Allow configuration of Rootlesskit's CopyUpDirs through an environment variable

[ludost]

Proposed Changes

In our use-case we are running an K3S Agent in a Rootless environment at the end-user's own workstation. This works well (enough), but we also want to provide access to the local folders (through e.g. HostPath volume mounts). But the Agent runs inside the rootless namespace, disallowing read/write access to the actual file-system.

This PR allows the Agent to setup extra "CopyUpDirs" entries, through an environment variable called "K3S_ROOTLESS_COPYUPDIRS". This environment variable can receive a comma-separated list absolute paths, which will be added to the existing list.

Types of Changes

This change is non-breaking, fully backwards compatible. It only adds a new feature and a new environment variable. The setup will not start (with a fatal error) if any of the added folders doesn't exist in the host system. Which is the normal behavior of the pre-existing code.

Verification

• Create a folder as your normal user: mkdir /home/<your_username>/containerShare
• Set an environment variable: export K3S_ROOTLESS_COPYUPDIRS=/home/<your_username>/containerShare
• Start a K3S agent (and/or server) with the --rootless parameter.
• Create a container with a hostPath volume mount like:

  volumeMounts:
    - mountPath:   /my_share/
      name: containerShare

   volumes: 
      - hostPath:
           path: /home/<your_username>/containerShare
           type: ""
        name: containerShare

• Try to write a file into the volume mount inside the container: touch /my_share/helloWorld
• From the host system, check if the file is there: ls /home/<your_username>/containerShare/

Testing

No, I currently have no unit test for this change.

Linked Issues

See #10385 for a further description.

User-Facing Change

Add new environment variable "K3S_ROOTLESS_COPYUPDIRS" to add folders to the Rootlesskit configuration.

Further Comments

[brandond]

kinda weird to both in-place modify the list, and also return it, but ok.

[ludost]

This should have been part of the standard libraries of Golang, but in the absence of a clear Map function I choose to just include a simple utility method I've found on: https://stackoverflow.com/a/49429437.
This leads me to 2 questions on K3S policy:

• Does K3S have a standard location of such utility methods?
• Can I use this example, or should I rewrite it myself for copyright purposes?

[brandond]

Do we really need this or can we just tell folks to, I don't know, not include unnecessary whitespace in the env var? We don't trim whitespace for any other args or env vars that take a comma-separated list, we just expect people to provide valid input.

[ludost]

FYI: to fix that DCO error, I've followed the advice of the DCO-page and rebased the commits, with proper sign-off statements.

[brandond]

Suggested change

	hasCUP := os.Getenv(copyUpDirsEnv) != ""
	if hasCUP {
	opt.CopyUpDirs = append(opt.CopyUpDirs, trimWhiteSpace(strings.Split(os.Getenv(copyUpDirsEnv), ","))...)
	if copyUpDirs := os.Getenv(copyUpDirsEnv); copyUpDirs != "" {
	opt.CopyUpDirs = append(opt.CopyUpDirs, strings.Split(copyUpDirs, ",")...)

[brandond]

Do we really need this or can we just tell folks to, I don't know, not include unnecessary whitespace in the env var? We don't trim whitespace for any other args or env vars that take a comma-separated list, we just expect people to provide valid input.