linux-kernel - Re: [PATCH] compiler.h: fix error in BUILD_BUG

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Message-ID: <dc53b8704ec674cba636b41d7f55bf507a7bd7aa.camel@perches.com>
Date:   Tue, 31 Mar 2020 11:20:07 -0700
From:   Joe Perches <joe@...ches.com>
To:     Vegard Nossum <vegard.nossum@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Daniel Santos <daniel.santos@...ox.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Ian Abbott <abbotti@....co.uk>
Subject: Re: [PATCH] compiler.h: fix error in BUILD_BUG_ON() reporting

On Tue, 2020-03-31 at 13:26 +0200, Vegard Nossum wrote:
> compiletime_assert() uses __LINE__ to create a unique function name.
> This means that if you have more than one BUILD_BUG_ON() in the same
> source line (which can happen if they appear e.g. in a macro), then
> the error message from the compiler might output the wrong condition.
> 
> For this source file:
> 
> 	#include <linux/build_bug.h>
> 
> 	#define macro() \
> 		BUILD_BUG_ON(1); \
> 		BUILD_BUG_ON(0);
> 
> 	void foo()
> 	{
> 		macro();
> 	}
> 
> gcc would output:
> 
> ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_9’ declared with attribute error: BUILD_BUG_ON failed: 0
>   _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
> 
> However, it was not the BUILD_BUG_ON(0) that failed, so it should say 1
> instead of 0. With this patch, we use __COUNTER__ instead of __LINE__, so
> each BUILD_BUG_ON() gets a different function name and the correct
> condition is printed:
> 
> ./include/linux/compiler.h:350:38: error: call to ‘__compiletime_assert_0’ declared with attribute error: BUILD_BUG_ON failed: 1
>   _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
[]
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
[]
> @@ -347,7 +347,7 @@ static inline void *offset_to_ptr(const int *off)
>   * compiler has support to do so.
>   */
>  #define compiletime_assert(condition, msg) \
> -	_compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
> +	_compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)

This might be better using something like __LINE__ ## _ ## __COUNTER__

as line # is somewhat useful to identify the specific assert in a file.