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a file system for mounting container images

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Composefs is a native Linux file system designed to help sharing filesystem contents, as well as ensuring said content is not modified. The initial target usecase are container images and ostree commits.

The basic idea is to have a single binary file that contains all the metadata of the filesystem, including the filenames, the permissions, the timestamps, etc. However, it doesn't contain the actual contents, but rather filenames to the real files that contain the contents. This is somewhat similar to overlayfs, which also doesn't store the file.

You pass the filename of the blob as well as the base directory for the content files when you mount the filesystem like this:

# mount composefs -t composefs -o descriptor=/path/to/blob,basedir=/path/to/content /mnt

This by itself doesn't seem very useful. You could use a single squashfs image, or regular directory with the files instead. However, the advantage comes if you want to store many such images. By storing the files content-adressed (e.g. using the hash of the content to name the file) shared files need only be stored once, yet can appear in multiple mounts. Since these are normal files they will also only be stored once in the page cache, meaning that the duplication is avoided both on disk and in ram.

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 on its own, in that it can only verify file data, not metadata.

Usecase: container images

When pulling a container image to the local storage we normally just untar each layer by itself. Instead we can store the file content in an content-addressed fashion, and then generate a composefs file for the layer (or perhaps the combined layers).

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: OSTree

OSTree already uses a content-address 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 reproducable, 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.

user space tools

The directory tools/ contains some user space tools to create the binary blob to pass to the client. They are all experimental and lack documentation.

  • mkcomposefs: Creates a composefs image given a directory pathname. Can also compute digests and create a content store directory.
  • writer-json: convert from a CRFS metadata file to the binary blob.
  • dump: prints the content of the binary blob.
  • converts an OSTree commit into a CRFS config file that writer-json can use.

kernel module

How to build:

# make -C $KERNEL_SOURCE modules M=$PWD &&  make -C $KERNEL_SOURCE modules_install M=$PWD
# insmod /lib/modules/$(uname -r)/extra/composefs.ko

Once it is loaded, it can be used as:

# mount composefs -t composefs -o descriptor=/path/to/blob,basedir=$BASE_DIR  /mnt

Mount options:

descriptor: the path to the binary blob that was generated with the user space tools. basedir: is the directory to use as a base when resolving relative content paths. 'noverity': Don't verify that target files have the right fs-verity digest. Useful if the fs doesn't support fs-verity but the descriptor has digests enabled. digest: A fs-verity sha256 digest that the descriptor file must match.

SELinux issues

Composefs support xattrs natively, and selinux normally uses xattrs to store selinux file contexts. However, this only works if the local policy allows a particular filesystem type to use xattrs for selinux, and the default is to not allow it. So, until the default selinux contexts supports composefs, you need to manually install a local policy for this.

To enable composefs selinux support, run:

# semodule -i composefs.cil

And, to later revert it, run:

# semodule -r composefs


a file system for mounting container images






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