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

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Message-ID: <BANLkTi=2kw+8ijdPDNva11Ct9vr0p_uhmA@mail.gmail.com>
Date:	Mon, 13 Jun 2011 23:05:33 +0600
From:	Rakib Mullick <rakib.mullick@...il.com>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Ingo Molnar <mingo@...e.hu>, 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 8:18 PM, Andrew Lutomirski <luto@....edu> wrote:
> 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.
>
Well, I think I already posted the GCC version in this thread. Anyway,
for you're convenience, here is my GCC version: gcc version 4.5.1
20100924 (Red Hat 4.5.1-4). I'm using Fedora Core 14.


Thanks,
Rakib
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