Fix select(2) exceptfds handling of FIFOs and pipes

Prevent select(2) from indicating an exceptional condition when the
other end of a FIFO or pipe is closed.

Originally, select(2) returned an exceptfds event only with a pty or
socket that has out-of-band data pending. millert@ says that OpenBSD
diverged from this by accident when poll(2) and select(2) were changed
to use the same backend code in year 2003.

OK millert@

4 files changed, 18 insertions, 7 deletions

diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
index 39a8ab7ae83..8fdfb8d8b84 100644
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: sys_generic.c,v 1.144 2021/11/30 02:58:33 visa Exp $	*/
+/*	$OpenBSD: sys_generic.c,v 1.145 2021/12/08 13:03:52 visa Exp $	*/
 /*	$NetBSD: sys_generic.c,v 1.24 1996/03/29 00:25:32 cgd Exp $	*/
 
 /*
@@ -762,7 +762,7 @@ pselregister(struct proc *p, fd_set *pibits[3], fd_set *pobits[3], int nfd,
 				DPRINTFN(2, "select fd %d mask %d serial %lu\n",
 				    fd, msk, p->p_kq_serial);
 				EV_SET(&kev, fd, evf[msk],
-				    EV_ADD|EV_ENABLE|__EV_POLL,
+				    EV_ADD|EV_ENABLE|__EV_POLL|__EV_SELECT,
 				    evff[msk], 0, (void *)(p->p_kq_serial));
 #ifdef KTRACE
 				if (KTRPOINT(p, KTR_STRUCT))
@@ -775,7 +775,8 @@ pselregister(struct proc *p, fd_set *pibits[3], fd_set *pobits[3], int nfd,
 				/* FALLTHROUGH */
 				case EOPNOTSUPP:/* No underlying kqfilter */
 				case EINVAL:	/* Unimplemented filter */
-				case EPERM:	/* Specific to FIFO */
+				case EPERM:	/* Specific to FIFO and
+						 * __EV_SELECT */
 					error = 0;
 					break;
 				case EPIPE:	/* Specific to pipes */
diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index 3ece79221b3..4da474aa257 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: sys_pipe.c,v 1.130 2021/12/07 14:06:16 visa Exp $	*/
+/*	$OpenBSD: sys_pipe.c,v 1.131 2021/12/08 13:03:52 visa Exp $	*/
 
 /*
  * Copyright (c) 1996 John S. Dyson
@@ -922,6 +922,11 @@ pipe_kqfilter(struct file *fp, struct knote *kn)
 		klist_insert_locked(&wpipe->pipe_sel.si_note, kn);
 		break;
 	case EVFILT_EXCEPT:
+		if (kn->kn_flags & __EV_SELECT) {
+			/* Prevent triggering exceptfds. */
+			error = EPERM;
+			break;
+		}
 		kn->kn_fop = &pipe_efiltops;
 		kn->kn_hook = rpipe;
 		klist_insert_locked(&rpipe->pipe_sel.si_note, kn);
diff --git a/sys/miscfs/fifofs/fifo_vnops.c b/sys/miscfs/fifofs/fifo_vnops.c
index c039fd411ec..ca8d40f6de0 100644
--- a/sys/miscfs/fifofs/fifo_vnops.c
+++ b/sys/miscfs/fifofs/fifo_vnops.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: fifo_vnops.c,v 1.85 2021/10/24 11:23:22 mpi Exp $	*/
+/*	$OpenBSD: fifo_vnops.c,v 1.86 2021/12/08 13:03:52 visa Exp $	*/
 /*	$NetBSD: fifo_vnops.c,v 1.18 1996/03/16 23:52:42 christos Exp $	*/
 
 /*
@@ -540,6 +540,10 @@ fifo_kqfilter(void *v)
 		sb = &so->so_snd;
 		break;
 	case EVFILT_EXCEPT:
+		if (ap->a_kn->kn_flags & __EV_SELECT) {
+			/* Prevent triggering exceptfds. */
+			return (EPERM);
+		}
 		ap->a_kn->kn_fop = &fifoexcept_filtops;
 		so = fip->fi_readsock;
 		sb = &so->so_rcv;
diff --git a/sys/sys/event.h b/sys/sys/event.h
index 20a1fd33c82..5004978e348 100644
--- a/sys/sys/event.h
+++ b/sys/sys/event.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: event.h,v 1.59 2021/11/29 15:54:04 visa Exp $	*/
+/*	$OpenBSD: event.h,v 1.60 2021/12/08 13:03:53 visa Exp $	*/
 
 /*-
  * Copyright (c) 1999,2000,2001 Jonathan Lemon <jlemon@FreeBSD.org>
@@ -74,7 +74,7 @@ struct kevent {
 #define EV_RECEIPT	0x0040          /* force EV_ERROR on success, data=0 */
 #define EV_DISPATCH	0x0080          /* disable event after reporting */
 
-#define EV_SYSFLAGS	0xF000		/* reserved by system */
+#define EV_SYSFLAGS	0xf800		/* reserved by system */
 #define EV_FLAG1	0x2000		/* filter-specific flag */
 
 /* returned values */
@@ -141,6 +141,7 @@ struct klist {
 #ifdef _KERNEL
 
 /* kernel-only flags */
+#define __EV_SELECT	0x0800		/* match behavior of select */
 #define __EV_POLL	0x1000		/* match behavior of poll & select */
 #define __EV_HUP	EV_FLAG1	/* device or socket disconnected */