composefs

The composefs project combines several underlying Linux features to provide a very flexible mechanism to support read-only mountable filesystem trees, stacking on top of an underlying "lower" Linux filesystem.

The key technologies composefs uses are:

• overlayfs as the kernel interface
• EROFS for a mountable metadata tree
• fs-verity (optional) from the lower filesystem

The manner in which these technologies are combined is important. First, to emphasize: composefs does not store any persistent data itself. The underlying metadata and data files must be stored in a valid "lower" Linux filesystem. Usually on most systems, this will be a traditional writable persistent Linux filesystem such as ext4, xfs,, btrfs etc.

Separation between metadata and data

A key aspect of the way composefs works is that it's designed to store "data" (i.e. non-empty regular files) distinct from "metadata" (i.e. everything else).

composefs reads and writes a filesystem image which is really just an EROFS which today is loopback mounted.

However, this EROFS filesystem tree is just metadata; the underlying non-empty data files can be shared in a distinct "backing store" directory. The EROFS filesystem includes trusted.overlay.redirect extended attributes which tell the overlayfs mount how to find the real underlying files.

Mounting multiple composefs with a shared backing store

The key targeted use case for composefs is versioned, immutable executable filesystem trees (i.e. container images and bootable host systems), where some of these filesystems may share parts of their storage (i.e. some files may be different, but not all).

Composefs ships with a mount helper that allows you to easily mount images by passing the image filename and the base directory for the content files like this:

mount -t composefs /path/to/image  -o basedir=/path/to/content /mnt

By storing the files content-addressed (e.g. using the hash of the content to name the file), shared files only need to be stored once, yet can appear in multiple mounts.

Backing store shared on disk and in page cache

A crucial advantage of composefs in contrast to other approaches is that data files are shared in the page cache.

This allows launching multiple container images that will reliably share memory.

Filesystem integrity

Composefs also supports fs-verity validation of the content files. When using this, the digest of the content files is stored in the image, and composefs will validate that the content file it uses has a matching enabled fs-verity digest. This means that the backing content cannot be changed in any way (by mistake or by malice) without this being detected when the file is used.

You can also use fs-verity on the image file itself, and pass the expected fs-verity digest as a mount option, which composefs will validate. In this case we have full trust of both data and metadata of the mounted file. This solves a weakness that fs-verity has when used on its own, in that it can only verify file data, not metadata.

Usecase: container images

There are multiple container image systems; for those using e.g. OCI a common approach (implemented by both docker and podman for example) is to just untar each layer by itself, and then use overlayfs to stitch them together at runtime. This is a partial inspiration for composefs; notably this approach does ensure that identical layers are shared.

However if instead we store the file content in a content-addressed fashion, and then we can generate a composefs file for each layer, continuing to mount them with a chain of overlayfs or we can generate a single composefs for the final merged filesystem tree.

This allows sharing of content files between images, even if the metadata (like the timestamps or file ownership) vary between images.

Together with something like zstd:chunked this will speed up pulling container images and make them available for usage, without the need to even create these files if already present!

Usecase: Bootable host systems (e.g. OSTree)

OSTree already uses a content-addressed object store. However, normally this has to be checked out into a regular directory (using hardlinks into the object store for regular files). This directory is then bind-mounted as the rootfs when the system boots.

OSTree already supports enabling fs-verity on the files in the store, but nothing can protect against changes to the checkout directories. A malicious user can add, remove or replace files there. We want to use composefs to avoid this.

Instead of checking out to a directory, we generate a composefs image pointing into the object store and mount that as the root fs. We can then enable fs-verity of the composefs image and embed the digest of that in the kernel commandline which specifies the rootfs. Since composefs generation is reproducible, we can even verify that the composefs image we generated is correct by comparing its digest to one in the ostree metadata that was generated when the ostree image was built.

For more information on ostree and composefs, see this tracking issue.

tools

Composefs installs two main tools:

• mkcomposefs: Creates a composefs image given a directory pathname. Can also compute digests and create a content store directory.
• mount.composefs: A mount helper that supports mounting composefs images.

mounting a composefs image

The mount.composefs helper allows you to mount composefs images (of both types).

The basic use is:

mount -t composefs /path/to/image.cfs -o basedir=/path/to/datafiles  /mnt

The default behaviour for fs-verity is that any image files that specifies an expected digest needs the backing file to match that fs-verity digest, at least if this is supported in the kernel. This can be modified with the verity and noverity options.

Mount options:

• basedir: is the directory to use as a base when resolving relative content paths.
• verity: All image files must specify a fs-verity image.
• noverity: Don't verfy fs-verity digests (useful for example if fs-verity is not supported on basedir).
• digest: A fs-verity sha256 digest that the image file must match. If set, verity_check defaults to 2.
• upperdir: Specify an upperdir for the overlayfs filesystem.
• workdir: Specify an upperdir for the overlayfs filesystem.
• idmap: Specify a path to a user namespace that is used as an idmap.