Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751323AbdH3XXW (ORCPT ); Wed, 30 Aug 2017 19:23:22 -0400 Received: from relay2-d.mail.gandi.net ([217.70.183.194]:51986 "EHLO relay2-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750972AbdH3XXV (ORCPT ); Wed, 30 Aug 2017 19:23:21 -0400 X-Originating-IP: 209.85.215.47 MIME-Version: 1.0 In-Reply-To: <20170830155910.4e2d43b92ff2d1e4492965d0@linux-foundation.org> References: <20170829230114.11662-1-joe@ovn.org> <20170830155910.4e2d43b92ff2d1e4492965d0@linux-foundation.org> From: Joe Stringer Date: Wed, 30 Aug 2017 16:22:58 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] compiler: Don't perform compiletime_assert with -O0. To: Andrew Morton Cc: LKML , Ian Abbott , Arnd Bergmann , Michal Nazarewicz , Kees Cook Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5157 Lines: 108 On 30 August 2017 at 15:59, Andrew Morton wrote: > 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? Local debugging, was trying to get a better understanding of the underlying assembly and the code I was using just happened to use container_of(). I doubt this is going to affect a large number of people, and most developers will rebase against something newish on a regular basis so I personally wouldn't push to apply against -stable. > 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 The commit message update looks fine, but it looks like this is v1 not v2 (see the #else part). Thanks, Joe