Make filt_dead() selectively inactive with EVFILT_EXCEPT

When a knote uses the dead event filter, the knote's file descriptor is
not supposed to point to an object with pending out-of-band data. Make
the knote inactive so that userspace will not receive a spurious event.
However, kqueue-based poll(2) should still receive HUP notifications.

This lets the system use dead_filtops with less strings attached
relative to the filter type.

diff --git a/sys/kern/kern_event.c b/sys/kern/kern_event.c
index fc86896..109f789 100644 (file)
--- a/sys/kern/kern_event.c
+++ b/sys/kern/kern_event.c
@@ -1,4 +1,4 @@
-/*     $OpenBSD: kern_event.c,v 1.176 2021/12/20 16:21:07 visa Exp $   */
+/*     $OpenBSD: kern_event.c,v 1.177 2021/12/20 16:24:32 visa Exp $   */
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -636,6 +636,17 @@ seltrue_kqfilter(dev_t dev, struct knote *kn)
 static int
 filt_dead(struct knote *kn, long hint)
 {
+       if (kn->kn_filter == EVFILT_EXCEPT) {
+               /*
+                * Do not deliver event because there is no out-of-band data.
+                * However, let HUP condition pass for poll(2).
+                */
+               if ((kn->kn_flags & __EV_POLL) == 0) {
+                       kn->kn_flags |= EV_DISABLE;
+                       return (0);
+               }
+       }
+
        kn->kn_flags |= (EV_EOF | EV_ONESHOT);
        if (kn->kn_flags & __EV_POLL)
                kn->kn_flags |= __EV_HUP;