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GDM on Ubuntu Core

This directory contains an image of Ubuntu Core 22 with the GDM display manager loaded into the boot file system. It can be launched in a Qemu virtual machine by following these instructions:

  1. Download and decompress the two image files and place them in the same directory.

  2. Add an image as a VM launchable from GNOME Boxes or virt-manager:

    # Delete VM if already registered
    virsh --connect qemu:///session undefine --nvram core-desktop
    virt-install --connect qemu:///session --name core-desktop \
      --memory 2048 --vcpus 2 --boot uefi --os-variant ubuntu24.04 \
      --video virtio,accel3d=no --graphics spice \
      --import --disk path=$(pwd)/pc.img,format=raw
    

    (We use the virt-install because the GNOME Boxes seems to create a legacy BIOS VM when adding the image).

  3. Let the VM boot and and automatically restart once as part of the setup process. Once it settles, you can close the virt-viewer window and manage the VM with GNOME Boxes.

  4. Follow the gnome-initial-setup wizard to create a user, and you'll be dropped into an unconfined desktop session.

At present, the wizard fails to make the created user an administrator, limiting what is possible. For now, I've left the root account open with a blank password as a stop-gap measure.

It is also possible to launch the image outside of GNOME Boxes or virt-manager with a command like the following:

    qemu-system-x86_64 -smp 2 -m 2048 -machine accel=kvm \
      -display gtk,gl=on \
      -net nic,model=virtio -net user,hostfwd=tcp::8022-:22 \
      -drive file=/usr/share/OVMF/OVMF_CODE.fd,if=pflash,format=raw,unit=0,readonly=on \
      -drive file=pc.img,cache=none,format=raw,id=main,if=none \
      -device virtio-blk-pci,drive=main,bootindex=1 \
      -device ac97 -audiodev pa,id=ac97

Other Repositories

Most of the code used to construct the image is now managed in other repositories. Namely:

Snap Repo Recipe Notes
core24-desktop core-base-desktop:24 via launchpad base snap, forked from core24 to integrate GDM graphical login
pc-desktop pc-amd64-gadget-desktop:24 via launchpad gadget snap, forked from pc, using core24-desktop as a base
ubuntu-desktop-session ubuntu-desktop-session-snap:24 via launchpad provides the confined desktop session
snapd ubuntu-core-desktop-snapd:master via launchpad a branch of snapd with additional changes not yet merged to mainline
Core 22 Repositories
Snap Repo Recipe Notes
core22-desktop core-base-desktop:22 via launchpad base snap, forked from core22 to integrate GDM graphical login
pc-desktop pc-amd64-gadget-desktop:22 via launchpad gadget snap, forked from pc, using core22-desktop as a base
pi-desktop pi-desktop via launchpad Pi gadget snap, forked from pi, using core22-desktop as a base
ubuntu-desktop-session ubuntu-desktop-session-snap:22 via launchpad provides the confined desktop session
snapd ubuntu-core-desktop-snapd via ~snappy-dev a branch of snapd with additional changes not yet merged to mainline

In addition, the base snap uses packages from the desktop-snappers core-desktop PPA. This is mostly to backport features we need that are not in jammy-updates.

Extracting the snaps from the image

The built image is a normal hard disk image, which means that it is possible to get the partition list with:

fdisk -lu pc.img

This will show a list like this one:

Disk pc2.img: 12 GiB, 12884901888 bytes, 25165824 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: gpt
Disk identifier: 6E3A867F-8F44-4880-BA2C-FABA633C36E6

Device     Start     End Sectors  Size Type
pc.img1    2048    4095    2048    1M BIOS boot
pc.img2    4096 7172095 7168000  3.4G EFI System

After running the system the first time, other partitions will be added for the system, the user data...

Now, using kpartx we can create loop devices for each of those partitions:

sudo kpartx -av pc-img

Two loop devices, /dev/mapper/loopXXX/p1 and /dev/mapper/loopXXX/p2, will be available. We want to mount the second one, which is the one that contains the EFI system and the base snaps:

sudo mount /dev/mapper/loopXXX/p2 /mnt

And now we can go to /mnt/snaps, and there are all the base snaps.

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