On Mon, Nov 19, 2018 at 4:37 AM Luc Van Oostenryck <luc.vanoostenr...@gmail.com> wrote: > > On Mon, Nov 19, 2018 at 07:31:43PM +0900, Masahiro Yamada wrote: > > The introduction of these dummy BUILD_BUG_ON stubs dates back to > > commit 903c0c7cdc21 ("sparse: define dummy BUILD_BUG_ON definition > > for sparse"). > > > > At that time, BUILD_BUG_ON() was implemented with the negative array > > trick *and* the link-time trick, like this: > > > > extern int __build_bug_on_failed; > > #define BUILD_BUG_ON(condition) \ > > do { \ > > ((void)sizeof(char[1 - 2*!!(condition)])); \ > > if (condition) __build_bug_on_failed = 1; \ > > } while(0) > > > > Sparse is more strict about the negative array trick than GCC because > > Sparse requires the array length to be really constant. > > > > Here is the simple test code for the macro above: > > > > static const int x = 0; > > BUILD_BUG_ON(x); > > > > GCC is absolutely fine with it (-Wvla was not enabled at that time), > > but Sparse warns like this: > > > > error: bad constant expression > > error: cannot size expression > > > > (If you are using a newer version of Sparse, you will see a different > > warning message, "warning: Variable length array is used".) > > > > Anyway, Sparse was producing many false positive warnings, hence > > silenced. > > > > With the previous commit, the leftover negative array trick is gone. > > Sparse is fine with the current BUILD_BUG_ON(), which is implemented > > by using the 'error' attribute. (assuming your Sparse version supports > > -Wno-unknown-attribute option) > > > > I am keeping the stub for BUILD_BUG_ON_ZERO(). Otherwise, Sparse > > would complain about the following code, which GCC is fine with: > > > > static const int x = 0; > > int y = BUILD_BUG_ON_ZERO(x); > > > > Signed-off-by: Masahiro Yamada <yamada.masah...@socionext.com> > > Acked-by: Kees Cook <keesc...@chromium.org> > > Reviewed-by: Luc Van Oostenryck <luc.vanoostenr...@gmail.com>
Clang builds not affected. Tested a quick arm64 defconfig build with Clang + this patch. Reviewed-by: Nick Desaulniers <ndesaulni...@google.com> Tested-by: Nick Desaulniers <ndesaulni...@google.com> -- Thanks, ~Nick Desaulniers