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

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Date:   Wed, 19 Feb 2020 13:24:26 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] kernel/extable: Use address-of operator on section
 symbols

On Wed, Feb 19, 2020 at 01:20:37PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> ../kernel/extable.c:37:52: warning: array comparison always evaluates to
> a constant [-Wtautological-compare]
>         if (main_extable_sort_needed && __stop___ex_table > __start___ex_table) {
>                                                           ^
> 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/892
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Gah this should have:

Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>

I can send a v3 if necessary.

> ---
> 
> v1 -> v2: https://lore.kernel.org/lkml/20200219045423.54190-3-natechancellor@gmail.com/
> 
> * No longer a series because there is no prerequisite patch.
> * Use address-of operator instead of casting to unsigned long.
> 
>  kernel/extable.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/extable.c b/kernel/extable.c
> index a0024f27d3a1..88f3251b05e3 100644
> --- a/kernel/extable.c
> +++ b/kernel/extable.c
> @@ -34,7 +34,8 @@ u32 __initdata __visible main_extable_sort_needed = 1;
>  /* Sort the kernel's built-in exception table */
>  void __init sort_main_extable(void)
>  {
> -	if (main_extable_sort_needed && __stop___ex_table > __start___ex_table) {
> +	if (main_extable_sort_needed &&
> +	    &__stop___ex_table > &__start___ex_table) {
>  		pr_notice("Sorting __ex_table...\n");
>  		sort_extable(__start___ex_table, __stop___ex_table);
>  	}
> -- 
> 2.25.1
>