diff -urBN /usr/src.orig/sys/fs/unionfs/union_subr.c /usr/src/sys/fs/unionfs/union_subr.c
--- /usr/src.orig/sys/fs/unionfs/union_subr.c	2007-10-22 23:28:17.000000000 +0900
+++ /usr/src/sys/fs/unionfs/union_subr.c	2007-10-23 01:44:18.000000000 +0900
@@ -990,8 +990,14 @@
 		uio.uio_resid = iov.iov_len;
 
 		error = VOP_READDIR(lvp, &uio, cred, &eofflag, NULL, NULL);
-		if (error)
+		if (error != 0)
 			break;
+		if (eofflag == 0 && uio.uio_resid == sizeof(buf)) {
+#ifdef DIAGNOSTIC
+			panic("bad readdir response from lower FS.");
+#endif
+			break;
+		}
 
 		edp = (struct dirent*)&buf[sizeof(buf) - uio.uio_resid];
 		for (dp = (struct dirent*)buf; !error && dp < edp;