Re: [Xen-devel] feature request - prevent user stupidity (eg mine)

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> > Given that there is almost no way you would ever want to publish the
> > same block device to the same virtual domain more than once, would
>
> there
>
> > be any value in flagging that as an error when you try and start the
> > domain?
>
> I'm surprised that the existing block device safety checks don't pick up
> on this, but I guess they're run before the guest is started rather than
> before each virtual disk is attached.

Looks like the hotplug scripts do contain the checking logic, and it does 
check each device at a time, but it explicitly ignores if the same VM already 
has access to a device.  Maybe checking for duplicate exports here would be 
too racey, since I guess the hotplug scripts are not run in any particular 
order...

> I certainly think there's a strong argument for protecting the user from
> this kind of error, at least unless they've specified 'w!' and hence
> indicated they want sharing.

Agreed.  It's something that - whilst fairly unlikely - is easy to do in the 
right circumstances.  And those circumstances usually lead to hosing off your 
filesystem quite effectively.

I looked at Kan's patch, which implements this checking within XendConfig.py.  
It's not an awful lot of extra code, however it does seem like this stuff 
would ideally be done in one place - either in Xend or in the hotplug 
scripts.

The nice thing about the hotplug scripts is that they Just Work for 
dynamically adding devices.  Unless I've misread Kan's code, it won't catch a 
user accidentally adding a duplicate blkdev to a running domain, which is a 
shame.

We should try to figure out if this can be added reliably to the blkdev 
hotplug scripts.  If not, maybe we should put all the checking into Xend, 
with hooks to validate added devices.

Cheers,
Mark

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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