[Libguestfs] [PATCH for discussion only] lib: libvirt: If root, run qemu subprocess as root.root.

[Richard W.M. Jones]

libvirt doesn't have a concept of "session qemu" for root:

  https://bugzilla.redhat.com/show_bug.cgi?id=890291

When a libguestfs-using process runs as root, and libvirt runs a qemu
subprocess, the qemu subprocess is run as a non-root user (typically
qemu.qemu).  This causes various problems, for example if we try to
open a file which is readable by root but unreadable by qemu.qemu then
the operation will fail.

This can be changed globally via a configuration file, but it can also
be changed by using a <seclabel/> clause in the XML (although I think
that's not the only effect):

  <seclabel type="static" model="dac" relabel="no">
    <label>0:0</label>
  </seclabel>

This patch makes that change.

I notice that after this change, qemu is indeed running as root.
However the file being examined still gets relabelled by SELinux (to
virt_content_t IIRC).  Maybe this relabelling is in fact desirable.

Also as you can see from the patch there are cases where we use
another <seclabel model='selinux'/> element to set labels to a known
value.  It's not clear if we can include both <seclabel/> elements.
The patch as shown overrides the selinux seclabel if running as root.

Rich.