lists.openwall.net   lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  PHC 
Open Source and information security mailing list archives
 
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date:	Tue,  1 Jan 2013 16:54:10 -0600
From:	danielfsantos@....net
To:	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Christopher Li <sparse@...isli.org>,
	David Daney <david.daney@...ium.com>,
	David Rientjes <rientjes@...gle.com>,
	Joe Perches <joe@...ches.com>,
	Josh Triplett <josh@...htriplett.org>,
	linux-sparse@...r.kernel.org,
	Michel Lespinasse <walken@...gle.com>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	Daniel Santos <daniel.santos@...ox.com>
Subject: [PATCH v8 8/9] compiler.h, bug.h: Prevent double error messages with BUILD_BUG{,_ON}

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@...ox.com>
---
 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)
 #endif
 
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);
 #endif
 #ifndef __compiletime_error
 # define __compiletime_error(message)
+# define __compiletime_error_fallback(condition) \
+	do { ((void)sizeof(char[1 - 2*!!(condition)])); } while (0)
+#else
+# define __compiletime_error_fallback(condition) do { } while (0)
 #endif
+
 /*
  * Prevent the compiler from merging or refetching accesses.  The compiler
  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
-- 
1.7.8.6

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Powered by blists - more mailing lists