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

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Date:	Mon, 13 Jun 2011 14:06:49 -0400
From:	Andrew Lutomirski <luto@....edu>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	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 4:45 AM, Rakib Mullick <rakib.mullick@...il.com> wrote:
> On Mon, Jun 13, 2011 at 10:54 AM, Rakib Mullick <rakib.mullick@...il.com> wrote:
>> On Mon, Jun 13, 2011 at 8:52 AM, Andrew Lutomirski <luto@....edu> wrote:
>>> On Sun, Jun 12, 2011 at 1:12 AM, Rakib Mullick <rakib.mullick@...il.com> wrote:
>>>> On Sat, Jun 11, 2011 at 5:01 PM, Andrew Lutomirski <luto@....edu> wrote:
>>>>> On Sat, Jun 11, 2011 at 3:31 AM, Rakib Mullick <rakib.mullick@...il.com> wrote:
>>>
>>> I think there are three separate issues here:
>>>
>>> 1. Can ret be used uninitialized?  I say no, even as seen by the
>>> compiler.  If vsyscall_nr is 0, 1, or 2, then ret is initialized.  If
>>> vsyscall_nr is 3, then the BUG gets hit.  BUG is defined as some
>>> assembly magic followed by unreachable(), and the compiler is supposed
>>> to know that code after unreachable() is qunreachable.  So how can ret
>>> be used uninitialized?
>>>
>> I don't have much knowledge of advance assembly, so I really don't
>> understand that part - how BUG handles this. If it really makes sure
>> that, it will handle it properly then I think you can drop this patch.
>>
>>> What version of gcc do you have?  gcc (GCC) 4.6.0 20110530 (Red Hat
>>> 4.6.0-9) does not produce this warning.
>>>
>> Currently, I'm replying from outside my home. I'll let you know when
>> I'm back home.
>>
> Here is my GCC version - gcc version 4.5.1 20100924 (Red Hat 4.5.1-4)
> (GCC). I'm using Fedora 14.

I also have gcc (GCC) 4.5.1 20100924 (Red Hat 4.5.1-4) on another box,
and I still can't reproduce this.

Can you tell me which git revision you're building and send me your
.config and the output of:

$ touch arch/x86/kernel/vsyscall_64.o
$ make V=1 arch/x86/kernel/vsyscall_64.o

Thanks,
Andy
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