Re: [PATCH v2] compiler.h: give up __compiletime_assert_fallback()

[Daniel Santos <daniel.santos@pobox.com>]

Hello Masahiro,


On 08/25/2018 01:16 PM, Masahiro Yamada wrote:
> __compiletime_assert_fallback() is supposed to stop building earlier
> by using the negative-array-size method in case the compiler does not
> support "error" attribute, but has never worked like that.
>
> You can simply try:
>
>     BUILD_BUG_ON(1);
>
> GCC immediately terminates the build, but Clang does not report
> anything because Clang does not support the "error" attribute now.
> It will later fail at link time, but __compiletime_assert_fallback()
> is not working at least.
>
> The root cause is commit 1d6a0d19c855 ("bug.h: prevent double evaluation
> of `condition' in BUILD_BUG_ON").

I didn't really think this particular patch was necessary, but it was
requested that I eliminate double evaluation and I didn't feel like
arguing it at the time. :)  In my philosophy however, one should *never*
use an expression with side effects in any type of assert.

> Prior to that commit, BUILD_BUG_ON()
> was checked by the negative-array-size method *and* the link-time trick.
> Since that commit, the negative-array-size is not effective because
> '__cond' is no longer constant.

Now we're back to the question of "what do you mean by 'constant'"?  If
you mean a C constant expression (as defined in the C standard) than
almost none of this code fits that criteria.  For these compile-time
assertions to work, we are concerned with the data flow analysis and
constant propagation performed by the compiler during optimization.  You
will notice in include/linux/compiler.h that __compiletime_assert is a
no-op when __OPTIMIZE__ is not defined.

> As the comment in <linux/build_bug.h>
> says, GCC (and Clang as well) only emits the error for obvious cases.
>
> When '__cond' is a variable,
>
>     ((void)sizeof(char[1 - 2 * __cond]))
>
> ... is not obvious for the compiler to know the array size is negative.
>
> Reverting that commit would break BUILD_BUG() because negative-size-array
> is evaluated before the code is optimized out.
>
> Let's give up __compiletime_assert_fallback().  This commit does not
> change the current behavior since it just rips off the useless code.

Clang is not the only target audience of
__compiletime_assert_fallback().  Instead of ripping out something that
may benefit builds with gcc 4.2 and earlier, why not override its
definition in compiler-clang.h with something that will break the build
for Clang?  It would need an #ifndef __compiletime_error_fallback here
though.

> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>
> Changes in v2:
>   - Rebase
>
>  include/linux/compiler.h | 17 +----------------
>  1 file changed, 1 insertion(+), 16 deletions(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 681d866..87c776c 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -314,29 +314,14 @@ static inline void *offset_to_ptr(const int *off)
>  #endif
>  #ifndef __compiletime_error
>  # define __compiletime_error(message)
> -/*
> - * Sparse complains of variable sized arrays due to the temporary variable in
> - * __compiletime_assert. Unfortunately we can't just expand it out to make
> - * sparse see a constant array size without breaking compiletime_assert on old
> - * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
> - */
> -# ifndef __CHECKER__
> -#  define __compiletime_error_fallback(condition) \
> -	do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
> -# endif
> -#endif
> -#ifndef __compiletime_error_fallback
> -# define __compiletime_error_fallback(condition) do { } while (0)
>  #endif
>  
>  #ifdef __OPTIMIZE__
>  # define __compiletime_assert(condition, msg, prefix, suffix)		\
>  	do {								\
> -		int __cond = !(condition);				\
>  		extern void prefix ## suffix(void) __compiletime_error(msg); \
> -		if (__cond)						\
> +		if (!(condition))					\
>  			prefix ## suffix();				\
> -		__compiletime_error_fallback(__cond);			\
>  	} while (0)
>  #else
>  # define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)

To give any more meaningful feedback I think I will need to experiment
with Clang, older GCC versions and icc.  It occurred to me that I should
probably clean up and publish my __builtin_constant_p test program and
also generate results for more recent compilers.  I can extend it to
test various negative sized array constructs and it could help inform
this decision.

IMO, the most ideal solution would be a set of C2x (or future)
extensions providing something similar to C++'s constexpr, GCC's
__builtin_constant_p and our BUILD_BUG_ON.  This would cross deeply into
territory traditionally considered to belong to the implementation, so
it's no small request.  A lot would have to be resolved for it to work
in the standard.

Daniel