From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1768216Ab2KOPHs (ORCPT ); Thu, 15 Nov 2012 10:07:48 -0500 Received: from mail.skyhub.de ([78.46.96.112]:56509 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1768160Ab2KOPHq (ORCPT ); Thu, 15 Nov 2012 10:07:46 -0500 Date: Thu, 15 Nov 2012 16:08:04 +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 Subject: Re: [PATCH v5 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON} Message-ID: <20121115150804.GF4956@x1.osrc.amd.com> 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 References: <1352844568-18826-1-git-send-email-daniel.santos@pobox.com> <1352844821-18952-8-git-send-email-daniel.santos@pobox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1352844821-18952-8-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 Tue, Nov 13, 2012 at 04:13:40PM -0600, 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. > > 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 > --- > 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.