Problem to solve
I’m developing a tool to manage btrfs snapshots. For testing, it requires a real btrfs filesystem to be mounted on the system. I haven’t found any meaningful way to stub or fake this out.
I want to run my tests on GitLab-hosted CI, but I can’t seem to mount btrfs filesystems from within docker, despite that the containers run with --privileged. I reckon the btrfs module isn’t mounted on the host, but I can’t find a way to load the module from the container.
Steps to reproduce / Configuration
A minimal gitlab-ci.yml to reproduce:
stages:
- test
repro:
stage: test
image: alpine
script:
- apk add btrfs-progs
- truncate -s $((1024*1024*1024)) /test.dd
- mkfs.btrfs /test.dd
- mount -o loop /test.dd /mnt
I ran this and got:
$ mount -o loop /test.dd /mnt
mount: mounting /dev/loop0 on /mnt failed: Invalid argument
Investigation, alternatives and workarounds
If I run the steps outlined above with docker run --privileged on my laptop, if the btrfs module is not loaded, I also get Invalid argument.
If I modprobe btrfs on the host, before or after the container is created, the mount succeeds.
In another gitlab-ci.yml test I confirmed that cat /proc/filesystems does not contain btrfs.
Elsewhere I’ve seen that when a docker container requires a specific module, one practice is to mount /lib/modules as a volume and let the container load modules as necessary. See linuxserver-wireguard for an example. On GitLab, I don’t see /lib/modules mounted in the runner, nor a way to cause this.
I imagine I could run my tests in qemu, which would remove the need to run tests as root, but would require a lot of work to make a test harness to do this, and possibly rewriting all my tests to make use of it.
I can’t think of any other workarounds.
Versions
- Self-managed
- GitLab.com SaaS
- Self-hosted Runners