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From: Nathan Chancellor <natechancellor@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
	Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2] tracing: Use address-of operator on section symbols
Date: Wed, 18 Mar 2020 19:00:04 -0700	[thread overview]
Message-ID: <20200319020004.GB8292@ubuntu-m2-xlarge-x86> (raw)
In-Reply-To: <20200220051011.26113-1-natechancellor@gmail.com>

On Wed, Feb 19, 2020 at 10:10:12PM -0700, Nathan Chancellor wrote:
> Clang warns:
> 
> ../kernel/trace/trace.c:9335:33: warning: array comparison always
> evaluates to true [-Wtautological-compare]
>         if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
>                                        ^
> 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 runtime result of the check (tested with some print
> statements compiled in with clang + ld.lld and gcc + ld.bfd in QEMU).
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/893
> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> 
> v1 -> v2: https://lore.kernel.org/lkml/20200219045423.54190-4-natechancellor@gmail.com/
> 
> * No longer a series because there is no prerequisite patch.
> * Use address-of operator instead of casting to unsigned long.
> 
> NOTE: The code generation does seem to change, unlike every other call
> site that I did this change to but the result of the check remains the
> same as noted in the commit message and I cannot really understand what
> has changed in the assembly. Please let me know if there is something
> catastrophically wrong.
> 
>  kernel/trace/trace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index c797a15a1fc7..78727dd9a6f5 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -9332,7 +9332,7 @@ __init static int tracer_alloc_buffers(void)
>  		goto out_free_buffer_mask;
>  
>  	/* Only allocate trace_printk buffers if a trace_printk exists */
> -	if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
> +	if (&__stop___trace_bprintk_fmt != &__start___trace_bprintk_fmt)
>  		/* Must be called before global_trace.buffer is allocated */
>  		trace_printk_init_buffers();
>  
> -- 
> 2.25.1
> 

Gentle ping for review/acceptance.

Cheers,
Nathan

  reply	other threads:[~2020-03-19  2:00 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-20  5:10 Nathan Chancellor
2020-03-19  2:00 ` Nathan Chancellor [this message]
2020-03-19 14:33   ` Steven Rostedt
2020-03-19 15:27     ` Nick Desaulniers
2020-03-19 16:01       ` Steven Rostedt
2020-03-26 19:46     ` Nathan Chancellor
2020-03-26 21:31       ` Steven Rostedt
2020-03-26 21:34         ` Nathan Chancellor

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