diff --git a/usr.bin/config/main.c b/usr.bin/config/main.c
index fa409b84e64c..a1aad388ef21 100644
--- a/usr.bin/config/main.c
+++ b/usr.bin/config/main.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: main.c,v 1.59 2014/10/10 07:08:26 uebayasi Exp $	*/
+/*	$NetBSD: main.c,v 1.60 2014/10/10 07:48:50 uebayasi Exp $	*/
 
 /*
  * Copyright (c) 1992, 1993
@@ -875,6 +875,13 @@ defopt(struct dlhash *ht, const char *fname, struct defoptlist *opts,
 				olddl = olddl->dl_next;
 			olddl->dl_next = dl;
 		}
+
+		/*
+		 * Implicit attribute definition for option.
+		 */
+		const char *n; 
+		n = strtolower(dl->dl_name);
+		refattr(n);
 	}
 }
 
diff --git a/usr.bin/config/sem.c b/usr.bin/config/sem.c
index 1daabecd657b..43599f4fcf47 100644
--- a/usr.bin/config/sem.c
+++ b/usr.bin/config/sem.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: sem.c,v 1.51 2014/10/10 07:08:26 uebayasi Exp $	*/
+/*	$NetBSD: sem.c,v 1.52 2014/10/10 07:48:50 uebayasi Exp $	*/
 
 /*
  * Copyright (c) 1992, 1993
@@ -236,12 +236,13 @@ defattr(const char *name, struct loclist *locs, struct attrlist *deps,
 		}
 	}
 
-	a = mkattr(name);
-	if (a == NULL) {
+	if (getrefattr(name, &a)) {
 		cfgerror("attribute `%s' already defined", name);
 		loclist_destroy(locs);
 		return (1);
 	}
+	if (a == NULL)
+		a = mkattr(name);
 
 	a->a_deps = deps;
 	expandattr(a, NULL);
@@ -674,6 +675,28 @@ refattr(const char *name)
 		(void)mkattr(name);
 }
 
+int
+getrefattr(const char *name, struct attr **ra)
+{
+	struct attr *a;
+
+	a = ht_lookup(attrtab, name);
+	if (a == NULL) {
+		*ra = NULL;
+		return (0);
+	}
+	/*
+	 * Check if the existing attr is only referenced, not really defined.
+	 */
+	if (a->a_deps == NULL &&
+	    a->a_iattr == 0 &&
+	    a->a_devclass == 0) {
+		*ra = a;
+		return (0);
+	}
+	return (1);
+}
+
 /*
  * Recursively expand an attribute and its dependencies, checking for
  * cycles, and invoking a callback for each attribute found.
diff --git a/usr.bin/config/sem.h b/usr.bin/config/sem.h
index ab8aa734de78..eb49d44d9b42 100644
--- a/usr.bin/config/sem.h
+++ b/usr.bin/config/sem.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: sem.h,v 1.12 2014/10/10 06:59:38 uebayasi Exp $	*/
+/*	$NetBSD: sem.h,v 1.13 2014/10/10 07:48:50 uebayasi Exp $	*/
 
 /*
  * Copyright (c) 1992, 1993
@@ -58,6 +58,7 @@ struct deva    *getdevattach(const char *);
 struct attr	*mkattr(const char *);
 struct attr    *getattr(const char *);
 void		refattr(const char *);
+int		getrefattr(const char *, struct attr **);
 void		expandattr(struct attr *, void (*)(struct attr *));
 void		selectattr(struct attr *);
 void		setmajor(struct devbase *, int);