[libvirt] [PATCH 00/30] storagefile, security: qcow2 data_file support

[Cole Robinson]

This series is the first steps to teaching libvirt about qcow2
data_file support, aka external data files or qcow2 external metadata.

A bit about the feature: it was added in qemu 4.0. It essentially
creates a two part image file: a qcow2 layer that just tracks the
image metadata, and a separate data file which is stores the VM
disk contents. AFAICT the driving use case is to keep a fully coherent
raw disk image on disk, and only use qcow2 as an intermediate metadata
layer when necessary, for things like incremental backup support.

The original qemu patch posting is here:
https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg07496.html

For testing, you can create a new qcow2+raw data_file image from an
existing image, like:

    qemu-img convert -O qcow2 \
        -o data_file=NEW.raw,data_file_raw=yes
        EXISTING.raw NEW.qcow2

The goal of this series is to teach libvirt enough about this case
so that we can correctly relabel the data_file on VM startup/shutdown.
The main functional changes are

  * Teach storagefile how to parse out data_file from the qcow2 header
  * Store the raw string as virStorageSource->externalDataStoreRaw
  * Track that as its out virStorageSource in externalDataStore
  * dac/selinux relabel externalDataStore as needed

>From libvirt's perspective, externalDataStore is conceptually pretty
close to a backingStore, but the main difference is its read/write
permissions should match its parent image, rather than being readonly
like backingStore.

This series has only been tested on top of the -blockdev enablement
series, but I don't think it actually interacts with that work at
the moment.


Future work:
  * Exposing this in the runtime XML. We need to figure out an XML
    schema. It will reuse virStorageSource obviously, but the main
    thing to figure out is probably 1) what the top element name
    should be ('dataFile' maybe?), 2) where it sits in the XML
    hierarchy (under <disk> or under <source> I guess)

  * Exposing this on the qemu -blockdev command line. Similar to how
    in the blockdev world we are explicitly putting the disk backing
    chain on the command line, we can do that for data_file too. Then
    like persistent <backingStore> XML the user will have the power
    to overwrite the data_file location for an individual VM run.

  * Figure out how we expect ovirt/rhev to be using this at runtime.
    Possibly taking a running VM using a raw image, doing blockdev-*
    magic to pivot it to qcow2+raw data_file, so it can initiate
    incremental backup on top of a previously raw only VM?


Known issues:
  * In the qemu driver, the qcow2 image metadata is only parsed
    in -blockdev world if no <backingStore> is specified in the
    persistent XML. So basically if there's a <backingStore> listed,
    we never parse the qcow2 header and detect the presence of
    data_file. Fixable I'm sure but I didn't look into it much yet.

Most of this is cleanups and refactorings to simplify the actual
functional changes.

Cole Robinson (30):
  storagefile: Make GetMetadataInternal static
  storagefile: qcow1: Check for BACKING_STORE_OK
  storagefile: qcow1: Fix check for empty backing file
  storagefile: qcow1: Let qcowXGetBackingStore fill in format
  storagefile: Check version to determine if qcow2 or not
  storagefile: Drop now unused isQCow2 argument
  storagefile: Use qcowXGetBackingStore directly
  storagefile: Push 'start' into qcow2GetBackingStoreFormat
  storagefile: Push extension_end calc to qcow2GetBackingStoreFormat
  storagefile: Rename qcow2GetBackingStoreFormat
  storagefile: Rename qcow2GetExtensions 'format' argument
  storagefile: Fix backing format \0 check
  storagefile: Add externalDataStoreRaw member
  storagefile: Parse qcow2 external data file
  storagefile: Fill in meta->externalDataStoreRaw
  storagefile: Don't access backingStoreRaw directly in
    FromBackingRelative
  storagefile: Split out virStorageSourceNewFromChild
  storagefile: Add externalDataStore member
  storagefile: Fill in meta->externalDataStore
  security: dac: Drop !parent handling in SetImageLabelInternal
  security: dac: Add is_toplevel to SetImageLabelInternal
  security: dac: Restore image label for externalDataStore
  security: dac: break out SetImageLabelRelative
  security: dac: Label externalDataStore
  security: selinux: Simplify SetImageLabelInternal
  security: selinux: Drop !parent handling in SetImageLabelInternal
  security: selinux: Add is_toplevel to SetImageLabelInternal
  security: selinux: Restore image label for externalDataStore
  security: selinux: break out SetImageLabelRelative
  security: selinux: Label externalDataStore

 src/libvirt_private.syms        |   1 -
 src/security/security_dac.c     |  63 +++++--
 src/security/security_selinux.c |  97 +++++++----
 src/util/virstoragefile.c       | 281 ++++++++++++++++++++------------
 src/util/virstoragefile.h       |  11 +-
 5 files changed, 290 insertions(+), 163 deletions(-)

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2.23.0