The default behavior for attaching processes to jails is that the jail's
cpuset augments the attaching processes, so that it cannot be used to
escalate a user's ability to take advantage of more CPUs than the
administrator wanted them to.
This is problematic when root needs to manage jails that have disjoint
sets with whatever process is attaching, as this would otherwise result
in a deadlock. Therefore, if we did not have an appropriate common
subset of cpus/domains for our new policy, we now allow the process to
simply take on the jail set *if* it has the privilege to widen its mask
anyways.
With the new logic, root can still usefully cpuset a process that
attaches to a jail with the desire of maintaining the set it was given
pre-attachment while still retaining the ability to manage child jails
without jumping through hoops.
PR: 253724
Differential Revision: https://reviews.freebsd.org/D28952