Mount /etc/machine-id in elastic-agent-managed-kubernetes.yaml

[daniel-almeida]

Describe the enhancement:

Change the yaml for managed Agent in k8s so that it has a volume for /etc/machine-id that is mounted in the Agent's container.

New volume mount for the elastic-agent container:

- name: etc-mid
  mountPath: /etc/machine-id
  readOnly: true

New volume:

- name: etc-mid
  hostPath:
    path: /etc/machine-id
    type: File

Describe a specific use case for the enhancement or feature:

To determine host ID, Agent needs the content of /etc/machine-id (see https://github.com/elastic/go-sysinfo/blob/main/providers/linux/machineid.go#L32).

Currently, the file is present in the container, but empty, which results in Agent using the value "" when determining the host ID, and sending an empty host ID to endpoint. That makes the policy status unhealthy because Endpoint requires a valid host ID.

[lrishi]

Cc: @norrietaylor @m-sample @qcorporation

[m-sample]

+1 on readOnly! Given that type: File means A file must exist at the given path we should test what happens when that file does not exist (e.g. on some mutant, hacked over Linux setup). If it's not a good response (e.g. Pod fails to launch) we may want to document it well for SDH avoidance or perhaps leave the type field as unspecified for 'no checks' behavior.

(Pod volume mount type field docs: https://kubernetes.io/docs/concepts/storage/volumes/#hostpath )