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>
