diff --git a/sys/kern/kern_prot.c b/sys/kern/kern_prot.c
--- a/sys/kern/kern_prot.c
+++ b/sys/kern/kern_prot.c
@@ -1277,36 +1277,43 @@
 }
 
 /*
- * Check if gid is a member of the group set.
+ * Returns whether gid designates a supplementary group in cred.
  */
-int
-groupmember(gid_t gid, struct ucred *cred)
+static int
+supplementary_group_member(gid_t gid, struct ucred *cred)
 {
-	int l;
-	int h;
-	int m;
-
-	if (cred->cr_groups[0] == gid)
-		return(1);
+	int l, h, m;
 
 	/*
-	 * If gid was not our primary group, perform a binary search
-	 * of the supplemental groups.  This is possible because we
-	 * sort the groups in crsetgroups().
+	 * Perform a binary search of the supplemental groups.  This is possible
+	 * because we sort the groups in crsetgroups().
 	 */
 	l = 1;
 	h = cred->cr_ngroups;
+
 	while (l < h) {
-		m = l + ((h - l) / 2);
+		m = l + (h - l) / 2;
 		if (cred->cr_groups[m] < gid)
-			l = m + 1; 
+			l = m + 1;
 		else
-			h = m; 
+			h = m;
 	}
-	if ((l < cred->cr_ngroups) && (cred->cr_groups[l] == gid))
+
+	return (l < cred->cr_ngroups && cred->cr_groups[l] == gid);
+}
+
+/*
+ * Check if gid is a member of the (effective) group set (i.e., effective and
+ * supplementary groups).
+ */
+int
+groupmember(gid_t gid, struct ucred *cred)
+{
+
+	if (cred->cr_groups[0] == gid)
 		return (1);
 
-	return (0);
+	return (supplementary_group_member(gid, cred));
 }
 
 /*