linux-kernel - Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit kernel

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Date:	Fri, 14 Dec 2012 13:21:48 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
CC:	Andy Lutomirski <luto@...capital.net>, aarcange@...hat.com,
	ak@...ux.intel.com, Pavel Emelyanov <xemul@...allels.com>,
	Stefani Seibold <stefani@...bold.net>, x86@...nel.org,
	linux-kernel@...r.kernel.org, criu@...nvz.org, mingo@...hat.com,
	john.stultz@...aro.org, tglx@...utronix.de
Subject: Re: [CRIU] [PATCH] Add VDSO time function support for x86 32-bit
 kernel

On 12/14/2012 01:20 PM, Cyrill Gorcunov wrote:
> On Fri, Dec 14, 2012 at 01:08:35PM -0800, H. Peter Anvin wrote:
>> On 12/14/2012 12:12 PM, Cyrill Gorcunov wrote:
>>>>>
>>>> The real issue is that happens if the process is checkpointed while
>>>> inside the vdso and now eip/rip or a stack frame points into the vdso.
>>>> This is not impossible or even unlikely, especially on 32 bits it is
>>>> downright likely.
>>>
>>> I fear if there are stacked ip which point to vdso -- we simply won't
>>> be able to restore properly if vdso internal format changed significantly
>>> between kernel versions. (At moment we restore vdso exactly at same position
>>> it was on checkpoint stage with same content, iirc).
>>>
>>
>> I don't think there is a way around that.  It is completely unreasonable
>> to say that the vdso cannot change between kernel versions, for obvious
>> reasons.  It's worse than "significantly"... changing even one
>> instruction makes it plausible your eip/rip will point into the middle
>> of an instruction.
> 
> Well, one idea was to try to escape dumping when a dumpee inside vdso area
> and wait until it leaves this zone, then proceed dumping. Then, if vdso is
> changed (say some new instructions were added) we zap original prologues
> with jmp to new symbols from fresh vdso provided us by a kernel. I'm not
> really sure if this would help us much but just saying (I must admit I
> didn't looked yet into vdso implementation details, so sorry if it sounds
> stupid).
> 

Well, if the vdso contains a system call you may be waiting indefinitely.

	-hpa


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