Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751346AbdH3W7M (ORCPT ); Wed, 30 Aug 2017 18:59:12 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:55912 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751016AbdH3W7L (ORCPT ); Wed, 30 Aug 2017 18:59:11 -0400 Date: Wed, 30 Aug 2017 15:59:10 -0700 From: Andrew Morton To: Joe Stringer Cc: linux-kernel@vger.kernel.org, Ian Abbott , Arnd Bergmann , Michal Nazarewicz , Kees Cook Subject: Re: [PATCH] compiler: Don't perform compiletime_assert with -O0. Message-Id: <20170830155910.4e2d43b92ff2d1e4492965d0@linux-foundation.org> In-Reply-To: <20170829230114.11662-1-joe@ovn.org> References: <20170829230114.11662-1-joe@ovn.org> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4389 Lines: 97 On Tue, 29 Aug 2017 16:01:14 -0700 Joe Stringer wrote: > Recent changes[0] to make use of __compiletime_assert() from > container_of() increased the usage of this macro, allowing developers to > notice type conflicts in usage of container_of() at compile time. > However, the implementation of __compiletime_assert relies on compiler > optimizations to report an error. This means that if a developer uses > "-O0" with any code that performs container_of(), the compiler will > always report an error regardless of whether there is an actual problem > in the code. > > This patch disables compile_time_assert when optimizations are disabled > to allow such code to compile with CFLAGS="-O0". I'm wondering if we should backport this into -stable. Probably not, as I doubt if many people use -O0 - it's a pretty weird thing to do. I used to use it a bit because it makes the ".lst" files (intermingled .c and .s files) make more sense. In fact I'm wondering how you even noticed this? So unless disagreed with, I think I'll leave this out of -stable. I redid the changelog somewhat, presenting it as a fix against c7acec713d14c6c: From: Joe Stringer Subject: include/linux/compiler.h: don't perform compiletime_assert with -O0 c7acec713d14c6c ("kernel.h: handle pointers to arrays better in container_of()") made use of __compiletime_assert() from container_of() thus increasing the usage of this macro, allowing developers to notice type conflicts in usage of container_of() at compile time. However, the implementation of __compiletime_assert relies on compiler optimizations to report an error. This means that if a developer uses "-O0" with any code that performs container_of(), the compiler will always report an error regardless of whether there is an actual problem in the code. This patch disables compile_time_assert when optimizations are disabled to allow such code to compile with CFLAGS="-O0". Example compilation failure: ./include/linux/compiler.h:547:38: error: call to `__compiletime_assert_94' declared with attribute error: pointer type mismatch in container_of() _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) ^ ./include/linux/compiler.h:530:4: note: in definition of macro `__compiletime_assert' prefix ## suffix(); \ ^~~~~~ ./include/linux/compiler.h:547:2: note: in expansion of macro `_compiletime_assert' _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__) ^~~~~~~~~~~~~~~~~~~ ./include/linux/build_bug.h:46:37: note: in expansion of macro `compiletime_assert' #define BUILD_BUG_ON_MSG(cond, msg) compiletime_assert(!(cond), msg) ^~~~~~~~~~~~~~~~~~ ./include/linux/kernel.h:860:2: note: in expansion of macro `BUILD_BUG_ON_MSG' BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \ ^~~~~~~~~~~~~~~~ Link: http://lkml.kernel.org/r/20170829230114.11662-1-joe@ovn.org Fixes: c7acec713d14c6c ("kernel.h: handle pointers to arrays better in container_of()") Signed-off-by: Joe Stringer Cc: Ian Abbott Cc: Arnd Bergmann Cc: Michal Nazarewicz Cc: Kees Cook Signed-off-by: Andrew Morton --- include/linux/compiler.h | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff -puN include/linux/compiler.h~compiler-dont-perform-compiletime_assert-with-o0 include/linux/compiler.h --- a/include/linux/compiler.h~compiler-dont-perform-compiletime_assert-with-o0 +++ a/include/linux/compiler.h @@ -517,7 +517,8 @@ static __always_inline void __write_once # define __compiletime_error_fallback(condition) do { } while (0) #endif -#define __compiletime_assert(condition, msg, prefix, suffix) \ +#ifdef __OPTIMIZE__ +# define __compiletime_assert(condition, msg, prefix, suffix) \ do { \ bool __cond = !(condition); \ extern void prefix ## suffix(void) __compiletime_error(msg); \ @@ -525,6 +526,9 @@ static __always_inline void __write_once prefix ## suffix(); \ __compiletime_error_fallback(__cond); \ } while (0) +#else +# define __compiletime_assert(condition, msg, prefix, suffix) +#endif #define _compiletime_assert(condition, msg, prefix, suffix) \ __compiletime_assert(condition, msg, prefix, suffix) _