How to install a custom package slice

When a specific package slice is not available on the upstream Chisel releases, you will more likely end up creating your own slice definition.

Once you have it though, the most obvious question is: how can I install this custom slice with Chisel?

Let’s assume you want to install the OpenSSL binaries slice created in this guide.

First, clone the Chisel releases repository:

# Let's assume we are working with Ubuntu 22.04
git clone -b ubuntu-22.04

This repository acts as the database of slice definitions files for each Chisel release (Chisel releases are named analogously to Ubuntu releases, and mapped into Git branches within the repository).

Chisel will only recognise slices belonging to a Chisel release, so you need to copy your slice definitions file - openssl.yaml in this example - into the chisel-releases/slices folder. Note that if a slice definitions file with the same name already exists, it most likely means that the package you’re slicing has already been sliced before, and in this case, you only need to merge your changes into that existing file.

At this point, you should be able to find your custom OpenSSL slice bins in the local Chisel release:

grep -q "bins" chisel-releases/slices/openssl.yaml && echo "My slice exists"

If you wanted to test it with Chisel alone, you could now simply run

# Testing with Chisel directly:
mkdir -p my-custom-openssl-fs
chisel cut --release ./chisel-releases --root my-custom-openssl-fs openssl_bins

You should end up with a folder named “my-custom-openssl-fs” containing a few folders, amongst which there would be ./usr/bin/openssl.

To install the custom package slice into a rock though, you need to use Rockcraft!

Start by initialising a new Rockcraft project:

rockcraft init

After this command, you should find a new rockcraft.yaml file in your current path.

Adjust the rockcraft.yaml file according to the following content (feel free to adjust the metadata, but pay special attention to the parts section):

name: custom-openssl-rock
base: bare
build_base: "[email protected]"
version: '0.0.1'
summary: A chiselled rock with a custom OpenSSL slice
description: |
    A rock containing only the binaries (and corresponding dependencies) from the OpenSSL package.
    Built from a custom Chisel release.
license: GPL-3.0
        plugin: nil
        source: chisel-releases/
        source-type: local
            chisel cut --release ./ --root ${CRAFT_PART_INSTALL} openssl_bins

The “build-context” part allows you to send the local chisel-releases folder into the builder. The “override-build” enables you to install your custom slice. Please not that this level of customisation is only needed when you want to install from a custom Chisel release. If the desired slice definitions are already upstream, then you can simply use stage-packages, as demonstrated in here.

Build your rock with:

rockcraft pack

The output will be:

Launching instance...
Retrieved base bare for amd64
Extracted bare:latest
Executed: pull build-context
Executed: pull pebble
Executed: overlay build-context
Executed: overlay pebble
Executed: build build-context
Executed: build pebble
Executed: stage build-context
Executed: stage pebble
Executed: prime build-context
Executed: prime pebble
Executed parts lifecycle
Exported to OCI archive 'custom-openssl-rock_0.0.1_amd64.rock'

Test that the OpenSSL binaries have been correctly installed with the following:

sudo rockcraft.skopeo --insecure-policy copy oci-archive:custom-openssl-rock_0.0.1_amd64.rock docker-daemon:chisel-openssl:latest

The output will be:

Getting image source signatures
Copying blob 253d707d7e97 done
Copying blob 7044a53e1935 done
Copying config c114b59704 done
Writing manifest to image destination
Storing signatures

And after:

docker run --rm chisel-openssl exec openssl

The output of the Docker command will be OpenSSL’s default help message:


Standard commands
asn1parse         ca                ciphers           cmp
cms               crl               crl2pkcs7         dgst
dhparam           dsa               dsaparam          ec
ecparam           enc               engine            errstr
fipsinstall       gendsa            genpkey           genrsa
help              info              kdf               list
mac               nseq              ocsp              passwd
pkcs12            pkcs7             pkcs8             pkey
pkeyparam         pkeyutl           prime             rand
rehash            req               rsa               rsautl
s_client          s_server          s_time            sess_id
<... many more lines of output>

And that’s it! You’ve now built your own rock from a custom Chisel release. Next step: share your slice definitions file with others!