Kubernetes fsGroup not changing file ownership on PersistentVolume - kubernetes

Question

On the host, everything in the mounted directory (/opt/testpod) is owned by uid=0 gid=0. I need those files to be owned by whatever the container decides, i.e. a different gid, to be able to write there. Resources I'm testing with:
---
apiVersion: v1
kind: PersistentVolume
metadata:
name: pv
labels:
name: pv
spec:
storageClassName: manual
capacity:
storage: 10Mi
accessModes:
- ReadWriteOnce
hostPath:
path: "/opt/testpod"
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: pvc
spec:
storageClassName: manual
selector:
matchLabels:
name: pv
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 10Mi
---
apiVersion: v1
kind: Pod
metadata:
name: testpod
spec:
nodeSelector:
foo: bar
securityContext:
runAsUser: 500
runAsGroup: 500
fsGroup: 500
volumes:
- name: vol
persistentVolumeClaim:
claimName: pvc
containers:
- name: testpod
image: busybox
command: [ "sh", "-c", "sleep 1h" ]
volumeMounts:
- name: vol
mountPath: /data
After the pod is running, I kubectl exec into it and ls -la /data shows everything still owned by gid=0. According to some Kuber docs, fsGroup is supposed to chown everything on the pod start but it doesn't happen. What am I doing wrong please?

Answer 1

The hostpath type PV doesn't support security context. You have to be root for the volume to be written in. It is described well in this github issue and this docs about hostPath:
The directories created on the underlying hosts are only writable by root. You either need to run your process as root in a privileged
container or modify the file permissions on the host to be able to write to a
hostPath volume
You may also want to check this github request describing why changing permission of host directory is dangerous.
The workaround people describe that it appears to be working is to grant your user sudo privileges but that actually makes the idea of running container as non root user useless.
Security context appears to be working well with emptyDir volume (described well in the k8s docs here)