linux-kernel - Re: [PATCH] arm64/mm: fix variable 'tag' set but not used

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Date:   Thu, 1 Aug 2019 13:01:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Qian Cai <cai@....pw>
Cc:     catalin.marinas@....com, andreyknvl@...gle.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/mm: fix variable 'tag' set but not used

On Wed, Jul 31, 2019 at 04:38:18PM -0400, Qian Cai wrote:
> When CONFIG_KASAN_SW_TAGS=n, set_tag() is compiled away. GCC throws a
> warning,
> 
> mm/kasan/common.c: In function '__kasan_kmalloc':
> mm/kasan/common.c:464:5: warning: variable 'tag' set but not used
> [-Wunused-but-set-variable]
>   u8 tag = 0xff;
>      ^~~
> 
> Fix it by making __tag_set() a static inline function.
> 
> Signed-off-by: Qian Cai <cai@....pw>
> ---
>  arch/arm64/include/asm/memory.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index b7ba75809751..9645b1340afe 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -210,7 +210,11 @@ static inline unsigned long kaslr_offset(void)
>  #define __tag_reset(addr)	untagged_addr(addr)
>  #define __tag_get(addr)		(__u8)((u64)(addr) >> 56)
>  #else
> -#define __tag_set(addr, tag)	(addr)
> +static inline const void *__tag_set(const void *addr, u8 tag)
> +{
> +	return addr;
> +}

Why doesn't this trigger a warning in page_to_virt(), which passes an
unsigned long for the address parameter?

Will