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migration corrupts qcow2 metadata when "backing file: json:{" is involve
Description of problem:
the bug happens when you have a qcow2 with backing file in json format
image: 2.qcow2
[...]
backing file: json:{"driver": "qcow2", "file": { "driver": "file", "filename": "1.qcow2"}}
backing file format: qcow2
[...]
if you want to migrate a VM that have that kind of qcow2 attached, the migration is gonna corrupted qcow2 metadata in memory and info block will look like this
json:{\"backing\": {\"backing\": {\"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"0.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"1.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"2.qcow2\"}}
later if you execute blockdev-snapshot-sync, the corrupt json will be write to the new qcow2 resulting with a unusable qcow2
Steps to reproduce:
/opt/qemu-7.0.0/bin/qemu-img create -f qcow2 0.qcow2 64G
/opt/qemu-7.0.0/bin/qemu-img create -F qcow2 -f qcow2 -b 'json:{"driver": "qcow2", "file": { "driver": "file", "filename": "0.qcow2"}}' 1.qcow2
/opt/qemu-7.0.0/bin/qemu-img create -F qcow2 -f qcow2 -b 'json:{"driver": "qcow2", "file": { "driver": "file", "filename": "1.qcow2"}}' 2.qcow2
#VM1
/opt/qemu-7.0.0/bin/qemu-system-x86_64 -enable-kvm -drive if=virtio,file=2.qcow2,node-name=drive0 -qmp stdio -display none
#VM2
/opt/qemu-7.0.0/bin/qemu-system-x86_64 -enable-kvm -drive if=virtio,file=2.qcow2,node-name=drive0 -qmp stdio -display none -incoming tcp::8082
#VM1 INFO BLOCK
{"QMP": {"version": {"qemu": {"micro": 0, "minor": 0, "major": 7}, "package": ""}, "capabilities": ["oob"]}}
{ "execute": "qmp_capabilities" }
{"return": {}}
{ "execute": "human-monitor-command", "arguments": {'command-line': 'info block'} }
{"return": "virtio0 (drive0): 2.qcow2 (qcow2)\r\n Attached to: /machine/peripheral-anon/device[0]/virtio-backend\r\n Cache mode: writethrough\r\n Backing file: 1.qcow2 (chain depth: 2)\r\n\r\nide1-cd0: [not inserted]\r\n Attached to: /machine/unattached/device[24]\r\n Removable device: not locked, tray closed\r\n\r\nfloppy0: [not inserted]\r\n Attached to: /machine/unattached/device[17]\r\n Removable device: not locked, tray closed\r\n\r\nsd0: [not inserted]\r\n Removable device: not locked, tray closed\r\n"}
#VM1 MIGRATE
{ "execute": "migrate", "arguments": { "uri": "tcp:localhost:8082" } }
{"return": {}}
{"timestamp": {"seconds": 1658491019, "microseconds": 233177}, "event": "STOP"}
#VM2 INFO BLOCK
{"QMP": {"version": {"qemu": {"micro": 0, "minor": 0, "major": 7}, "package": ""}, "capabilities": ["oob"]}}
{ "execute": "qmp_capabilities" }
{"return": {}}
{ "execute": "human-monitor-command", "arguments": {'command-line': 'info block'} }
{"return": "virtio0 (drive0): 2.qcow2 (qcow2)\r\n Attached to: /machine/peripheral-anon/device[0]/virtio-backend\r\n Cache mode: writeback\r\n Backing file: 1.qcow2 (chain depth: 2)\r\n\r\nide1-cd0: [not inserted]\r\n Attached to: /machine/unattached/device[24]\r\n Removable device: not locked, tray closed\r\n\r\nfloppy0: [not inserted]\r\n Attached to: /machine/unattached/device[17]\r\n Removable device: not locked, tray closed\r\n\r\nsd0: [not inserted]\r\n Removable device: not locked, tray closed\r\n"}
#VM2 MIGRATE
{"timestamp": {"seconds": 1658491019, "microseconds": 249760}, "event": "RESUME"}
#VM2 MIGRATION DONE, INFO BLOCK
{ "execute": "human-monitor-command", "arguments": {'command-line': 'info block'} }
