Re: linux next: Native Linux KVM tool inclusion request

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On 08/24/2011 12:19 PM, Pekka Enberg wrote:
On 8/24/11 11:31 AM, Avi Kivity wrote:
On 08/23/2011 08:08 AM, Pekka Enberg wrote:
As for changes, we've implemented rootfs over 9p with "kvm run"
booting to host filesystem "/bin/sh" by default.

Isn't this dangerous? Users expect virtualization to land them in sandbox, but here an rm -rf / in the guest will happily junk the host filesystem.

Not really because I never run the tool as root. However, you're right that
we should not default to /bin/sh if you're root.

Well, just trashing /home/penberg would be bad too, no? (my recent experience indicates it's not that catastrophic - anything important sits on a server somewhere and the local data is just a cache).


Still dangerous (but just to the guest), since it's not a true snapshot. If the host filesystem changes underneath the guest, it will see partial and incoherent updates. Copy-on-write only works if the host filesystem doesn't change.

That's a generic problem with overlayfs based solutions, isn't it? We're planning to use copy-on-write only on files that aren't supposed to change that often -
like /usr and /lib.

So the guest won't see bad data that often?

Overlay works fine if the host tree is readonly. So if you have a separate tree for guests, you can share it with any number of them. Just don't share the host root.

Note this probably isn't a problem booting to /bin/bash, just booting a full-featured guest with package management and other database-like apps that expect exclusive control over their data.

I suppose we should force shared files to be read-only in
the guest.


Yes, that's safer.


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