Messages in this thread Patch in this message | | | Date | Wed, 30 Aug 2017 15:59:10 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] compiler: Don't perform compiletime_assert with -O0. |
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On Tue, 29 Aug 2017 16:01:14 -0700 Joe Stringer <joe@ovn.org> 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 <joe@ovn.org> 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 <joe@ovn.org> Cc: Ian Abbott <abbotti@mev.co.uk> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Michal Nazarewicz <mina86@mina86.com> Cc: Kees Cook <keescook@chromium.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> ---
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) _
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