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

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Date:	Sat, 15 Dec 2012 01:20:23 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	"H. Peter Anvin" <hpa@...or.com>
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 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).

	Cyrill
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