linux-kernel - Re: [PATCH v2] dynamic_debug: Use address-of operator on section symbols

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Date:   Wed, 18 Mar 2020 18:59:09 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jason Baron <jbaron@...mai.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] dynamic_debug: Use address-of operator on section
 symbols

On Wed, Feb 19, 2020 at 10:13:20PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> ../lib/dynamic_debug.c:1034:24: warning: array comparison always
> evaluates to false [-Wtautological-compare]
>         if (__start___verbose == __stop___verbose) {
>                               ^
> 1 warning generated.
> 
> These are not true arrays, they are linker defined symbols, which are
> just addresses. Using the address of operator silences the warning and
> does not change the resulting assembly with either clang/ld.lld or
> gcc/ld (tested with diff + objdump -Dr).
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/894
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
> v1 -> v2: https://lore.kernel.org/lkml/20200219045423.54190-5-natechancellor@gmail.com/
> 
> * No longer a series because there is no prerequisite patch.
> * Use address-of operator instead of casting to unsigned long.
> 
>  lib/dynamic_debug.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
> index aae17d9522e5..8f199f403ab5 100644
> --- a/lib/dynamic_debug.c
> +++ b/lib/dynamic_debug.c
> @@ -1031,7 +1031,7 @@ static int __init dynamic_debug_init(void)
>  	int n = 0, entries = 0, modct = 0;
>  	int verbose_bytes = 0;
>  
> -	if (__start___verbose == __stop___verbose) {
> +	if (&__start___verbose == &__stop___verbose) {
>  		pr_warn("_ddebug table is empty in a CONFIG_DYNAMIC_DEBUG build\n");
>  		return 1;
>  	}
> -- 
> 2.25.1
> 

Gentle ping for review/acceptance.

Cheers,
Nathan