Initrd for Ubuntu Core

This repository contains source code to build initial ramdisk used in Ubuntu Core Embedded Linux OS.

Purpose

See: https://en.wikipedia.org/wiki/Initial_ramdisk

Architecture

In Ubuntu Core, initrd.img file is part of Kernel Snap as a binary. This file is brought to Kernel snap from a PPA. See integration below.

In UC20 and further, initrd is migrated from script based implementation (UC16/18) to systemd based. See the architecture document for more details.

Building initrd for Ubuntu Core

TODO: Write documentation for how to build initrd locally

Prequisities

Preparation

Building

Testing & Debugging

See Hacking

Releasing

The UC initrd is included in kernel snaps. However, the process to get it inside the kernel snap is not immediate and consists of a few steps. First, we need to build the ubuntu-core-initramfs debian package in the snappy-dev/image PPA by following these steps:

1. Update the changelog with latest changes since last release (use dch -i for this)

2. Commit using debcommit --release -a which will commit the changelog update & tag the release

3. Propose a PR to the repo with the new changelog, get it reviewed and merged

4. Push the tag to the repository with the new version (GitHub pull requests do not update tags)

5. Build the source package by running (note that the clean command removes all untracked files, including subtrees with .git folders)

    git clean -ffdx
    gbp buildpackage -S -sa -d --git-ignore-branch
   

6. Compare with the latest package that was uploaded to the snappy-dev PPA to make sure that the changes are correct. For this, you can download the .dsc file and the tarball from the PPA, then run debdiff to find out the differences:

    dget https://launchpad.net/~snappy-dev/+archive/ubuntu/image/+sourcefiles/ubuntu-core-initramfs/<old_version>/ubuntu-core-initramfs_<old_version>.dsc
    debdiff ubuntu-core-initramfs_<old_version>.dsc ubuntu-core-initramfs_<new_version>.dsc > diff.txt
   

7. Upload, or request sponsorship, to the snappy-dev PPA

    dput ppa:snappy-dev/image ubuntu-core-initramfs_<new_version>_source.changes
   

8. Make sure that the package has been built correctly. If not, make changes appropriately and repeat these steps, including creating a new changelog entry.

Note that ubuntu-core-initramfs gets some files from its build dependencies while being built, including for instance snap-bootstrap, so we need to make sure that the snappy-dev PPA already contains the desired version of the snapd deb package (or others) when we upload the package.

After this, the initrd changes will be included in future kernel snaps releases automatically, following the usual 3 weeks cadence, as the snappy-dev PPA is included when these build happen.

Updating systemd

git-subtree is used to synchronize with Ubuntu systemd. To update it, run

git subtree pull --prefix vendor/systemd https://git.launchpad.net/ubuntu/+source/systemd ubuntu/<series>-updates --squash

where series is the ubuntu series. Note that we are pulling from the updates pocket, that is usually what we want, but it could be different depending on the stage of development of the series.

To create the subtree from scratch, which is useful when bumping to a new series, run

git rm -r vendor/systemd
git commit -a -m "Remove <old_series> systemd"
git subtree add --prefix vendor/systemd https://git.launchpad.net/ubuntu/+source/systemd ubuntu/<series> --squash

where we use the default pocket although again it might vary.

Bootchart

It is possible to enable bootcharts by adding ubuntu_core.bootchart to the kernel command line. The sample collector will run until the systemd switches root, and the chart will be saved in /run/log. If bootcharts are also enabled for the core snap, that file will be eventually moved to the ubuntu-save partition (see Core snap documentation).