I was curious what this was for; see it was added in this PR;

• converter: support appending files to bootstrap layer containerd/nydus-snapshotter#591

Not sure how it's controlled though; the mention of "user can add some files" scared me a bit; do you have more context on this @imeoer (from a security perspective?)

Having a quick look, and it looks like this may be a continuation of containerd/nydus-snapshotter@a3fccd0, which (IIUC) allows for a custom image to be injected in the rootfs, for accelerated performance (containerd/nydus-snapshotter#244)

It allows nydus lazily load OCI targz layer with the nydus zran ref image.

So the previous code would allow for a "nydus zran" bootstrap image / tar to be used, from which the EntryBootstrap bootstrap-image would be loaded an injected into the init layer;

And the new option allows for custom files to be specified through AppendFiles to be added; https://github.com/containerd/nydus-snapshotter/blob/v0.13.13/pkg/converter/convert_unix.go#L628-L644

	bootstrapRa, err := local.OpenReader(targetBootstrapPath)
	if err != nil {
		return nil, errors.Wrap(err, "open bootstrap reader")
	}
	defer bootstrapRa.Close()

	files := append([]File{
		{
			Name:   EntryBootstrap,
			Reader: content.NewReader(bootstrapRa),
			Size:   bootstrapRa.Size(),
		},
	}, opt.AppendFiles...)
	var rc io.ReadCloser

	if opt.WithTar {
		rc = packToTar(files, false)

I guess my concern here ia; how can this behavior be controlled, as (IIUC, but I'm not super-familiar with all of this) this could result in arbitrary files to be injected?

Hi @thaJeztah, here is a nydus image manifest example, the manifest consists of a bootstrap layer (containerd.io/snapshot/nydus-bootstrap) and several blob layers (containerd.io/snapshot/nydus-blob).

• The bootstrap: it contains meta-information (inode, attrs...) about files and directories of all layers.
• The blob: is the data of each layer.

The option AppendFiles allows image build / conversion tools to append some files to the bootstrap layer, the format like:

gzip(tar(bootstrap + appended_files...))

so that nydus runtime (for example nydus snapshotter) can get more information about image, but the image build / conversion tools can't inject arbitrary files to image filesystem (bootstrap) by the converter package. :)