{"return": "virtio0 (drive0): json:{\"backing\": {\"backing\": {\"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"0.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"1.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"2.qcow2\"}} (qcow2)\r\n Attached to: /machine/peripheral-anon/device[0]/virtio-backend\r\n Cache mode: writethrough\r\n Backing file: json:{\"backing\": {\"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"0.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"1.qcow2\"}} (chain depth: 2)\r\n\r\nide1-cd0: [not inserted]\r\n Attached to: /machine/unattached/device[24]\r\n Removable device: not locked, tray closed\r\n\r\nfloppy0: [not inserted]\r\n Attached to: /machine/unattached/device[17]\r\n Removable device: not locked, tray closed\r\n\r\nsd0: [not inserted]\r\n Removable device: not locked, tray closed\r\n"}
#VM2 SNAPSHOT AFTER MIGRATION
{ "execute": "blockdev-snapshot-sync", "arguments": { "format": "qcow2", "snapshot-file": "3.qcow2", "node-name": "drive0", "snapshot-node-name": "drive0-snap" }}
Formatting '3.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=68719476736 backing_file=json:{"backing": {"backing": {"driver": "qcow2",, "file": {"driver": "file",, "filename": "0.qcow2"}},, "driver": "qcow2",, "file": {"driver": "file",, "filename": "1.qcow2"}},, "driver": "qcow2",, "file": {"driver": "file",, "filename": "2.qcow2"}} backing_fmt=qcow2 lazy_refcounts=off refcount_bits=16
{"return": {}}
#VM2 INFO BLOCK AFTER SNAPSHOT
{ "execute": "human-monitor-command", "arguments": {'command-line': 'info block'} }
{"return": "virtio0 (drive0-snap): 3.qcow2 (qcow2)\r\n Attached to: /machine/peripheral-anon/device[0]/virtio-backend\r\n Cache mode: writethrough\r\n Backing file: json:{\"backing\": {\"backing\": {\"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"0.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"1.qcow2\"}}, \"driver\": \"qcow2\", \"file\": {\"driver\": \"file\", \"filename\": \"2.qcow2\"}} (chain depth: 3)\r\n\r\nide1-cd0: [not inserted]\r\n Attached to: /machine/unattached/device[24]\r\n Removable device: not locked, tray closed\r\n\r\nfloppy0: [not inserted]\r\n Attached to: /machine/unattached/device[17]\r\n Removable device: not locked, tray closed\r\n\r\nsd0: [not inserted]\r\n Removable device: not locked, tray closed\r\n"}
#INFO
/opt/qemu-7.0.0/bin/qemu-img info --backing-chain 3.qcow2
qemu-img: Could not open 'json:{"backing": {"backing": {"driver": "qcow2", "file": {"driver": "file", "filename": "0.qcow2"}}, "driver": "qcow2", "file": {"driver": "file", "filename": "1.qcow2"}}, "driver": "qcow2", "file": {"driver": "file", "filename": "2.qcow2"}}': Block format 'qcow2' does not support the option 'backing.backing.driver'
Additional information:
Even if the bug is scary it's very simple to fix it
/opt/qemu-7.0.0/bin/qemu-img info --backing-chain 3.qcow2
qemu-img: Could not open 'json:{"backing": {"backing": {"driver": "qcow2", "file": {"driver": "file", "filename": "0.qcow2"}}, "driver": "qcow2", "file": {"driver": "file", "filename": "1.qcow2"}}, "driver": "qcow2", "file": {"driver": "file", "filename": "2.qcow2"}}': Block format 'qcow2' does not support the option 'backing.backing.driver'
root@lenovo2:/data# /opt/qemu-7.0.0/bin/qemu-img rebase -f qcow2 -F qcow2 -u -b 2.qcow2 3.qcow2
root@lenovo2:/data# /opt/qemu-7.0.0/bin/qemu-img info --backing-chain 3.qcow2
image: 3.qcow2
file format: qcow2
virtual size: 64 GiB (68719476736 bytes)
disk size: 24 KiB
cluster_size: 65536
backing file: 2.qcow2
backing file format: qcow2
Format specific information:
compat: 1.1
compression type: zlib
lazy refcounts: false
refcount bits: 16
corrupt: false
extended l2: false
image: 2.qcow2
file format: qcow2
virtual size: 64 GiB (68719476736 bytes)
disk size: 24 KiB
cluster_size: 65536
[..........]
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