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

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Message-ID: <4b766edb-73e2-c295-22eb-b501405baa9f@akamai.com>
Date:   Thu, 19 Mar 2020 16:03:39 -0400
From:   Jason Baron <jbaron@...mai.com>
To:     Nathan Chancellor <natechancellor@...il.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] dynamic_debug: Use address-of operator on section
 symbols



On 3/18/20 9:59 PM, Nathan Chancellor wrote:
> 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

Works for me.

Acked-by: Jason Baron <jbaron@...mai.com>