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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 d7a516c6eeae ("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>

authored by

Vincent Mailhol and committed by
Yury Norov 243c90e9 e289b488

+7 -7
+5 -5
include/linux/build_bug.h
··· 4 4 5 5 #include <linux/compiler.h> 6 6 7 - #ifdef __CHECKER__ 8 - #define BUILD_BUG_ON_ZERO(e) (0) 9 - #else /* __CHECKER__ */ 10 7 /* 11 8 * Force a compilation error if condition is true, but also produce a 12 9 * result (of value 0 and type int), so the expression can be used 13 10 * e.g. in a structure initializer (or where-ever else comma expressions 14 11 * aren't permitted). 12 + * 13 + * Take an error message as an optional second argument. If omitted, 14 + * default to the stringification of the tested expression. 15 15 */ 16 - #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) 17 - #endif /* __CHECKER__ */ 16 + #define BUILD_BUG_ON_ZERO(e, ...) \ 17 + __BUILD_BUG_ON_ZERO_MSG(e, ##__VA_ARGS__, #e " is true") 18 18 19 19 /* Force a compilation error if a constant expression is not a power of 2 */ 20 20 #define __BUILD_BUG_ON_NOT_POWER_OF_2(n) \
+2 -2
include/linux/compiler.h
··· 192 192 }) 193 193 194 194 #ifdef __CHECKER__ 195 - #define __BUILD_BUG_ON_ZERO_MSG(e, msg) (0) 195 + #define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) (0) 196 196 #else /* __CHECKER__ */ 197 - #define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);})) 197 + #define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) ((int)sizeof(struct {_Static_assert(!(e), msg);})) 198 198 #endif /* __CHECKER__ */ 199 199 200 200 /* &a[0] degrades to a pointer: a different type from an array */