From d1d5acb9cfe37705e80195008bc98f2074b61563 Mon Sep 17 00:00:00 2001
From: visa <visa@openbsd.org>
Date: Tue, 9 May 2023 14:22:17 +0000
Subject: [PATCH] Don't return EPIPE from pipe kqfilter

Proceed with the registration of an EVFILT_WRITE filter even if the
pipe's other end has been closed, instead of failing with error EPIPE.
The filter will trigger immediately. This reduces the possible outcomes
when a kevent(2) call and a close(2) call race on the different ends
of a pipe.

This fixes a failure of lang/ghc unit test as reported by gnezdo@.

OK gnezdo@ mpi@
---
 sys/kern/sys_generic.c | 12 +-----------
 sys/kern/sys_pipe.c    | 12 ++++++++----
 2 files changed, 9 insertions(+), 15 deletions(-)

diff --git a/sys/kern/sys_generic.c b/sys/kern/sys_generic.c
index 9eccb48a3f1..e012d086ba6 100644
--- a/sys/kern/sys_generic.c
+++ b/sys/kern/sys_generic.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: sys_generic.c,v 1.155 2023/02/25 09:55:46 mvs Exp $	*/
+/*	$OpenBSD: sys_generic.c,v 1.156 2023/05/09 14:22:17 visa Exp $	*/
 /*	$NetBSD: sys_generic.c,v 1.24 1996/03/29 00:25:32 cgd Exp $	*/
 
 /*
@@ -769,12 +769,6 @@ pselregister(struct proc *p, fd_set *pibits[3], fd_set *pobits[3], int nfd,
 						 * __EV_SELECT */
 					error = 0;
 					break;
-				case EPIPE:	/* Specific to pipes */
-					KASSERT(kev.filter == EVFILT_WRITE);
-					FD_SET(kev.ident, pobits[1]);
-					(*ncollected)++;
-					error = 0;
-					break;
 				case ENXIO:	/* Device has been detached */
 				default:
 					goto bad;
@@ -1073,10 +1067,6 @@ again:
 				goto again;
 			}
 			break;
-		case EPIPE:	/* Specific to pipes */
-			KASSERT(kevp->filter == EVFILT_WRITE);
-			pl->revents |= POLLHUP;
-			break;
 		default:
 			DPRINTFN(0, "poll err %lu fd %d revents %02x serial"
 			    " %lu filt %d ERROR=%d\n",
diff --git a/sys/kern/sys_pipe.c b/sys/kern/sys_pipe.c
index 8958157f18c..b96d5081dba 100644
--- a/sys/kern/sys_pipe.c
+++ b/sys/kern/sys_pipe.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: sys_pipe.c,v 1.145 2023/02/12 10:41:00 mvs Exp $	*/
+/*	$OpenBSD: sys_pipe.c,v 1.146 2023/05/09 14:22:17 visa Exp $	*/
 
 /*
  * Copyright (c) 1996 John S. Dyson
@@ -857,9 +857,13 @@ pipe_kqfilter(struct file *fp, struct knote *kn)
 		break;
 	case EVFILT_WRITE:
 		if (wpipe == NULL) {
-			/* other end of pipe has been closed */
-			error = EPIPE;
-			break;
+			/*
+			 * The other end of the pipe has been closed.
+			 * Since the filter now always indicates a pending
+			 * event, attach the knote to the current side
+			 * to proceed with the registration.
+			 */
+			wpipe = rpipe;
 		}
 		kn->kn_fop = &pipe_wfiltops;
 		kn->kn_hook = wpipe;
-- 
2.20.1