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

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__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 d7a516c6ee ("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>
This commit is contained in:
Vincent Mailhol 2025-03-29 01:48:50 +09:00 committed by Yury Norov
parent e289b48825
commit 243c90e917
2 changed files with 7 additions and 7 deletions
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10
include/linux/build_bug.h
View File

@ -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) \

4
include/linux/compiler.h
View File

@ -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 */