Re: [Qemu-devel] Live migration - exec: support to be reintroduced?

[Daniel P. Berrange]

On Wed, Nov 05, 2008 at 08:14:01AM -0600, Anthony Liguori wrote:
> Avi Kivity wrote:
> >Daniel P. Berrange wrote:
> >>We used it to implement save-to-file, exec'ing 'dd'. I'd rather prefer
> >>that we had a more explicit 'migrate to a file' capability rather than
> >>the hack of execing dd.
> >>  
> >
> >(qemu) acceptfd blah
> >receives an file descriptor from the monitor using SCM_RIGHTS and 
> >assigns it to the tag 'blah'
> >(qemu) migrate fd:blah
> >migrates to the fd denominated by the tag 'blah'
> >(qemu) closefd blah
> 
> This could be generally useful for other things too (like char device 
> redirection).
> 
> I'm not sure how many people would really use it though, you're talking 
> about serious unix-fu here.

It is useful from a security point of view - it means QEMU doesn't
need to be given permissions to create files, merely append to an
opened file handle.

On a related note, Avi pointed out to me that SCM_RIGHTS fd passing
would be important for NIC hotplug to allow parity with -net arg
on the command line. If the QEMU process is running unprivileged,
it will not have rights to create TAP devices & giving it a setuid()
network script is not desirable. The management app invoking QEMU
could open the TAP device, and do any setup before passing the FD
to the NIC hotplug command in the monitor. 

Daniel
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