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