On Thu, May 30, 2019 at 11:01:10PM -0700, Nathan Chancellor wrote:
> From: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
>
> 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@xxxxxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
>
> 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
