linux-kernel - [PATCH v3 2/3] build_bug.h: remove negative-array fallback for BUILD_BUG

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Date:   Mon, 19 Nov 2018 19:31:42 +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>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3 2/3] build_bug.h: remove negative-array fallback for BUILD_BUG_ON()

The kernel can only be compiled with an optimization option (-O2, -Os,
or the currently proposed -Og). Hence, __OPTIMIZE__ is always defined
in the kernel source.

The fallback for the -O0 case is just hypothetical and pointless.
Moreover, commit 0bb95f80a38f ("Makefile: Globally enable VLA warning")
enabled -Wvla warning. The use of variable length arrays is banned.

Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
Acked-by: Kees Cook <keescook@...omium.org>
---

Changes in v3: None
Changes in v2: None

 include/linux/build_bug.h | 14 --------------
 1 file changed, 14 deletions(-)

diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
index 43d1fd5..d415c64 100644
--- a/include/linux/build_bug.h
+++ b/include/linux/build_bug.h
@@ -51,23 +51,9 @@
  * If you have some code which relies on certain constants being equal, or
  * some other compile-time-evaluated condition, you should use BUILD_BUG_ON to
  * detect if someone changes it.
- *
- * The implementation uses gcc's reluctance to create a negative array, but gcc
- * (as of 4.4) only emits that error for obvious cases (e.g. not arguments to
- * inline functions).  Luckily, in 4.3 they added the "error" function
- * attribute just for this type of case.  Thus, we use a negative sized array
- * (should always create an error on gcc versions older than 4.4) and then call
- * an undefined function with the error attribute (should always create an
- * error on gcc 4.3 and later).  If for some reason, neither creates a
- * compile-time error, we'll still have a link-time error, which is harder to
- * track down.
  */
-#ifndef __OPTIMIZE__
-#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)]))
-#else
 #define BUILD_BUG_ON(condition) \
 	BUILD_BUG_ON_MSG(condition, "BUILD_BUG_ON failed: " #condition)
-#endif
 
 /**
  * BUILD_BUG - break compile if used.
-- 
2.7.4