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Date: Sat, 24 Nov 2018 09:29:19 +0100 From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com> To: Masahiro Yamada <yamada.masahiro@...ionext.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Luc Van Oostenryck <luc.vanoostenryck@...il.com>, Nick Desaulniers <ndesaulniers@...gle.com>, Kees Cook <keescook@...omium.org>, josh@...htriplett.org, linux-kernel <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 3/3] build_bug.h: remove most of dummy BUILD_BUG_ON stubs for Sparse On Thu, Nov 22, 2018 at 5:08 PM Masahiro Yamada <yamada.masahiro@...ionext.com> 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 <yamada.masahiro@...ionext.com> > Acked-by: Kees Cook <keescook@...omium.org> > Reviewed-by: Luc Van Oostenryck <luc.vanoostenryck@...il.com> > Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com> > Tested-by: Nick Desaulniers <ndesaulniers@...gle.com> Nice to see those CHECKER blocks are being reduced! Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@...il.com> Cheers, Miguel
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