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Date:   Wed, 30 Aug 2017 12:26:44 +0200
From:   Michal Nazarewicz <mina86@...a86.com>
To:     Joe Stringer <joe@....org>, linux-kernel@...r.kernel.org
Cc:     Ian Abbott <abbotti@....co.uk>, Arnd Bergmann <arnd@...db.de>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] compiler: Don't perform compiletime_assert with -O0.

On Tue, Aug 29 2017, 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".
>
> 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) && \
>   ^~~~~~~~~~~~~~~~
>
> [0] http://lkml.kernel.org/r/20170525120316.24473-7-abbotti@mev.co.uk
>
> Signed-off-by: Joe Stringer <joe@....org>
> Cc: Ian Abbott <abbotti@....co.uk>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Michal Nazarewicz <mina86@...a86.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>  include/linux/compiler.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index eca8ad75e28b..b67e5ec9b810 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -517,7 +517,8 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>  # 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_size(volatile void *p, void *res, int s
>  			prefix ## suffix();				\
>  		__compiletime_error_fallback(__cond);			\
>  	} while (0)
> +#else
> +# define __compiletime_assert(condition, msg, prefix, suffix)

+# define __compiletime_assert(condition, msg, prefix, suffix) \
+	do { } while (0)

With that fix, 

Acked-by: Michal Nazarewicz <mina86@...a86.com>

> +#endif
>  
>  #define _compiletime_assert(condition, msg, prefix, suffix) \
>  	__compiletime_assert(condition, msg, prefix, suffix)

-- 
Best regards
ミハウ “𝓶𝓲𝓷𝓪86” ナザレヴイツ
«If at first you don’t succeed, give up skydiving»

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