This is a helper utility to extract to exact kernel release from docker.iso. With this information you are able to build external kernel modules for docker-for-desktop linux kernel.
When trying to compile the ZFSonLinux kernel module I recognized the additional string RANDSTRUCT_PLUGIN_${sha256_hash}
with any kernel module bundled within the Docker-for-Desktop installation. I found, that this is a security measure and
the correct RANDSTRCUCT_PLUGIN secret is needed to build loadable kernel modules.
Turns out, that the the kernel-sources from docker.io/linuxkit/kernel are not the ones use for the Docker-for-Desktop
linux kernels. Instead docker.io/docker/for-desktop-kernel included the kernels. A simple uname -r does not
reveal the information needed, as (at the time of writing this) there are no less than 13 images for kernel 4.19.76.
After building the ZFS kernel module with a larger part of those kernel images I found out, that the appended hash in
the docker image tag can be found in docker.iso (which is mapped to /dev/sr0) in the file /etc/linuxkit.yml.
Unfortunately this file is not directly accessible which your docker-for-desktop-virtual-machine. Fortunately there is a
pure-golang library for reading ISO9660 file systems.
Wrapping all together:
- build the Dockerimage:
docker build -t docker-for-desktop-get-kernel-image . - Extact the information:
docker run --rm --device=/dev/sr0 docker-for-desktop-get-kernel-image - Use this image to build kernel modules for your currently running docker-for-desktop kernel.