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SubjectRe: [PATCH v3 1/3] compiler_types.h: make __builtin_types_compatible_p() noop for Sparse
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.

> 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.

Regards,
-- Luc

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