From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Wed, 18 Sep 2013 15:45:44 +0200 (CEST) Received: from localhost.localdomain ([127.0.0.1]:51194 "EHLO linux-mips.org" rhost-flags-OK-OK-OK-FAIL) by eddie.linux-mips.org with ESMTP id S6826582Ab3IRNpjVwV2G (ORCPT ); Wed, 18 Sep 2013 15:45:39 +0200 Received: from scotty.linux-mips.net (localhost.localdomain [127.0.0.1]) by scotty.linux-mips.net (8.14.7/8.14.4) with ESMTP id r8IDja8b030107; Wed, 18 Sep 2013 15:45:36 +0200 Received: (from ralf@localhost) by scotty.linux-mips.net (8.14.7/8.14.7/Submit) id r8IDjXf8030106; Wed, 18 Sep 2013 15:45:33 +0200 Date: Wed, 18 Sep 2013 15:45:33 +0200 From: Ralf Baechle To: linux-kernel@vger.kernel.org, Arnd Bergmann , David Howells , Markos Chandras Cc: linux-mips@linux-mips.org Subject: Re: [PATCH v2] MIPS: ath79: Avoid using unitialized 'reg' variable Message-ID: <20130918134533.GN22468@linux-mips.org> References: <1377082042-4219-1-git-send-email-markos.chandras@imgtec.com> <20130903133839.GA10563@linux-mips.org> <5225EC3B.1070701@imgtec.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <5225EC3B.1070701@imgtec.com> User-Agent: Mutt/1.5.21 (2010-09-15) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 37865 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: ralf@linux-mips.org Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Tue, Sep 03, 2013 at 03:03:39PM +0100, Markos Chandras wrote: > >Was this triggered by CONFIG_BUG=n? > > > > Ralf > > > > Hi Ralf, > > Yes it was triggered by CONFIG_BUG=n So here's a small test case to demonstrate the issue: /* * Definition of BUG taken from asm-generic/bug.h for the CONFIG_BUG=n case */ #define BUG() do {} while(0) int foo(int arg) { int res; if (arg == 1) res = 23; else if (arg -= 2) res = 42; else BUG(); return res; } [ralf@h7 linux-mips]$ gcc -O2 -Wall -c bug.c bug.c: In function ‘foo’: bug.c:17:2: warning: ‘res’ may be used uninitialized in this function [-Wmaybe-uninitialized] return res; ^ It's fairly obvious to see what's happening here - GCC doesn't know that the else case can not be reached, thus razorsharply concludes that res may be used uninitialized. I think the definition of BUG should be changed to something like #define BUG() unreachable() This has the disadvantage of of expanding into a while (1) loop for older compilers - but that's only for older compilers, relativly minor in bloat and last I checked BUG() wasn't performance critical ;-) Ralf Signed-off-by: Ralf Baechle include/asm-generic/bug.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h index 7d10f96..6f78771 100644 --- a/include/asm-generic/bug.h +++ b/include/asm-generic/bug.h @@ -108,7 +108,7 @@ extern void warn_slowpath_null(const char *file, const int line); #else /* !CONFIG_BUG */ #ifndef HAVE_ARCH_BUG -#define BUG() do {} while(0) +#define BUG() unreachable() #endif #ifndef HAVE_ARCH_BUG_ON