From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759658Ab2J3QTm (ORCPT ); Tue, 30 Oct 2012 12:19:42 -0400 Received: from mail.skyhub.de ([78.46.96.112]:34702 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753423Ab2J3QTj (ORCPT ); Tue, 30 Oct 2012 12:19:39 -0400 Date: Tue, 30 Oct 2012 17:19:33 +0100 From: Borislav Petkov To: Daniel Santos Cc: LKML , Andi Kleen , Andrea Arcangeli , Andrew Morton , Christopher Li , Daniel Santos , David Daney , David Howells , Joe Perches , Josh Triplett , Konstantin Khlebnikov , linux-sparse@vger.kernel.org, Michel Lespinasse , Paul Gortmaker , Pavel Pisa , Peter Zijlstra , Steven Rostedt , David Rientjes Subject: Re: [PATCH v4 6/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON} Message-ID: <20121030161933.GD28499@liondog.tnic> Mail-Followup-To: Borislav Petkov , Daniel Santos , LKML , Andi Kleen , Andrea Arcangeli , Andrew Morton , Christopher Li , David Daney , David Howells , Joe Perches , Josh Triplett , Konstantin Khlebnikov , linux-sparse@vger.kernel.org, Michel Lespinasse , Paul Gortmaker , Pavel Pisa , Peter Zijlstra , Steven Rostedt , David Rientjes References: <1351457648-7453-1-git-send-email-daniel.santos@pobox.com> <1351457835-7553-6-git-send-email-daniel.santos@pobox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1351457835-7553-6-git-send-email-daniel.santos@pobox.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 28, 2012 at 03:57:12PM -0500, danielfsantos@att.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. > > Signed-off-by: Daniel Santos > --- > include/linux/bug.h | 4 ++-- > include/linux/compiler.h | 7 +++++++ > 2 files changed, 9 insertions(+), 2 deletions(-) > > diff --git a/include/linux/bug.h b/include/linux/bug.h > index 03259d7..da03dc1 100644 > --- a/include/linux/bug.h > +++ b/include/linux/bug.h > @@ -57,13 +57,13 @@ struct pt_regs; > * track down. > */ > #ifndef __OPTIMIZE__ > -#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) > +#define BUILD_BUG_ON(condition) __compiletime_error_fallback(condition) > #else > #define BUILD_BUG_ON(condition) \ > do { \ > extern void __build_bug_on_failed(void) \ > __compiletime_error("BUILD_BUG_ON failed"); \ > - ((void)sizeof(char[1 - 2*!!(condition)])); \ > + __compiletime_error_fallback(condition); \ > if (condition) \ > __build_bug_on_failed(); \ If we're defining a fallback, shouldn't it come second? I.e.: if (condition) __build_bug_on_failed(); __compiletime_error_fallback(condition); Also, the error message from __build_bug_on_failed is much more informative: arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’: arch/x86/kernel/cpu/amd.c:486:2: error: call to ‘__build_bug_on_failed’ declared with attribute error: BUILD_BUG_ON failed make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [arch/x86/kernel/cpu/] Error 2 than arch/x86/kernel/cpu/amd.c: In function ‘early_init_amd’: arch/x86/kernel/cpu/amd.c:486:2: error: size of unnamed array is negative make[1]: *** [arch/x86/kernel/cpu/amd.o] Error 1 make[1]: *** Waiting for unfinished jobs.... make: *** [arch/x86/kernel/cpu/] Error 2 Finally, you need to do: bool __cond = !!(condition); and use __cond so that condition doesn't get evaluated multiple times (possibly with side effects). Thanks. -- Regards/Gruss, Boris.