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When an image is modified to point to the new backing file, the backing file format is set to NULL, which means auto-probe.  This is wrong, in fact it is a small security problem.

How to test:

(qemu) snapshot_blkdev ide0-hd0 test.qcow2 qcow2
(qemu) snapshot_blkdev ide0-hd0 fff.qcow2 qcow2
(qemu) block_stream ide0-hd0 base.qcow2

On fixed QEMU, the fff.qcow2 will show something like ^Eqcow2^@^@^@base.qcow2 when viewed with less.

On broken QEMU, it will show something like base.qcow2ow2^@^@^@test.qcow2.

Verified this issue with qemu-kvm-rhev-0.12.1.2-2.288.el6

steps

1. boot guest with /root/zhang/rhel6.3-64-virtio.qcow2
2. snapshot_blkdev virtio0 /root/zhang/test1 qcow2
3. snapshot_blkdev virtio0 /root/zhang/test2 qcow2
4. block-stream virtio0   /root/zhang/rhel6.3-64-virtio.qcow2
5. view /root/test2 | less

Results
Eqcow2^@^@^@/root/zhang/rhel6.3-64-virtio.qcow2

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No Documentation Needed

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

http://rhn.redhat.com/errata/RHBA-2012-1017.html