Get diff directory path in core

[Jonas-Heinrich]

I am new to the containerd project and therefore unfamiliar with the code base. Nevertheless, I am working on a custom modification of the checkpoint procedure.

My Question: What is the correct way to get the diff directory path (used by the overlay2 storage driver) for the given process here:

func (p *Init) checkpoint(ctx context.Context, r *CheckpointConfig) error {
    // ...
}

p.Rootfs is empty. I also tried to find a similar example somewhere else (e.g. snapshot), however it didn't really match.