Re: [PATCH] powerpc: Avoid clang uninitialized warning in __get_user_size_allowed

[Christophe Leroy]

Le 26/04/2021 à 22:35, Nathan Chancellor a écrit :
> Commit 9975f852ce1b ("powerpc/uaccess: Remove calls to __get_user_bad()
> and __put_user_bad()") switch to BUILD_BUG() in the default case, which
> leaves x uninitialized. This will not be an issue because the build will
> be broken in that case but clang does static analysis before it realizes
> the default case will be done so it warns about x being uninitialized
> (trimmed for brevity):
>
>   In file included from mm/mprotect.c:13:
>   In file included from ./include/linux/hugetlb.h:28:
>   In file included from ./include/linux/mempolicy.h:16:
>   ./include/linux/pagemap.h:772:16: warning: variable '__gu_val' is used
>   uninitialized whenever switch default is taken [-Wsometimes-uninitialized]
>                   if (unlikely(__get_user(c, uaddr) != 0))
>                                ^~~~~~~~~~~~~~~~~~~~
>   ./arch/powerpc/include/asm/uaccess.h:266:2: note: expanded from macro '__get_user'
>           __get_user_size_allowed(__gu_val, __gu_addr, __gu_size, __gu_err);      \
>           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>   ./arch/powerpc/include/asm/uaccess.h:235:2: note: expanded from macro
>   '__get_user_size_allowed'
>          default: BUILD_BUG();                                   \
>          ^~~~~~~
>
> Commit 5cd29b1fd3e8 ("powerpc/uaccess: Use asm goto for get_user when
> compiler supports it") added an initialization for x because of the same
> reason. Do the same thing here so there is no warning across all
> versions of clang.

Ah yes, I tested with Clang 11 which has CONFIG_CC_HAS_ASM_GOTO_OUTPUT, that's the reason why I hit 
that warning only in the CONFIG_CC_HAS_ASM_GOTO_OUTPUT branch.

But regardless, is that normal that Clang warns that on a never taken branch ? That's puzzling.

> Link: https://github.com/ClangBuiltLinux/linux/issues/1359
> Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>

Acked-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>

> ---
>   arch/powerpc/include/asm/uaccess.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index a4e791bcd3fe..a09e4240c5b1 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -232,7 +232,7 @@ do {								\
>   	case 2: __get_user_asm(x, (u16 __user *)ptr, retval, "lhz"); break;	\
>   	case 4: __get_user_asm(x, (u32 __user *)ptr, retval, "lwz"); break;	\
>   	case 8: __get_user_asm2(x, (u64 __user *)ptr, retval);  break;	\
> -	default: BUILD_BUG();					\
> +	default: x = 0; BUILD_BUG();				\
>   	}							\
>   } while (0)
>   
>
> base-commit: ee6b25fa7c037e42cc5f3b5c024b2a779edab6dd