The kqueue_register() function assumes that it is called from the top of

the syscall code and acquires various event subsystem locks as needed.
The handling of the NOTE_TRACK for EVFILT_PROC is currently done by
calling the kqueue_register() from filt_proc() filter, causing recursive
entrance of the kqueue code. This results in the LORs and recursive
acquisition of the locks.

Implement the variant of the knote() function designed to only handle
the fork() event. It mostly copies the knote() body, but also handles
the NOTE_TRACK, removing the handling from the filt_proc(), where it
causes problems described above. The function is called from the fork1()
instead of knote().

When encountering NOTE_TRACK knote, it marks the knote as influx
and drops the knlist and kqueue lock. In this context call to
kqueue_register is safe from the problems.

An error from the kqueue_register() is reported to the observer as
NOTE_TRACKERR fflag.

PR:	108201
Reviewed by:	jhb, Pramod Srinivasan <pramod juniper net> (previous version)
Discussed with:	jmg
Tested by:	pho
MFC after:	2 weeks

1 files changed, 2 insertions, 4 deletions

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index 8387532..6da07d4 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -728,14 +728,12 @@ again:
 	 */
 	PROC_LOCK(p1);
 	_PRELE(p1);
+	PROC_UNLOCK(p1);
 
 	/*
 	 * Tell any interested parties about the new process.
 	 */
-	KNOTE_LOCKED(&p1->p_klist, NOTE_FORK | p2->p_pid);
-
-	PROC_UNLOCK(p1);
-
+	knote_fork(&p1->p_klist, p2->p_pid);
 	SDT_PROBE(proc, kernel, , create, p2, p1, flags, 0, 0);
 
 	/*