Apply by doing:
	cd /usr/src
	patch -p0 < 005_ptrace.patch

Then build and install a new kernel.

Index: sys/compat/hpux/hpux_compat.c
===================================================================
RCS file: /cvs/src/sys/compat/hpux/hpux_compat.c,v
retrieving revision 1.30
retrieving revision 1.30.6.1
diff -u -p -r1.30 -r1.30.6.1
--- sys/compat/hpux/hpux_compat.c	1 Nov 2008 05:59:21 -0000	1.30
+++ sys/compat/hpux/hpux_compat.c	29 Jan 2010 21:33:13 -0000	1.30.6.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: hpux_compat.c,v 1.30 2008/11/01 05:59:21 deraadt Exp $	*/
+/*	$OpenBSD: hpux_compat.c,v 1.30.6.1 2010/01/29 21:33:13 sthen Exp $	*/
 /*	$NetBSD: hpux_compat.c,v 1.35 1997/05/08 16:19:48 mycroft Exp $	*/
 
 /*
@@ -966,7 +966,7 @@ hpux_sys_getpgrp2(cp, v, retval)
 	if (p == 0)
 		return (ESRCH);
 	if (cp->p_ucred->cr_uid && p->p_ucred->cr_uid != cp->p_ucred->cr_uid &&
-	    !inferior(p))
+	    !inferior(p, cp))
 		return (EPERM);
 	*retval = p->p_pgid;
 	return (0);
Index: sys/sys/proc.h
===================================================================
RCS file: /cvs/src/sys/sys/proc.h,v
retrieving revision 1.119
retrieving revision 1.119.4.1
diff -u -p -r1.119 -r1.119.4.1
--- sys/sys/proc.h	5 Jun 2009 00:30:05 -0000	1.119
+++ sys/sys/proc.h	29 Jan 2010 21:33:13 -0000	1.119.4.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: proc.h,v 1.119 2009/06/05 00:30:05 guenther Exp $	*/
+/*	$OpenBSD: proc.h,v 1.119.4.1 2010/01/29 21:33:13 sthen Exp $	*/
 /*	$NetBSD: proc.h,v 1.44 1996/04/22 01:23:21 christos Exp $	*/
 
 /*-
@@ -417,7 +417,7 @@ int	chgproccnt(uid_t uid, int diff);
 int	enterpgrp(struct proc *p, pid_t pgid, struct pgrp *newpgrp,
 	    struct session *newsess);
 void	fixjobc(struct proc *p, struct pgrp *pgrp, int entering);
-int	inferior(struct proc *p);
+int	inferior(struct proc *, struct proc *);
 int	leavepgrp(struct proc *p);
 void	yield(void);
 void	preempt(struct proc *);
Index: sys/kern/kern_proc.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_proc.c,v
retrieving revision 1.40
retrieving revision 1.40.4.1
diff -u -p -r1.40 -r1.40.4.1
--- sys/kern/kern_proc.c	15 Apr 2009 10:47:46 -0000	1.40
+++ sys/kern/kern_proc.c	29 Jan 2010 21:33:13 -0000	1.40.4.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_proc.c,v 1.40 2009/04/15 10:47:46 art Exp $	*/
+/*	$OpenBSD: kern_proc.c,v 1.40.4.1 2010/01/29 21:33:13 sthen Exp $	*/
 /*	$NetBSD: kern_proc.c,v 1.14 1996/02/09 18:59:41 christos Exp $	*/
 
 /*
@@ -151,14 +151,14 @@ chgproccnt(uid_t uid, int diff)
 }
 
 /*
- * Is p an inferior of the current process?
+ * Is p an inferior of parent?
  */
 int
-inferior(struct proc *p)
+inferior(struct proc *p, struct proc *parent)
 {
 
-	for (; p != curproc; p = p->p_pptr)
-		if (p->p_pid == 0)
+	for (; p != parent; p = p->p_pptr)
+		if (p->p_pid == 0 || p->p_pid == 1)
 			return (0);
 	return (1);
 }
Index: sys/kern/kern_prot.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_prot.c,v
retrieving revision 1.39
retrieving revision 1.39.4.1
diff -u -p -r1.39 -r1.39.4.1
--- sys/kern/kern_prot.c	2 Jun 2009 20:03:59 -0000	1.39
+++ sys/kern/kern_prot.c	29 Jan 2010 21:33:13 -0000	1.39.4.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: kern_prot.c,v 1.39 2009/06/02 20:03:59 guenther Exp $	*/
+/*	$OpenBSD: kern_prot.c,v 1.39.4.1 2010/01/29 21:33:13 sthen Exp $	*/
 /*	$NetBSD: kern_prot.c,v 1.33 1996/02/09 18:59:42 christos Exp $	*/
 
 /*
@@ -282,7 +282,7 @@ sys_setpgid(struct proc *curp, void *v, 
 	newpgrp = pool_get(&pgrp_pool, PR_WAITOK);
 
 	if (pid != 0 && pid != curp->p_pid) {
-		if ((targp = pfind(pid)) == 0 || !inferior(targp)) {
+		if ((targp = pfind(pid)) == 0 || !inferior(targp, curp)) {
 			error = ESRCH;
 			goto out;
 		}
Index: sys/kern/sys_process.c
===================================================================
RCS file: /cvs/src/sys/kern/sys_process.c,v
retrieving revision 1.43
retrieving revision 1.43.6.1
diff -u -p -r1.43 -r1.43.6.1
--- sys/kern/sys_process.c	31 Oct 2008 17:29:51 -0000	1.43
+++ sys/kern/sys_process.c	29 Jan 2010 21:33:13 -0000	1.43.6.1
@@ -1,4 +1,4 @@
-/*	$OpenBSD: sys_process.c,v 1.43 2008/10/31 17:29:51 deraadt Exp $	*/
+/*	$OpenBSD: sys_process.c,v 1.43.6.1 2010/01/29 21:33:13 sthen Exp $	*/
 /*	$NetBSD: sys_process.c,v 1.55 1996/05/15 06:17:47 tls Exp $	*/
 
 /*-
@@ -167,6 +167,14 @@ sys_ptrace(struct proc *p, void *v, regi
 		 */
 		if ((t->p_pid == 1) && (securelevel > -1))
 			return (EPERM);
+
+		/*
+		 *	(6) it's an ancestor of the current process and
+		 *	    not init (because that would create a loop in
+		 *	    the process graph).
+		 */
+		if (t->p_pid != 1 && inferior(p, t))
+			return (EINVAL);
 		break;
 
 	case  PT_READ_I: