Re: [libvirt] backing file chains, -blockdev, and storage file autodetection

IIUC there are three scenarios at play here 1. qcow2 file, first level raw backing 2. qcow2 file, first level qcow2 backing, no backing 3. qcow2 file, first level qcow2 backing, with second level backing Historically with the SELinux driver, if no backing_fmt is set, then we blindly assume that scenario (1) is in effect. We cannot tell QEMU that we treated it as scenario (1) though, so QEMU will still probe format and potentially open the first level backing as qcow2 still. IOW, despite our SELinux & QEMU driver assumption for scenario (1), scenario (2) would still suceed in the past. Scenario (3) would always have failed, because SELinux won't have labelled the second level backing.

Essentially scenario (2) worked by accident, not design, but IIUC we have not been warning users about this. With new libvirt and -blockdev we still assume the backing_fmt is raw if not set in the SELinux driver, but we now have the ability to tell QEMU about our assumption via -blockdev. As such we've not ensured scenario (2) fails. Users who were silently relying on scenario (2) are now broken and have three options IIUC - Run qemu-img rebase to set the backing_fmt or - Update the guest XML to set the <backingStore> format value or - Update the /etc/libvirt/qemu.conf to set capability_filters to disable "blockdev"

Always assuming the format is raw certainly avoids the security danger, but is unhelpful to users who relied on scenario (2). I'm inclined to say we should make scenario (2)/(3) into a hard error. Only allow scenario (1) to run normally. ie, we should probe the disk format for the backing file, and if it is *not* raw, then report an immediate error, with the error message pointing to the kbase page explaining how to fix this. This will help the 99% common case identify the fix more quickly, with no obvious downside that I see.