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Subject[PATCH v2] compiler.h: give up __compiletime_assert_fallback()
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__compiletime_assert_fallback() is supposed to stop building earlier
by using the negative-array-size method in case the compiler does not
support "error" attribute, but has never worked like that.

You can simply try:

BUILD_BUG_ON(1);

GCC immediately terminates the build, but Clang does not report
anything because Clang does not support the "error" attribute now.
It will later fail at link time, but __compiletime_assert_fallback()
is not working at least.

The root cause is commit 1d6a0d19c855 ("bug.h: prevent double evaluation
of `condition' in BUILD_BUG_ON"). Prior to that commit, BUILD_BUG_ON()
was checked by the negative-array-size method *and* the link-time trick.
Since that commit, the negative-array-size is not effective because
'__cond' is no longer constant. As the comment in <linux/build_bug.h>
says, GCC (and Clang as well) only emits the error for obvious cases.

When '__cond' is a variable,

((void)sizeof(char[1 - 2 * __cond]))

... is not obvious for the compiler to know the array size is negative.

Reverting that commit would break BUILD_BUG() because negative-size-array
is evaluated before the code is optimized out.

Let's give up __compiletime_assert_fallback(). This commit does not
change the current behavior since it just rips off the useless code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
---

Changes in v2:
- Rebase

include/linux/compiler.h | 17 +----------------
1 file changed, 1 insertion(+), 16 deletions(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 681d866..87c776c 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -314,29 +314,14 @@ static inline void *offset_to_ptr(const int *off)
#endif
#ifndef __compiletime_error
# define __compiletime_error(message)
-/*
- * Sparse complains of variable sized arrays due to the temporary variable in
- * __compiletime_assert. Unfortunately we can't just expand it out to make
- * sparse see a constant array size without breaking compiletime_assert on old
- * versions of GCC (e.g. 4.2.4), so hide the array from sparse altogether.
- */
-# ifndef __CHECKER__
-# define __compiletime_error_fallback(condition) \
- do { ((void)sizeof(char[1 - 2 * condition])); } while (0)
-# endif
-#endif
-#ifndef __compiletime_error_fallback
-# define __compiletime_error_fallback(condition) do { } while (0)
#endif

#ifdef __OPTIMIZE__
# define __compiletime_assert(condition, msg, prefix, suffix) \
do { \
- int __cond = !(condition); \
extern void prefix ## suffix(void) __compiletime_error(msg); \
- if (__cond) \
+ if (!(condition)) \
prefix ## suffix(); \
- __compiletime_error_fallback(__cond); \
} while (0)
#else
# define __compiletime_assert(condition, msg, prefix, suffix) do { } while (0)
--
2.7.4
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