I have the following nullfs mounts:
/var/jail/builder/src /usr/src nullfs ro,late 0 0
/var/jail/builder/obj /usr/obj nullfs ro,late 0 0
/var/jail/builder/src and /var/jail/builder/obj are ZFS datasets.
Exporting them like this:
/usr/src -maproot=root -network=192.168.0.0/16 -ro
/usr/obj -maproot=root -network=192.168.0.0/16 -ro
worked fine on any 11.x and 12.x version. It stopped working after upgrade to 13.0-RC2, giving these errors:
mountd: can't change attributes for /usr/src: netcred already exists for given addr/mask
mountd: bad exports list line '/usr/src -maproot'
Workaround for now is to directly export the datasets instead.