linux-kernel - [PATCH v3 1/3] compiler_types.h: make __builtin_types_compatible

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Date:   Mon, 19 Nov 2018 19:31:41 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Josh Triplett <josh@...htriplett.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        Paul Burton <paul.burton@...s.com>
Subject: [PATCH v3 1/3] compiler_types.h: make __builtin_types_compatible_p() noop for Sparse

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. Make it no-op.

Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
---

Changes in v3:
  - New patch

Changes in v2: None

 include/linux/compiler_types.h | 1 +
 1 file changed, 1 insertion(+)

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)
 #else /* __CHECKER__ */
 # ifdef STRUCTLEAK_PLUGIN
 #  define __user __attribute__((user))
-- 
2.7.4