ktrace: Fix a race with fork()
ktrace(2) may toggle trace points in any of
- a single process
- all members of a process group
- all descendents of the processes in 1 or 2
In the first two cases, we do not permit the operation if the process is
being forked or not visible. However, in case 3 we did not enforce this
restriction for descendents. As a result, the assertions about the child
in ktrprocfork() may be violated.
Move these checks into ktrops() so that they are applied consistently.
Allow KTROP_CLEAR for nascent processes. Otherwise, there is a window
where we cannot clear trace points for a nascent child if they are
inherited from the parent.
Reported by: email@example.com
Reported by: firstname.lastname@example.org
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30481