[libvirt-users] virsh snapshot-create-as: change umask on snapshots

[Peter Krempa]

On Wed, Mar 20, 2019 at 15:48:43 -0500, Eric Blake wrote:
> On 3/20/19 1:50 PM, Mircea Husz wrote:
> > I scripted the creation of snapshots and it works fine. Now I'd like to run the script as non-root.
> > 
> > virsh snapshot-create-as --domain hq-live-v01 \
> >      --name snappy \
> >      --diskspec vda,file=/var/lib/libvirt/images/hq-live-v01.snappy,snapshot=external \
> >      --diskspec vdb,file=/var/lib/libvirt/images/hq-live-storage.snappy,snapshot=external \
> >      --disk-only --quiesce --atomic
> > 
> > This fragment creates the snapshots, but get created with mode 0600:
> > -rw-------  1 qemu qemu     393216 Mar 19 17:08 hq-live-storage.snappy
> > -rw-------  1 qemu qemu    1048576 Mar 19 17:08 hq-live-v01.snappy
> > 
> > The user account is in the libvirt group and has permissions to do everything except delete the files created by the snapshot, all I need is to get the snapshots created with 0660 mode.
> > 
> > This is on a Centos 7.6 installation.  What knobs do I need to turn to control the umask?
> 
> I'm not sure if you can force libvirt to create the files with a
> different mask, but perhaps a workaround would be to pre-create the
> files yourself with desired permissions, then tell virsh to
> --reuse-external (so that libvirt no longer has to try and create the
> files, and thus doesn't mess with permissions).

--reuse-external is good only for using a custom-formatted image.
Libvirt will chown the image to qemu:qemu if you don't disable
relabelling. This is possible to do via the <seclabel> even in a
snapshot <disk> definition.

Note that it's not documented yet and also does not conform to the
schema, but the parser happily parses it and the code uses the correct
<seclabel> then. I have a not-sufficiently-tested patch that adds the
schema (and IIRC also docs) which I planned to send after testing.
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