Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS

[James Morse]

Hi Arnd,

On 3/16/22 6:38 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
>
> The newly introduced TRAMP_VALIAS definition causes a build warning
> with clang-14:
>
> arch/arm64/include/asm/vectors.h:66:31: error: arithmetic on a null pointer treated as a cast from integer to pointer is a GNU extension [-Werror,-Wnull-pointer-arithmetic]
>                  return (char *)TRAMP_VALIAS + SZ_2K * slot;

It is? Good to know!
(clang 11 doesn't seem to be so fussy)

Just for some more background:
TRAMP_VALIAS existed before, but it wasn't defined if KPTI is disabled at compile time, you can't
do that without CONFIG_EXPERT.
I thought these dummy definitions would be cleaner than #ifdef'ing the code.


> Change the addition to something clang does not complain about.

Acked-by: James Morse <james.morse@xxxxxxx>


Thanks!

James



> I see this warning on 5.17-rc8, but did not test it on linux-next,
> which may already have a fix.
>
> diff --git a/arch/arm64/include/asm/vectors.h b/arch/arm64/include/asm/vectors.h
> index f64613a96d53..bc9a2145f419 100644
> --- a/arch/arm64/include/asm/vectors.h
> +++ b/arch/arm64/include/asm/vectors.h
> @@ -56,14 +56,14 @@ enum arm64_bp_harden_el1_vectors {
>   DECLARE_PER_CPU_READ_MOSTLY(const char *, this_cpu_vector);
>   
>   #ifndef CONFIG_UNMAP_KERNEL_AT_EL0
> -#define TRAMP_VALIAS	0
> +#define TRAMP_VALIAS	0ul
>   #endif
>   
>   static inline const char *
>   arm64_get_bp_hardening_vector(enum arm64_bp_harden_el1_vectors slot)
>   {
>   	if (arm64_kernel_unmapped_at_el0())
> -		return (char *)TRAMP_VALIAS + SZ_2K * slot;
> +		return (char *)(TRAMP_VALIAS + SZ_2K * slot);
>   
>   	WARN_ON_ONCE(slot == EL1_VECTOR_KPTI);
>