linux-kernel - Re: [PATCH v5 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,

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Message-ID: <20121115150804.GF4956@x1.osrc.amd.com>
Date:	Thu, 15 Nov 2012 16:08:04 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Daniel Santos <daniel.santos@...ox.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christopher Li <sparse@...isli.org>,
	Daniel Santos <daniel.santos@...ox.com>,
	David Daney <david.daney@...ium.com>,
	David Howells <dhowells@...hat.com>,
	Joe Perches <joe@...ches.com>,
	Josh Triplett <josh@...htriplett.org>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Pavel Pisa <pisa@....felk.cvut.cz>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v5 8/9] compiler.h, bug.h: Prevent double error messages
 with BUILD_BUG{,_ON}

On Tue, Nov 13, 2012 at 04:13:40PM -0600, danielfsantos@....net wrote:
> Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
> creating compile-time errors required a little trickery.
> BUILD_BUG{,_ON} uses this attribute when available to generate
> compile-time errors, but also uses the negative-sized array trick for
> older compilers, resulting in two error messages in some cases.  The
> reason it's "some" cases is that as of gcc 4.4, the negative-sized array
> will not create an error in some situations, like inline functions.
> 
> This patch replaces the negative-sized array code with the new
> __compiletime_error_fallback() macro which expands to the same thing
> unless the the error attribute is available, in which case it expands to
> do{}while(0), resulting in exactly one compile-time error on all
> versions of gcc.
> 
> Note that we are not changing the negative-sized array code for the
> unoptimized version of BUILD_BUG_ON, since it has the potential to catch
> problems that would be disabled in later versions of gcc were
> __compiletime_error_fallback used.  The reason is that that an
> unoptimized build can't always remove calls to an error-attributed
> function call (like we are using) that should effectively become dead
> code if it were optimized.  However, using a negative-sized array with a
> similar value will not result in an false-positive (error). The only
> caveat being that it will also fail to catch valid conditions, which we
> should be expecting in an unoptimized build anyway.
> 
> Signed-off-by: Daniel Santos <daniel.santos@...ox.com>
> ---
>  include/linux/bug.h      |    2 +-
>  include/linux/compiler.h |    5 +++++
>  2 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/bug.h b/include/linux/bug.h
> index dd4f506..125e744 100644
> --- a/include/linux/bug.h
> +++ b/include/linux/bug.h
> @@ -66,7 +66,7 @@ struct pt_regs;
>  			__compiletime_error("BUILD_BUG_ON failed");	\
>  		if (__cond)						\
>  			__build_bug_on_failed();			\
> -		((void)sizeof(char[1 - 2*!!(__cond)]));			\
> +		__compiletime_error_fallback(__cond);			\

We're passing an already evaluated __cond here which gets doubly-negated
again in __compiletime_error_fallback. If __compiletime_error_fallback
is going to be called only from BUILD_BUG_ON, then its definition should
be:

	do { ((void)sizeof(char[1 - 2 * (condition)])); } while (0)

i.e., without the !!.


>  	} while(0)
>  #endif
>  
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index cbf6d9d..8e5b9d5 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -298,7 +298,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>  #endif
>  #ifndef __compiletime_error
>  # define __compiletime_error(message)
> +# define __compiletime_error_fallback(condition) \
> +	do { ((void)sizeof(char[1 - 2*!!(condition)])); } while (0)
> +#else
> +# define __compiletime_error_fallback(condition) do { } while (0)
>  #endif
> +
>  /*
>   * Prevent the compiler from merging or refetching accesses.  The compiler
>   * is also forbidden from reordering successive instances of ACCESS_ONCE(),
> -- 
> 1.7.3.4

Thanks.

-- 
Regards/Gruss,
Boris.
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