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

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Date:   Wed, 18 Mar 2020 19:00:04 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] tracing: Use address-of operator on section symbols

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@...gle.com>
> Signed-off-by: Nathan Chancellor <natechancellor@...il.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