[Qemu-devel] [PATCH 0/2] Introducing qcow2 extensions + keep backing file format (v4)

[Uri Lublin <uril@redhat.com>]

This patch set is an implementation of keeping a backing file format, such
that we would not need probing to figure it out.

I introduce qcow2 extensions which can be used in the future.
I use one such an extension to keep the backing file format,
and use it when opening a file.

Qcow2 extensions are built of magic (id) len (in bytes) and data (len bytes).
They reside between the end of the header and the filename.

We keep the backing file format in a such a qcow2 extension, to
1. Provide a way to know the backing file format without probing
   it (setting the format at creation time).
2. Enable using qcow2 format over host block devices.
   (only if the user specifically asks for it, by providing the format
    at image creation time).

This patchset is backward compatible in two ways:
1. It does not break old images (no extensions -> probe).
2. New images can be opened by old Qemu (extensions skipped -> probe).

I've added bdrv_create2 and drv->bdrv_create2 (implemented only
by block-qcow2 currently) to pass the backing-format to create.

Based on a work done by Shahar Frank.

Also fixes a security flaw found by Daniel P. Berrange on [1]
which summarizes: "Autoprobing: just say no."

[1] http://lists.gnu.org/archive/html/qemu-devel/2008-12/msg01083.html

First patch implements qcow2 extensions + bdrv_create2
Second patch modifies qemu-img such that the user can use it.

Currently this is implemented only for qcow2.

Changes from v3:
   Rebased to current HEAD.
   Removed some unneeded comments.

Thanks,
     Uri.