union_copyup
extern int union_copyup(struct union_node *, int, kauth_cred_t,
error = union_copyup(un, 1, cnp->cn_cred, curlwp);
error = union_copyup(un, (mode&O_TRUNC) == 0, cred, l);
error = union_copyup(un, 1, ap->a_cred, curlwp);
error = union_copyup(un, (vap->va_size != 0),