Re: [PATCH v5 03/12] x86: implement array_idx_mask

[Ingo Molnar]

* Dan Williams <dan.j.williams@xxxxxxxxx> wrote:

> 'array_idx' uses a mask to sanitize user controllable array indexes,
> i.e. generate a 0 mask if idx >= sz, and a ~0 mask otherwise. While the
> default array_idx_mask handles the carry-bit from the (index - size)
> result in software. The x86 'array_idx_mask' does the same, but the
> carry-bit is handled in the processor CF flag without conditional
> instructions in the control flow.

Same style comments apply as for patch 02.

> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  arch/x86/include/asm/barrier.h |   22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
> index 01727dbc294a..30419b674ebd 100644
> --- a/arch/x86/include/asm/barrier.h
> +++ b/arch/x86/include/asm/barrier.h
> @@ -24,6 +24,28 @@
>  #define wmb()	asm volatile("sfence" ::: "memory")
>  #endif
>  
> +/**
> + * array_idx_mask - generate a mask for array_idx() that is ~0UL when
> + * the bounds check succeeds and 0 otherwise
> + *
> + *     mask = 0 - (idx < sz);
> + */
> +#define array_idx_mask array_idx_mask
> +static inline unsigned long array_idx_mask(unsigned long idx, unsigned long sz)

Please put an extra newline between definitions (even if they are closely related 
as these).

> +{
> +	unsigned long mask;
> +
> +#ifdef CONFIG_X86_32
> +	asm ("cmpl %1,%2; sbbl %0,%0;"
> +#else
> +	asm ("cmpq %1,%2; sbbq %0,%0;"
> +#endif

Wouldn't this suffice:

	asm ("cmp %1,%2; sbb %0,%0;"

... as the word width should automatically be 32 bits on 32-bit kernels and 64 
bits on 64-bit kernels?

Thanks,

	Ingo