As of Linux 6.6-rc1, which contains the overlayfs fs-verity support, all the kernel changes that was required for composefs are upstream. This allows us to finalize the composefs image format and give guarantees of its future stability.

This means that we are happy to welcome Composefs 1.0 to the world!

The main feature of 1.0 is the stability of the file format and the library API, however, there are a few new major features in 1.0:

• Various tweaks make the image format more efficient.
• The library and the tools now has the ability to inspect composefs image files. This includes listing what basedir object files they refer to which makes it easy to figure out what objects are missing (and has to be downloaded).
• The use of the built-in kernel fs-verity signature verification has been dropped on recommendation from the fs-verity maintainer. Instead we recommended to use userspace libraries to verify fs-verity digests.

For more details and download links, see the release notes.  For a short introduction to composefs, see this earlier blog entry.

There is also ongoing work in the wider community to use composefs:

Ostree 2023.6 and rpm-ostree 2023.6 together allow for end-to-end signed and validated composefs ostree deployments. The code is still marked experimental and composefs needs to be enabled manually on the host, but the feature is compiled in and available by default.

containers/storage contains initial work on supporting composefs in the overlayfs backend. Once this is finalized and used in podman, it will be possible to use the cross-image de-duplication and tamper-proofing features of composefs for all podman containers. This will lead to improved container density and security.