Index: lib/libc/gen/opendir.c
===================================================================
--- lib/libc/gen/opendir.c
+++ lib/libc/gen/opendir.c
@@ -273,6 +273,28 @@
 	return (true);
 }
 
+static int
+is_unionstack(int fd)
+{
+	struct statfs sfb;
+	int unionstack;
+
+	unionstack = _fcntl(fd, F_ISUNIONSTACK);
+	if (unionstack != -1)
+		return (unionstack);
+
+	/*
+	 * Temporary compat for kernels which don't provide F_ISUNIONSTACK.
+	 */
+	if (_fstatfs(fd, &sfb) < 0) {
+		unionstack = -1;
+		goto fail;
+	}
+	unionstack = !strcmp(sfb.f_fstypename, "unionfs")
+	    || (sfb.f_flags & MNT_UNION);
+fail:
+	return (unionstack);
+}
 
 /*
  * Common routine for opendir(3), __opendir2(3) and fdopendir(3).
@@ -310,15 +332,11 @@
 	/*
 	 * Determine whether this directory is the top of a union stack.
 	 */
+	unionstack = 0;
 	if (flags & DTF_NODUP) {
-		struct statfs sfb;
-
-		if (_fstatfs(fd, &sfb) < 0)
+		unionstack = is_unionstack(fd);
+		if (unionstack == -1)
 			goto fail;
-		unionstack = !strcmp(sfb.f_fstypename, "unionfs")
-		    || (sfb.f_flags & MNT_UNION);
-	} else {
-		unionstack = 0;
 	}
 
 	if (unionstack) {
Index: sys/kern/kern_descrip.c
===================================================================
--- sys/kern/kern_descrip.c
+++ sys/kern/kern_descrip.c
@@ -489,6 +489,7 @@
 	struct filedescent *fde;
 	struct proc *p;
 	struct vnode *vp;
+	struct mount *mp;
 	int error, flg, seals, tmp;
 	uint64_t bsize;
 	off_t foffset;
@@ -816,6 +817,48 @@
 		fdrop(fp, td);
 		break;
 
+	case F_ISUNIONSTACK:
+		/*
+		 * See if the vnode is part of a union stack (either the "union"
+		 * flag from mount(2) or unionfs).
+		 *
+		 * Prior to introduction of this op libc's readdir would call
+		 * fstatfs(2), in effect unnecessarily copying kilobytes of
+		 * data just to check fs name and a mount flag.
+		 *
+		 * Fixing the code to handle everything in the kernel instead
+		 * is a non-trivial endeavor and has low priority, thus this
+		 * horrible kludge facilitates the current behavior in a much
+		 * cheaper manner until someone(tm) sorts this out.
+		 */
+		error = fget_unlocked(fdp, fd, &cap_no_rights, &fp, NULL);
+		if (error != 0)
+			break;
+		if (fp->f_type != DTYPE_VNODE) {
+			fdrop(fp, td);
+			error = EBADF;
+			break;
+		}
+		/*
+		 * Since we don't prevent dooming the vnode even non-null mp
+		 * found can become immediately stale. This is tolerable since
+		 * mount points are type-stable (providing safe memory access)
+		 * and any vfs op on this vnode going forward will return an
+		 * error (meaning return value in this case is meaningless).
+		 */
+		mp = (struct mount *)atomic_load_ptr(&fp->f_vnode->v_mount);
+		if (__predict_false(mp == NULL)) {
+			fdrop(fp, td);
+			error = EBADF;
+			break;
+		}
+		td->td_retval[0] = 0;
+		if (strcmp(mp->mnt_vfc->vfc_name, "unionfs") == 0 ||
+		    mp->mnt_flag & MNT_UNION)
+			td->td_retval[0] = 1;
+		fdrop(fp, td);
+		break;
+
 	default:
 		error = EINVAL;
 		break;
Index: sys/sys/fcntl.h
===================================================================
--- sys/sys/fcntl.h
+++ sys/sys/fcntl.h
@@ -250,6 +250,7 @@
 #define	F_DUP2FD_CLOEXEC 18		/* Like F_DUP2FD, but FD_CLOEXEC is set */
 #define	F_ADD_SEALS	19
 #define	F_GET_SEALS	20
+#define	F_ISUNIONSTACK	21		/* Kludge for libc, don't use it. */
 
 /* Seals (F_ADD_SEALS, F_GET_SEALS). */
 #define	F_SEAL_SEAL	0x0001		/* Prevent adding sealings */