Re: [PATCH 4.19] compiler.h: give up __compiletime_assert_fallback()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Thu, May 30, 2019 at 11:01:10PM -0700, Nathan Chancellor wrote:
> From: Masahiro Yamada <yamada.masahiro@socionext.com>
> 
> commit 81b45683487a51b0f4d3b29d37f20d6d078544e4 upstream.
> 
> __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").  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.  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.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> 
> Hi Greg and Sasha,
> 
> Please pick up this patch for 4.19. It fixes an insane amount of spam
> from the drivers/gpu/drm/i915 subsystem because they enable the -Wvla
> warning and we have been carrying it in our CI for a while.

Now applied, thanks.

greg k-h