linux-kernel - Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall

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Message-ID: <20110613141408.GA16126@elte.hu>
Date:	Mon, 13 Jun 2011 16:14:08 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Rakib Mullick <rakib.mullick@...il.com>, hpa@...or.com,
	tglx@...utronix.de, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86, vsyscall: Fix build warning in vsyscall_64.c


* Andrew Lutomirski <luto@....edu> wrote:

> On Mon, Jun 13, 2011 at 5:29 AM, Ingo Molnar <mingo@...e.hu> wrote:
> >
> > * Andrew Lutomirski <luto@....edu> wrote:
> >
> >> On Sat, Jun 11, 2011 at 3:31 AM, Rakib Mullick <rakib.mullick@...il.com> wrote:
> >> > Due to commit 5cec93c216db77 (x86-64: Emulate legacy vsyscalls), we get the following warning:
> >> >
> >> >   arch/x86/kernel/vsyscall_64.c: In function ‘do_emulate_vsyscall’:
> >> >   arch/x86/kernel/vsyscall_64.c:111:7: warning: ‘ret’ may be used uninitialized in this function
> >>
> >> What's the code path that uses ret without initializing it?
> >
> > If the code is correct but GCC got confused then please use the
> > simplest possible patch to help GCC find its way around the code.
> 
> The simplest patch is to mark ret as uninitialized_var.

No - that primitive really sucks as it might hide *future* debug 
warnings and silently break code.

The problem with uninitialized_var() is that such code:

	int test(void)
	{
		int uninitialized_var(ret);

		return ret;
	}

Builds without a single warning but it is very broken code.

So if we use uninitialized_var() and the code is changed in the 
future to have the above broken sequence, we'll have a silent runtime 
failure ...

So we try to avoid using uninitialized_var() in arch/x86/ and use 
explicit initialization instead.

That way GCC that can see through the flow will optimize away the 
superfluous initialization - GCC versions that are older will 
generate one more instruction but that's OK.

Thanks,

	Ingo
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