +
> +#ifndef __ASSEMBLER__
> +_Static_assert(sizeof(long) * 8 == __BITS_PER_LONG,
> +	       "Inconsistent word size. Check uapi/asm/bitsperlong.h");

nak...

You can't assume the compiler has _Static_assert().
All the ones that do probably define __SIZEOF_LONG__.
You could use something 'old-school' like:
typedef char __inconsistent_long_size[1 - 2 * (sizeof(long) * 8 != __BITS_PER_LONG))];

	David

> +#endif
> +#endif /* __KERNEL__ */
> +
>  #endif /* _UAPI__ASM_GENERIC_BITS_PER_LONG */
>