Fix race between a process registering a NOTE_EXIT EVFILT_PROC event and

the target process exiting which causes attempts to register the kevent
to randomly fail depending on whether the target runs to completion before
the parent can call kevent(2).  The bug actually effects EVFILT_PROC
events on any zombie process, but the most common manifestation is with
parents trying to monitor child processes.

MFC after:	2 weeks
Sponsored by:	NTT Multimedia Communications Labs

1 files changed, 15 insertions, 0 deletions

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index 8c552d1..c8501a2 100644
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -190,11 +190,17 @@ static int
 filt_procattach(struct knote *kn)
 {
 	struct proc *p;
+	int immediate;
 	int error;
 
+	immediate = 0;
 	p = pfind(kn->kn_id);
 	if (p == NULL)
 		return (ESRCH);
+	if (p == NULL && (kn->kn_sfflags & NOTE_EXIT)) {
+		p = zpfind(kn->kn_id);
+		immediate = 1;
+	}
 	if ((error = p_cansee(curthread, p))) {
 		PROC_UNLOCK(p);
 		return (error);
@@ -213,6 +219,15 @@ filt_procattach(struct knote *kn)
 	}
 
 	SLIST_INSERT_HEAD(&p->p_klist, kn, kn_selnext);
+
+	/*
+	 * Immediately activate any exit notes if the target process is a
+	 * zombie.  This is necessary to handle the case where the target
+	 * process, e.g. a child, dies before the kevent is registered.
+	 */
+	if (immediate && filt_proc(kn, NOTE_EXIT))
+		KNOTE_ACTIVATE(kn);
+
 	PROC_UNLOCK(p);
 
 	return (0);