build_bug.h: more user friendly error messages in BUILD_BUG_ON_ZERO()

__BUILD_BUG_ON_ZERO_MSG(), as introduced in [1], makes it possible to
do a static assertions in expressions. The direct benefit is to
provide a meaningful error message instead of the cryptic negative
bitfield size error message currently returned by BUILD_BUG_ON_ZERO():

  ./include/linux/build_bug.h:16:51: error: negative width in bit-field '<anonymous>'
     16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
        |                                                   ^

Get rid of BUILD_BUG_ON_ZERO()'s bitfield size hack. Instead rely on
__BUILD_BUG_ON_ZERO_MSG() which in turn relies on C11's
_Static_assert().

Use some macro magic, similarly to static_assert(), to either use an
optional error message provided by the user or, when omitted, to
produce a default error message by stringifying the tested
expression. With this, for example:

  BUILD_BUG_ON_ZERO(1 > 0)

would now throw:

  ./include/linux/compiler.h:197:62: error: static assertion failed: "1 > 0 is true"
    197 | define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
        |                                                             ^~~~~~~~~~~~~~

Finally, __BUILD_BUG_ON_ZERO_MSG() is already guarded by an:

  #ifdef __CHECKER__

So no need any more for that guard clause for BUILD_BUG_ON_ZERO().
Remove it.

[1] commit d7a516c ("compiler.h: Fix undefined BUILD_BUG_ON_ZERO()")
Link: https://git.kernel.org/torvalds/c/d7a516c6eeae

Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Link: https://git.kernel.org/next/linux-next/c/b88937277df
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>

Author: vincent-mailhol

include/linux/build_bug.h

@@ -4,17 +4,17 @@
 
 #include <linux/compiler.h>
 
-#ifdef __CHECKER__
-#define BUILD_BUG_ON_ZERO(e) (0)
-#else /* __CHECKER__ */
 /*
  * Force a compilation error if condition is true, but also produce a
  * result (of value 0 and type int), so the expression can be used
  * e.g. in a structure initializer (or where-ever else comma expressions
  * aren't permitted).
+ *
+ * Take an error message as an optional second argument. If omitted,
+ * default to the stringification of the tested expression.
  */
-#define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
-#endif /* __CHECKER__ */
+#define BUILD_BUG_ON_ZERO(e, ...) \
+	__BUILD_BUG_ON_ZERO_MSG(e, ##__VA_ARGS__, #e " is true")
 
 /* Force a compilation error if a constant expression is not a power of 2 */
 #define __BUILD_BUG_ON_NOT_POWER_OF_2(n)	\

include/linux/compiler.h

@@ -192,9 +192,9 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 })
 
 #ifdef __CHECKER__
-#define __BUILD_BUG_ON_ZERO_MSG(e, msg) (0)
+#define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) (0)
 #else /* __CHECKER__ */
-#define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
+#define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
 #endif /* __CHECKER__ */
 
 /* &a[0] degrades to a pointer: a different type from an array */