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Re: [libvirt] [Qemu-devel] live snapshot wiki updated

On 07/20/2011 11:27 AM, Blue Swirl wrote:
We've already told you - qemu must have a way to be passed fds which are
associated with names, and when a file refers to another backing file by
name, then qemu falls back on its fd/name mapping to use the already-passed
fd instead.  Which implies that someone else, either libvirt or a
qemu-maintained libblockformat.so, needs to have a stable interface for
parsing the backing file name out of an arbitrary qcow2 file, and that this
interface must work no matter how many other extensions are added to qcow2.

I'd avoid any name based access in this case. If QEMU has write access
to main file, it could forge the backing file name in main file to
point to for example /etc/shadow and then request libvirt to perform
the opening.

Won't work. Well, it might work within the context of a single qemu process. But when that process ends, then libvirt would have to touch up the qcow2 headers of that file to replace the /etc/shadow name with the real backing file name, otherwise, the next time you restart qemu-img or a new qemu guest using the same image, the information has been lost, since the fd has been closed in the meantime.

We really _do_ need a way to give qemu both an fd and the name of the file that the fd is tied to. On Linux, qemu could use /proc/self/fd to reconstruct the name from fd, but that's not portable to other OS. And we've already discussed how in the libvirt model, that libvirt is deemed more secure than qemu. Therefore, I think it is reasonable for qemu to make the assumptions that if it exposes a monitor command where the supervisor (libvirt or otherwise) can pass in both an fd and a file name, that either the supervisor is passing in correct information, or that the bug is in the supervisor and not in qemu if the supervisor passes in wrong information and things blow up.

And the snapshot_blkdev monitor command is a case where qemu needs to create a new qcow2 image on the fly, while referencing the name of an existing file. What backing name do you put in the new qcow2 file unless you already have a name association for all fds already open for the existing backing file?

Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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