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containerd-to-vm

What

A recent article from the team at fly.io described how they build VMs for firecracker from the docker image provided by their customers. They outline the following steps:

  1. Pull the matching container from the registry.
  2. Create a loop device to store the container’s filesystem on.
  3. Unpack the container (in this case, using Docker’s Go libraries) into the mounted loop device.
  4. Create a second block device and inject our init, kernel, configuration, and other goop into.
  5. Track down any persistent volumes attached to the application, unlock them with LUKS, and collect their unlocked block devices.
  6. Create a TAP device, configure it for our network, and attach BPF code to it.
  7. Hand all this stuff off to Firecracker and tell it to boot .

As I’ve been interested in playing with both containerd’s API and firecracker, I thought it would be a good opportunity to try to implement this.

How

You’ll need a few things before you can run this tool.

Kernel

You’ll need a kernel to run this. The following instructions should do it (to get a 5.8 kernel):

git clone https://github.com/torvalds/linux.git linux.git
cd linux.git
git checkout v5.8
curl -o .config -s https://github.com/firecracker-microvm/firecracker/blob/main/resources/microvm-kernel-x86_64.config
make menuconfig
make vmlinux -j8

CNI

You need CNI plugins and configurations installed. The recommended configuration is stored under hack/cni. It needs to be copied to /etc/cni/conf.d.

Firecracker binaries

Running make firecracker-binary will download a version of firecracker under the directory hack/firecracker.