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Subject: Re: [PATCH] arm64: fix clang warning about TRAMP_VALIAS
To:     Arnd Bergmann <arnd@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        "Russell King (Oracle)" <rmk+kernel@armlinux.org.uk>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev
References: <20220316183833.1563139-1-arnd@kernel.org>
From:   James Morse <james.morse@arm.com>
Message-ID: <92859987-5088-3b2c-3e51-e61222724d23@arm.com>
Date:   Fri, 18 Mar 2022 12:37:12 +0000
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Hi Arnd,

On 3/16/22 6:38 PM, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@arndb.de>
> 
> 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@arm.com>


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);
>   
>