Re: [Xen-devel] [PATCH] Prevent xl save from segfaulting when control/shutdown key is removed

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2011-11-23 at 12:59 +0000, Paul Durrant wrote:
> > -----Original Message-----
> > From: Ian Campbell
> > Sent: 23 November 2011 11:24
> > To: Paul Durrant
> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> > Subject: RE: [Xen-devel] [PATCH] Prevent xl save from segfaulting
> > when control/shutdown key is removed
> > 
> > On Wed, 2011-11-23 at 11:19 +0000, Paul Durrant wrote:
> > > what is the reason for creating control ro to the guest?
> > 
> > In general libxl prefers to whitelist paths which the guest can
> > write too, just to prevent a complete free for all, keep things
> > somewhat under control and to help avoid situations where tools
> > might inadvertently rely on a guest-writeable key in an unsafe way..
> > 
> > >  In XenServer we allow the guest to write the control key to
> > advertise
> > > feature-shutdown, feature-suspend etc. so that the tools know what
> > > values of control/shutdown the guest will respond to.
> > 
> > The libxl way would be to create these at build time (perhaps empty)
> > with the appropriate permissions.
> > 
> > It's not clear how that functionality can be added in a way which is
> > compatible with existing guests though, e.g. no Linux guest writes
> > those but many can be suspended etc.
> > 
> 
> So the simple solution, for compatibility's sake, is to make control rw isn't 
> it?

The problem I'm thinking of exists even if control is rw.

Given an empty control directory how do you know if a guest supports
suspend or not, given that most existing guests which do support suspend
do not write any key there?

Ian.


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