How to customize golang-docker image to use golang for scripting?

Question

I came across this blog: using go as a scripting language and tried to create a custom image that I can use to run golang scripts ie

FROM golang:1.15
RUN go get github.com/erning/gorun
RUN mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc
RUN echo ':golang:E::go::/go/bin/gorun:OC' | tee /proc/sys/fs/binfmt_misc/register

It fails with error:

mount: /proc/sys/fs/binfmt_misc: permission denied.
ERROR: Service 'go_saga' failed to build : The command '/bin/sh -c mount binfmt_misc -t binfmt_misc /proc/sys/fs/binfmt_misc' returned a non-zero code: 32

It's readonly file system so can't change the permissions as well. The task I'm trying to achieve here is well documented here . Please help me with following questions:

1. Is that even possible ie mount /proc/sys/fs/binfmt_misc and write to the file: /proc/sys/fs/binfmt_misc/register ?
2. If Yes, how to do that ?

I guess, it would be great, if we could run golang scripts in the container.

Answer 1

First a quick disclaimer that I haven't done this binfmt trick to run go scripts. I suppose it might work, but I just use go run when I want to run something on the fly.

There's a lot to unpack in this. Container isolation runs an application with a shared kernel in an isolated environment. The namespaces, cgroups, and security settings are designed to prevent one container from impacting other containers or the host.

Why is that important? Because /proc/sys/fs/binfmt_misc is interacting with the kernel, and pushing a change to that would be considered a container escape since you're modifying the underlying host.

The next thing to cover is building an image vs running a container. When you build an image with the Dockerfile, you are defining the image filesystem and some metadata (labels, entrypoint, exposed ports, etc). Each RUN command executes that command inside a temporary container, based on the previous step's result, and when the command finishes it captures the changes to the container filesystem. When you mount another filesystem, that doesn't change the underlying container filesystem, so even if you could, the mount command would be a noop during the image build.

So if this is possible, you'll need to do it inside the container rather than during build time, that container will need to be privileged since doing things like mounting filesystems and modifying /proc requires access not normally given to containers, and you'll be modifying the host kernel in the process. You'd need to make the container entrypoint run the mount and register the binfmt_misc entry, and figure out what to do if the entry is already setup/registered, but possibly to a different directory in another container.

As an aside, when dealing with binfmt_misc and containers, the F flag is very important, though in your use case it's important that you don't have it. Typically you need the F flag so the binary is found on the host filesystem rather than searched for within the container filesystem namespace. The typical use case of binfmt_misc and containers is configuring the host to be able to run containers for different architectures, eg Docker Desktop can run amd64, arm64, and a bunch of other platforms today using this.

In the end, if you want to run a container as a one off to run a go command as a script, I'd skip the binfmt misc trick and make an entrypoint that does a go run instead. But if you're using the container for longer run processes where you want to periodically run a go file as a script, you'll need to do that in the container, and as a privileged container that has the ability to escape to the host.