This has some tools to build a (fedora-based) disk image that can be used to boot a custom kernel and then test composefs inside it using qemu and virtiofs to share files with the host.
To use this, first install the dependencies:
$ dnf install virtiofsd osbuild osbuild-tools
Then build the test image:
$ ./build_image.sh
Then build a kernel with composefs and virtio support build in, and run it like this:
$ ./runkernel.sh /path/to/linux/arch/x86/boot/bzImage
This will give you a VM exposing the directory "shared" in this directory as /shared in the VM. Log in as "root" or "guest" both with password "password".