On Mon, Nov 19, 2018 at 9:35 PM Luc Van Oostenryck
<luc.vanoostenr...@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
