xattr names for unprivileged stacking?

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

Hi,
  Are there any standards for mapping xattr names/classes when
a restricted view of the filesystem needs to think it's root?

e.g. VMs that mount host filesystems, remote filesystems etc and the
client kernel tries to set a trusted. or security. xattr and you want
to store that on an underlying normal filesystem, but your
VM system doesn't want to have CAP_SYS_ADMIN and/or doesn't want to
interfere with the real hosts security.

I can see some existing examples:

  9p in qemu
     maps system.posix_acl_* to user.virtfs.system.posix_acl_*
          stops the guest accessing any user.virtfs.*

   overlayfs
      uses trusted.overlay.* on upper layer and blocks that from 
           clients

   fuse-overlayfs
      uses trusted.overlay.* for compatibiltiy if it has perms,
      otherwise falls back to user.fuseoverlayfs.*

   crosvm's virtiofs
      maps "security.sehash" to "user.virtiofs.security.sehash"
      and blocks the guest from accessing user.virtiofs.*

Does anyone know of any others?

It all seems quite adhoc;  these all fall to bits when you
stack them or when you write a filesystem using one of these
schemes and then mount it with another.

(I'm about to do a similar mapping for virtiofs's C daemon)

Thanks in advance,

Dave 

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK