code: func (kl *Kubelet) podFieldSelectorRuntimeValue(fs *v1.ObjectFieldSelector, pod *v1.Pod, podIP string) (string, error) {
  	internalFieldPath, _, err := podshelper.ConvertDownwardAPIFieldLabel(fs.APIVersion, fs.FieldPath, "")
  	if err != nil {
  		return "", err
  	}
  	switch internalFieldPath {
  	case "spec.nodeName":
  		return pod.Spec.NodeName, nil
  	case "spec.serviceAccountName":
  		return pod.Spec.ServiceAccountName, nil
  	case "status.hostIP":
  		hostIP, err := kl.getHostIPAnyWay()
  		if err != nil {
  			return "", err
  		}
  		return hostIP.String(), nil
  	case "status.podIP":
  		return podIP, nil
  	}
  	return fieldpath.ExtractFieldPathAsString(pod, internalFieldPath)
  }
docs: // podFieldSelectorRuntimeValue returns the runtime value of the given
  // selector for a pod.