Re: [PATCH] arm64: atomics: Dereference matching size

[Ard Biesheuvel]

On Sat, 8 Jan 2022 at 00:28, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> When building with -Warray-bounds (which is desired to be enabled
> globally), the following warning is generated:
>
> In file included from ./arch/arm64/include/asm/lse.h:16,
>                  from ./arch/arm64/include/asm/cmpxchg.h:14,
>                  from ./arch/arm64/include/asm/atomic.h:16,
>                  from ./include/linux/atomic.h:7,
>                  from ./include/asm-generic/bitops/atomic.h:5,
>                  from ./arch/arm64/include/asm/bitops.h:25,
>                  from ./include/linux/bitops.h:33,
>                  from ./include/linux/kernel.h:22,
>                  from kernel/printk/printk.c:22:
> ./arch/arm64/include/asm/atomic_lse.h:247:9: warning: array subscript 'long unsigned int[0]' is partly outside array bounds of 'atomic_t[1]' [-Warray-bounds]
>   247 |         asm volatile(                                                   \
>       |         ^~~
> ./arch/arm64/include/asm/atomic_lse.h:266:1: note: in expansion of macro '__CMPXCHG_CASE'
>   266 | __CMPXCHG_CASE(w,  , acq_, 32,  a, "memory")
>       | ^~~~~~~~~~~~~~
> kernel/printk/printk.c:3606:17: note: while referencing 'printk_cpulock_owner'
>  3606 | static atomic_t printk_cpulock_owner = ATOMIC_INIT(-1);
>       |                 ^~~~~~~~~~~~~~~~~~~~
>
> This is due to the compiler seeing an unsigned long * cast against
> something (atomic_t) that is int sized. Replace the cast with the
> matching size cast. This results in no change in binary output.
>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

Acked-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

> ---
>  arch/arm64/include/asm/atomic_lse.h | 2 +-
>  arch/arm64/include/asm/cmpxchg.h    | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/atomic_lse.h b/arch/arm64/include/asm/atomic_lse.h
> index d955ade5df7c..5d460f6b7675 100644
> --- a/arch/arm64/include/asm/atomic_lse.h
> +++ b/arch/arm64/include/asm/atomic_lse.h
> @@ -249,7 +249,7 @@ __lse__cmpxchg_case_##name##sz(volatile void *ptr,                  \
>         "       mov     %" #w "[tmp], %" #w "[old]\n"                   \
>         "       cas" #mb #sfx "\t%" #w "[tmp], %" #w "[new], %[v]\n"    \
>         "       mov     %" #w "[ret], %" #w "[tmp]"                     \
> -       : [ret] "+r" (x0), [v] "+Q" (*(unsigned long *)ptr),            \
> +       : [ret] "+r" (x0), [v] "+Q" (*(u##sz *)ptr),                    \
>           [tmp] "=&r" (tmp)                                             \
>         : [old] "r" (x1), [new] "r" (x2)                                \
>         : cl);                                                          \
> diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
> index f9bef42c1411..497acf134d99 100644
> --- a/arch/arm64/include/asm/cmpxchg.h
> +++ b/arch/arm64/include/asm/cmpxchg.h
> @@ -243,7 +243,7 @@ static inline void __cmpwait_case_##sz(volatile void *ptr,          \
>         "       cbnz    %" #w "[tmp], 1f\n"                             \
>         "       wfe\n"                                                  \
>         "1:"                                                            \
> -       : [tmp] "=&r" (tmp), [v] "+Q" (*(unsigned long *)ptr)           \
> +       : [tmp] "=&r" (tmp), [v] "+Q" (*(u##sz *)ptr)                   \
>         : [val] "r" (val));                                             \
>  }
>
> --
> 2.30.2
>
>
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