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

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Message-ID: <20200331223638.GA53668@ubuntu-m2-xlarge-x86>
Date:   Tue, 31 Mar 2020 15:36:38 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Daniel Santos <daniel.santos@...ox.com>
Cc:     Vegard Nossum <vegard.nossum@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        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, Mar 31, 2020 at 05:25:38PM -0500, Daniel Santos wrote:
> On 3/31/20 6:26 AM, 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__)
> >
> > Cc: Daniel Santos <daniel.santos@...ox.com>
> > Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> > Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > Cc: Ian Abbott <abbotti@....co.uk>
> > Signed-off-by: Vegard Nossum <vegard.nossum@...cle.com>
> > ---
> >  include/linux/compiler.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 5e88e7e33abec..034b0a644efcc 100644
> > --- 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__)
> >  
> >  #define compiletime_assert_atomic_type(t)				\
> >  	compiletime_assert(__native_word(t),				\
> 
> This will break builds using gcc 4.2 and earlier and the expectation was
> that you don't put two of them on the same line -- not helpful in macros
> where it all must be on the same line.  Is gcc 4.2 still supported?  If
> so, I recommend using another macro for the unique number that uses
> __COUNTER__ if available and __LINE__ otherwise.  This was the decision
> for using __LINE__ when I wrote the original anyway.
> 
> Also note that this construct:
> 
> BUILD_BUG_ON_MSG(0, "I like chicken"); BUILD_BUG_ON_MSG(1, "I don't like
> chicken");
> 
> will incorrectly claim that I like chicken.  This is because of how
> __attribute__((error)) works -- gcc will use the first declaration to
> define the error message.
> 
> I couple of years ago, I almost wrote a gcc extension to get rid of this
> whole mess and just __builtin_const_assert(cond, msg).  Maybe I'll
> finish that this year.
> 
> Daniel

No, GCC 4.6 is the minimum required version and it is highly likely that
the minimum version of GCC will be raised to 4.8 soon:

https://lore.kernel.org/lkml/20200123153341.19947-10-will@kernel.org/
https://git.kernel.org/peterz/queue/c/0e75b883b400ac4b1dafafe3cbd2e0a39b714232

Cheers,
Nathan