Re: [PATCH] arm64: Fix warning in mte_get_random_tag()

[Ard Biesheuvel]

On Thu, 11 Feb 2021 at 13:57, Vincenzo Frascino
<vincenzo.frascino@xxxxxxx> wrote:
>
> The simplification of mte_get_random_tag() caused the introduction of the
> warning below:
>
> In file included from arch/arm64/include/asm/kasan.h:9,
>                  from include/linux/kasan.h:16,
>                  from mm/kasan/common.c:14:
> mm/kasan/common.c: In function ‘mte_get_random_tag’:
> arch/arm64/include/asm/mte-kasan.h:45:9: warning: ‘addr’ is used
>                                          uninitialized [-Wuninitialized]
>    45 |         asm(__MTE_PREAMBLE "irg %0, %0"
>       |
>
> Fix the warning initializing the address to NULL.
>
> Note: mte_get_random_tag() returns a tag and it never dereferences the address,
> hence 'addr' can be safely initialized to NULL.
>
> Fixes: c8f8de4c0887 ("arm64: kasan: simplify and inline MTE functions")
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@xxxxxxx>
> ---
>
> This patch is based on linux-next/akpm
>
>  arch/arm64/include/asm/mte-kasan.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/mte-kasan.h b/arch/arm64/include/asm/mte-kasan.h
> index 3d58489228c0..b2850b750726 100644
> --- a/arch/arm64/include/asm/mte-kasan.h
> +++ b/arch/arm64/include/asm/mte-kasan.h
> @@ -40,7 +40,12 @@ static inline u8 mte_get_mem_tag(void *addr)
>  /* Generate a random tag. */
>  static inline u8 mte_get_random_tag(void)
>  {
> -       void *addr;
> +       /*
> +        * mte_get_random_tag() returns a tag and it
> +        * never dereferences the address, hence addr
> +        * can be safely initialized to NULL.
> +        */
> +       void *addr = NULL;
>
>         asm(__MTE_PREAMBLE "irg %0, %0"
>                 : "+r" (addr));
> --
> 2.30.0
>

Might it be better to simply change the asm constraint to "=r" ?