On Thu, Nov 22, 2018 at 5:08 PM Masahiro Yamada <[email protected]> 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 enabled only very recently), > 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 positives, and noisier than > it should be at that time. > > 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. > > 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 <[email protected]> > Acked-by: Kees Cook <[email protected]> > Reviewed-by: Luc Van Oostenryck <[email protected]> > Reviewed-by: Nick Desaulniers <[email protected]> > Tested-by: Nick Desaulniers <[email protected]>
Nice to see those CHECKER blocks are being reduced! Acked-by: Miguel Ojeda <[email protected]> Cheers, Miguel
