  We discuss the stability of highly degenerate zero-energy states tha appear
at the surface of a nodal superconductor preserving time-reversal symmetry. The
existence of such surface states is a direct consequence of the nontrivial
topological numbers defined in the restricted Brillouin zones in the clean
limit. In experiments, however, potential disorder is inevitable near the
surface of a real superconductor, which may lift the high degeneracy at zero
energy. We show that an index defined in terms of the chiral eigenvalues of the
zero-energy states can be used to measure the degree of degeneracy at zero
energy in the presence of potential disorder. We also discuss the relationship
between the index and the topological numbers.
