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

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Message-ID: <BANLkTimctV0Tp7PP0CaGvmxmxh=yw6dRvw@mail.gmail.com>
Date:	Mon, 13 Jun 2011 10:18:12 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, Jun 13, 2011 at 10:14 AM, Ingo Molnar <mingo@...e.hu> wrote:
>
> * 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.

Fair enough.

Unfortunately there doesn't seem to be an EKERNELBUG error code, and
initializing to EFAULT seems silly.  0 is probably harmless.

I'll wait awhile longer for that GCC version, since there might be a
better fix.  In any case, it would be nice for the changelog entry to
say which version has a warning that's being worked around.

--Andy
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