在Kubernetes中将ConfigMap挂载为卷时，如何计算defaultMode的取值？

在Kubernetes的Deployment里给Volume设置defaultMode确实有点绕，它要求输入三位十进制数，这个数值对应UNIX权限的二进制值。举个例子，如果想让挂载的ConfigMap拥有r------（仅所有者可读）的权限，就得把defaultMode设为256。

下面是完整的配置示例，包含Deployment和对应的ConfigMap：

apiVersion: apps/v1
kind: Deployment
metadata:
  name: foo
  namespace: foo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: foo
  template:
    metadata:
      labels:
        app: foo
    spec:
      containers:
        - image: php-fpm:latest
          volumeMounts:
            - name: phpini
              mountPath: /usr/local/etc/php/conf.d/99-settings.ini
              readOnly: true
              subPath: 99-settings.ini
      volumes:
        - configMap:
            defaultMode: 256
            name: phpini-configmap
            optional: false
          name: phpini
---
apiVersion: v1
kind: ConfigMap
metadata:
  labels:
    app: foo
  namespace: foo
  name: phpini-configmap
data:
  99-settings.ini: |
    ; Enable Zend OPcache extension module
    zend_extension = opcache

内容的提问来源于stack exchange，提问作者Jesus Iniesta