--- sys/sys/procdesc.h	2012-11-27 12:15:40.872728289 +0100
+++ sys/sys/procdesc.h	2012-11-27 12:14:43.956385574 +0100
@@ -87,6 +87,7 @@
 #define	PDF_SELECTED	0x00000002	/* Issue selwakeup(). */
 #define	PDF_EXITED	0x00000004	/* Process exited. */
 #define	PDF_DAEMON	0x00000008	/* Don't exit when procdesc closes. */
+#define	PDF_KNOTE	0x00000010	/* Issue KNOTE_LOCKED(). */
 
 /*
  * In-kernel interfaces to process descriptors.
--- sys/kern/sys_procdesc.c	2012-11-16 10:06:49.712336836 +0100
+++ sys/kern/sys_procdesc.c	2012-11-27 12:12:55.879727484 +0100
@@ -116,6 +116,16 @@
 	.fo_flags = DFLAG_PASSABLE,
 };
 
+static int      procdesc_kqread(struct knote *note, long hint);
+static void     procdesc_kqdetach(struct knote *note);
+
+static struct filterops procdesc_filterops = {
+	.f_isfd =	1,
+	.f_attach =	NULL,
+	.f_detach =	procdesc_kqdetach,
+	.f_event =	procdesc_kqread,
+};
+
 /*
  * Initialize with VFS so that process descriptors are available along with
  * other file descriptor types.  As long as it runs before init(8) starts,
@@ -237,6 +247,7 @@
 	if (flags & PD_DAEMON)
 		pd->pd_flags |= PDF_DAEMON;
 	PROCDESC_LOCK_INIT(pd);
+	knlist_init_mtx(&pd->pd_selinfo.si_note, &pd->pd_lock);
 
 	/*
 	 * Process descriptors start out with two references: one from their
@@ -311,6 +322,10 @@
 		procdesc_free(pd);
 		return (1);
 	}
+	if (pd->pd_flags & PDF_KNOTE) {
+		pd->pd_flags &= ~PDF_KNOTE;
+		KNOTE_LOCKED(&pd->pd_selinfo.si_note, 0);
+	}
 	if (pd->pd_flags & PDF_SELECTED) {
 		pd->pd_flags &= ~PDF_SELECTED;
 		selwakeup(&pd->pd_selinfo);
@@ -462,8 +477,48 @@
 static int
 procdesc_kqfilter(struct file *fp, struct knote *kn)
 {
+	struct procdesc *pd;
 
-	return (EOPNOTSUPP);
+	if (kn->kn_filter != EVFILT_READ)
+		return (EINVAL);
+
+	kn->kn_fop = &procdesc_filterops;
+	kn->kn_hook = NULL;
+
+	pd = fp->f_data;
+	PROCDESC_LOCK(pd);
+	knlist_add(&pd->pd_selinfo.si_note, kn, 1);
+	pd->pd_flags |= PDF_KNOTE;
+	PROCDESC_UNLOCK(pd);
+	return (0);
+}
+
+static void
+procdesc_kqdetach(struct knote *kn)
+{
+	struct procdesc *pd;
+
+	pd = kn->kn_fp->f_data;
+	PROCDESC_LOCK(pd);
+	knlist_remove(&pd->pd_selinfo.si_note, kn, 1);
+	if (knlist_empty(&pd->pd_selinfo.si_note))
+		pd->pd_flags &= ~PDF_KNOTE;
+	PROCDESC_UNLOCK(pd);
+}
+
+static int
+procdesc_kqread(struct knote *kn, long hint)
+{
+	struct procdesc *pd;
+
+	pd = kn->kn_fp->f_data;
+	mtx_assert(&pd->pd_lock, MA_OWNED);
+
+	if (pd->pd_flags & PDF_EXITED) {
+		kn->kn_flags |= EV_EOF;
+		return (1);
+	}
+	return (0);
 }
 
 static int