Mount() in cros_disks DBus service doesn't return any values, but instead sends a signal when it's finished mounting (or failed) currently the signal doesn't have information about what mount options (ro/rw, especially) was applied for each mount path.
Current implementation is to have DiskMountManager memorizes what parameter it passed when requesting mount to cros_disks for each volume (identified by its device path).
It can break by a race condition when multiple mount operations are conducted for the same mount path / volume in a short time. It's cleaner to have MountComplete signal convey the actual mount options it applied.