Messages in this thread | | | From | Masahiro Yamada <> | Date | Tue, 20 Nov 2018 10:32:40 +0900 | Subject | Re: [PATCH v3 1/3] compiler_types.h: make __builtin_types_compatible_p() noop for Sparse |
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On Mon, Nov 19, 2018 at 9:35 PM Luc Van Oostenryck <luc.vanoostenryck@gmail.com> wrote: > > On Mon, Nov 19, 2018 at 07:31:41PM +0900, Masahiro Yamada wrote: > > When I tried to delete BUILD_BUG_ON stubs for sparse, the kbuild test > > robot reported lots of Sparse warnings from container_of(), which > > seem false positive. > > > > The following checker in container_of() seems to be causing something > > strange for Sparse. > > > > BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) && \ > > !__same_type(*(ptr), void), \ > > "pointer type mismatch in container_of()"); \ > > > > I narrowed down the problem into the following test code: > > > > --------------------(test_code.c begin)-------------------- > > struct foo { > > int (*callback)(void); > > }; > > > > void assert(int); > > > > static inline struct foo *get_foo(void) > > { > > assert(__builtin_types_compatible_p(void, void)); > > > > return (struct foo *)0; > > } > > > > int test(void); > > int test(void) > > { > > return get_foo()->callback(); > > } > > ---------------------(test_code.c end)--------------------- > > > > Of course, GCC (and Clang as well) can compile it: > > > > $ gcc -Wall -c -o test_code.o test_code.c > > > > However, Sparse complains about this obviously correct code: > > > > $ sparse test_code.c > > test_code.c:9:45: warning: unknown expression (4 0) > > test_code.c:9:51: warning: unknown expression (4 0) > > > > Interstingly, just removing the 'inline' keyword in the test code > > makes Sparse happy. > > > > I concluded that Sparse cannot handle __builtin_types_compatible_p() > > correctly. > > I think it's only caused by comparing 'void' (which is never > an l-value). > I'll investigate. Thanks for the small test-case.
Yes, please.
> > Make it no-op. > > ... > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h > > index 4a3f9c0..9e7da0b 100644 > > --- a/include/linux/compiler_types.h > > +++ b/include/linux/compiler_types.h > > @@ -23,6 +23,7 @@ > > extern void __chk_user_ptr(const volatile void __user *); > > extern void __chk_io_ptr(const volatile void __iomem *); > > # define ACCESS_PRIVATE(p, member) (*((typeof((p)->member) __force *) &(p)->member)) > > +# define __builtin_types_compatible_p(t1, t2) (1) > > Now, BUILD_BUG_ON() becomes a no-op for sparse but all the other usages > of __builtin_types_compatible_p() become potentially wrong and can now > create their onw false warnings.
You are right. This patch is probably a bad idea.
Thanks.
-- Best Regards Masahiro Yamada
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