cr_cansee
if (cr_cansee(cred, sc->ucred))
return (cr_cansee(td->td_ucred, p->p_ucred));
if (cr_cansee(req->td->td_ucred,
cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
error = cr_cansee(req->td->td_ucred,
int cr_cansee(struct ucred *u1, struct ucred *u2);