[PATCH v8 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

From: danielfsantos
Date: Tue Jan 01 2013 - 17:55:30 EST

Prior to the introduction of __attribute__((error("msg"))) in gcc 4.3,
creating compile-time errors required a little trickery.
BUILD_BUG{,_ON} uses this attribute when available to generate
compile-time errors, but also uses the negative-sized array trick for
older compilers, resulting in two error messages in some cases. The
reason it's "some" cases is that as of gcc 4.4, the negative-sized array
will not create an error in some situations, like inline functions.

This patch replaces the negative-sized array code with the new
__compiletime_error_fallback() macro which expands to the same thing
unless the the error attribute is available, in which case it expands to
do{}while(0), resulting in exactly one compile-time error on all
versions of gcc.

Note that we are not changing the negative-sized array code for the
unoptimized version of BUILD_BUG_ON, since it has the potential to catch
problems that would be disabled in later versions of gcc were
__compiletime_error_fallback used. The reason is that that an
unoptimized build can't always remove calls to an error-attributed
function call (like we are using) that should effectively become dead
code if it were optimized. However, using a negative-sized array with a
similar value will not result in an false-positive (error). The only
caveat being that it will also fail to catch valid conditions, which we
should be expecting in an unoptimized build anyway.

Signed-off-by: Daniel Santos <daniel.santos@xxxxxxxxx>
include/linux/bug.h | 2 +-
include/linux/compiler.h | 5 +++++
2 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/include/linux/bug.h b/include/linux/bug.h
index 08d97e9..57c7688 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -67,7 +67,7 @@ struct pt_regs;
__compiletime_error("BUILD_BUG_ON failed"); \
if (__cond) \
__build_bug_on_failed(); \
- ((void)sizeof(char[1 - 2 * __cond])); \
+ __compiletime_error_fallback(__cond); \
} while(0)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 4c638be..423bb6b 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -307,7 +307,12 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
#ifndef __compiletime_error
# define __compiletime_error(message)
+# define __compiletime_error_fallback(condition) \
+ do { ((void)sizeof(char[1 - 2*!!(condition)])); } while (0)
+# define __compiletime_error_fallback(condition) do { } while (0)
* Prevent the compiler from merging or refetching accesses. The compiler
* is also forbidden from reordering successive instances of ACCESS_ONCE(),

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