Re: [Qemu-devel] simulating a chroot-like interface with qemu

[Avi Kivity <avi@redhat.com>]

Liraz Siri wrote:
> I'd like to run by you guys an idea I've been playing around with.
>
> We've recently cut down in our use of qemu/kvm in our development
> toolchain for TurnKey Linux and instead switched to using chroot for
> many things, mainly because it is lighter and easier to script which
> translates into reduced overhead during development.
>
> On the flip side there are many downsides to using chroot:
>
> * requires root privileges. You can get around this by giving a program
>   suid privileges but that's dangerous because...
> * root processes inside the chroot can easily break out
> * processes inside the chroot share the network stack with processes
>   outside the chroot.
>
>   So for example, if mysql is running with the default configuration
>   inside a VM and binds to port 3306 that will work even if the host is
>   also running mysql listening to 3306. If you're using chroot  there is
>   an additional overhead requiring you to reconfigure things.
>
> * similarly, processes inside the chroot share the same abstract unix
>   socket namespace, which complicates some usage scenarios...
>
>   

All of these (and many more) will be fixed by the namespace work that's 
being merged into Linux bit by bit.

> I'm thinking maybe for some uses it would be useful to simulate an
> interface that looks and functions like chroot but is magically
> implemented with qemu/kvm behind the scenes (e.g., separate kernel,
> network stack, etc.).  Sort of a power chroot that offers stronger
> isolation/compartmentalization but with a similar unixish interface
> (e.g., pipeable, scriptable, etc.)
>
> Perhaps rather than mounting a block device, the guest could access its
> root in the host filesystem transparently via a network filesystem of
> some kind.
>   

You could easily use nfsroot in the guest together with -kernel and 
-initrd to load a guest without a block device.  With -append and 
'init=' you can have the guest start any script you like.


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