fiber_check_cfunc(mrb_state *mrb, struct mrb_context *c)
{
  mrb_callinfo *ci;

  for (ci = c->ci; ci >= c->cibase; ci--) {
    if (ci->acc < 0) {
      mrb_raise(mrb, E_FIBER_ERROR, "can't cross C function boundary");
    }
  }
}

The product reads data past the end, or before the beginning, of the intended buffer.