Containerd on Luther Monson

Bind Mounts Across Mount Namespaces: Four PRs and a Container Escape

Thu, 24 Sep 2026 21:02:00 -0700

GitHub Actions has a container: key that runs a job’s steps inside an image of your choosing. It’s popular because it gives you a reproducible toolchain without building a custom runner image. On a self-hosted runner it works by having the runner drive Docker. On ephemerd, “Docker” is a shim that speaks the Docker API and creates containers in containerd. Getting bind mounts right through that shim took four PRs in a week. A couple of months later the version that worked turned out to be an escape.

Kata Containers for CI: Boot Cost Isn't the Problem, File I/O Is

Thu, 03 Sep 2026 21:35:00 -0700

Of the three platforms ephemerd runs jobs on, Linux has the weakest isolation. Windows jobs get a Hyper-V isolated container with its own kernel. macOS jobs get a whole VM. A Linux job is an ordinary runc container sharing the host kernel. A kernel escape from a Linux job is a host compromise. Kata Containers fixes that by putting each container in a lightweight VM with its own kernel. It plugs into containerd as just another runtime handler. So I added it in #155, measured it, and left it off by default. This post is why.

ephemerd: One Daemon, a Fresh Runner Per Job, Every OS

Tue, 18 Aug 2026 21:05:00 -0700

I have CI that needs Linux amd64, Linux arm64, Windows and macOS. GitHub-hosted runners for all four get expensive fast, especially the arm64 and Mac minutes. Self-hosted runners fix the bill and create a different problem. A stock actions/runner on a box you own runs whatever the workflow says. It keeps whatever the last job left behind. And it’s one fork PR away from somebody running code on your LAN.