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 13:12 +0000, Paul Durrant wrote:
> True. I was conflating the key rm issue with the fact that the
> XenServer Windows PV guest agent gets unhappy because it can't
> advertise its features (which I hadn't mentioned before).
> 
> I suppose that libxl will just have to continue to speculatively write
> control/shutdown and timeout if it's not NUL-ed or rm-ed as it does
> now.

I suppose one option might be a "feature-will-write-features" flag. If
it is not written to true then assume everything is supported, if it
does get set to true then look for the flags and follow them. However if
a protocol is going to be invented then it should be discussed here and
documented etc.

Ian.

> 
>   Paul
> 
> > -----Original Message-----
> > From: Ian Campbell
> > Sent: 23 November 2011 13:05
> > 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 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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