Index: sys/kern/kern_ktrace.c
===================================================================
--- sys/kern/kern_ktrace.c
+++ sys/kern/kern_ktrace.c
@@ -1006,7 +1006,7 @@
 	int facs = uap->facs & ~KTRFAC_ROOT;
 	int ops = KTROP(uap->ops);
 	int descend = uap->ops & KTRFLAG_DESCEND;
-	int nfound, ret = 0;
+	int ret = 0;
 	int flags, error = 0;
 	struct nameidata nd;
 	struct ktr_io_params *kiop, *old_kiop;
@@ -1080,42 +1080,31 @@
 			error = ESRCH;
 			goto done;
 		}
+
 		/*
 		 * ktrops() may call vrele(). Lock pg_members
 		 * by the proctree_lock rather than pg_mtx.
 		 */
 		PGRP_UNLOCK(pg);
-		nfound = 0;
+		if (LIST_EMPTY(&pg->pg_members)) {
+			sx_sunlock(&proctree_lock);
+			error = ESRCH;
+			goto done;
+		}
 		LIST_FOREACH(p, &pg->pg_members, p_pglist) {
 			PROC_LOCK(p);
-			if (p->p_state == PRS_NEW ||
-			    p_cansee(td, p) != 0) {
-				PROC_UNLOCK(p); 
-				continue;
-			}
-			nfound++;
 			if (descend)
 				ret |= ktrsetchildren(td, p, ops, facs, kiop);
 			else
 				ret |= ktrops(td, p, ops, facs, kiop);
 		}
-		if (nfound == 0) {
-			sx_sunlock(&proctree_lock);
-			error = ESRCH;
-			goto done;
-		}
 	} else {
 		/*
 		 * by pid
 		 */
 		p = pfind(uap->pid);
-		if (p == NULL)
+		if (p == NULL) {
 			error = ESRCH;
-		else
-			error = p_cansee(td, p);
-		if (error) {
-			if (p != NULL)
-				PROC_UNLOCK(p);
 			sx_sunlock(&proctree_lock);
 			goto done;
 		}
@@ -1187,8 +1176,20 @@
 		PROC_UNLOCK(p);
 		return (0);
 	}
-	if (p->p_flag & P_WEXIT) {
-		/* If the process is exiting, just ignore it. */
+	if ((ops == KTROP_SET && p->p_state == PRS_NEW) || !p_cansee(td, p)) {
+		/*
+		 * Disallow setting trace points if the process is being born.
+		 * This avoids races with trace point inheritance in
+		 * ktrprocfork().
+		 */
+		PROC_UNLOCK(p);
+		return (0);
+	}
+	if ((p->p_flag & P_WEXIT) != 0) {
+		/*
+		 * There's nothing to do if the process is exiting, but avoid
+		 * signaling an error.
+		 */
 		PROC_UNLOCK(p);
 		return (1);
 	}
@@ -1240,16 +1241,18 @@
 		 * otherwise do any siblings, and if done with this level,
 		 * follow back up the tree (but not past top).
 		 */
-		if (!LIST_EMPTY(&p->p_children))
+		if (!LIST_EMPTY(&p->p_children)) {
 			p = LIST_FIRST(&p->p_children);
-		else for (;;) {
-			if (p == top)
-				return (ret);
-			if (LIST_NEXT(p, p_sibling)) {
-				p = LIST_NEXT(p, p_sibling);
-				break;
+		} else {
+			for (;;) {
+				if (p == top)
+					return (ret);
+				if (LIST_NEXT(p, p_sibling) != NULL) {
+					p = LIST_NEXT(p, p_sibling);
+					break;
+				}
+				p = p->p_pptr;
 			}
-			p = p->p_pptr;
 		}
 		PROC_LOCK(p);
 	}