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Re: [PATCH v2] MIPS: ath79: Avoid using unitialized 'reg' variable

To: linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Markos Chandras <Markos.Chandras@imgtec.com>
Subject: Re: [PATCH v2] MIPS: ath79: Avoid using unitialized 'reg' variable
From: Ralf Baechle <ralf@linux-mips.org>
Date: Wed, 18 Sep 2013 15:45:33 +0200
Cc: linux-mips@linux-mips.org
In-reply-to: <5225EC3B.1070701@imgtec.com>
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References: <1377082042-4219-1-git-send-email-markos.chandras@imgtec.com> <20130903133839.GA10563@linux-mips.org> <5225EC3B.1070701@imgtec.com>
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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 <ralf@linux-mips.org>

 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

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