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

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Date:	Sat, 15 Dec 2012 02:25:17 +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 02:00:17PM -0800, H. Peter Anvin wrote:
> On 12/14/2012 01:27 PM, Andy Lutomirski wrote:
> > 
> > I don't know all that much about the linux vm.  Can we create a
> > special vdso address_space or struct inode or something so that a
> > single vma can contain pages with different flags?
> > 
> 
> No, that is still different vmas, but it probably isn't a big deal.
> 
> The advantage of having an inode/namespace is that it lets you use
> mmap() as opposed to mremap() with it, which might be useful, I don't know.
> 
> One option for the checkpoint people might actually be to not use the
> vdso for a process that needs to be checkpointed and restarted on a
> different machine or different kernel version.  Instead they can install
> a pseudo-vdso which just calls normal system calls, and is simply a
> static piece of code that makes normal system calls ... since the
> internals of the kernel are hidden from userspace it is "clean" that way.
> 
> With any actual vdso you risk something like:
> 

Is there a chance to make it something like that (assuming the
dumpee is ptraced)

> 	-> vdso entry

mark task as vdso-entered

> 	-> signal received, transfer to signal handler
> 	-> signal handler exit

before task leave vdso the task mark vdso-entered get cleaned
and if ptraced, the ptracing task is notified

> ... and now you return to the address in the old vdso, but the internals
> of the vdso may have changed.

this would allow us to defer checkpoint until task finish vdso code. Peter,
if I understand you correctly you propose we provide some own proxy-vdso
which would redirect calls to real ones, right? But the main problem
is that is exactly the idea to be able to c/r existing programs without
recompiling and such (or I miss something here?).

	Cyrill
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