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

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Date:   Thu, 19 Mar 2020 14:43:31 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Jason Baron <jbaron@...mai.com>,
        Andrew Morton <akpm@...ux-foundation.org>
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 Thu, Mar 19, 2020 at 04:03:39PM -0400, Jason Baron wrote:
> 
> 
> 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>

Thank you!

Andrew, I assume you'll pick this up?

Cheers,
Nathan