cloud-hypervisor/docs/custom-image.md

How to create a custom Ubuntu image

In the context of adding more utilities to the Ubuntu cloud image being used for integration testing, this quick guide details how to achieve the proper modification of an official Ubuntu cloud image.

Create the image

Let's go through the steps on how to extend an official Ubuntu image. These steps can be applied to other distributions (with a few changes regarding package management).

Get latest Ubuntu cloud image

wget https://cloud-images.ubuntu.com/focal/current/focal-server-cloudimg-amd64.img

Check the file format is QCOW2

file focal-server-cloudimg-amd64.img
focal-server-cloudimg-amd64.img: QEMU QCOW2 Image (v2), 2361393152 bytes

Convert QCOW2 into RAW

qemu-img convert -p -f qcow2 -O raw focal-server-cloudimg-amd64.img focal-server-cloudimg-amd64.raw

Identify the Linux partition

The goal is to mount the image rootfs so that it can be modified as needed. That's why we need to identify where the Linux filesystem partition is located in the image.

sudo fdisk -l focal-server-cloudimg-amd64.raw
Disk focal-server-cloudimg-amd64.raw: 2.2 GiB, 2361393152 bytes, 4612096 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: A1171ABA-2BEA-4218-A467-1B2B607E5953

Device                             Start     End Sectors  Size Type
focal-server-cloudimg-amd64.raw1  227328 4612062 4384735  2.1G Linux filesystem
focal-server-cloudimg-amd64.raw14   2048   10239    8192    4M BIOS boot
focal-server-cloudimg-amd64.raw15  10240  227327  217088  106M EFI System

Partition table entries are not in disk order.

Mount the Linux partition

mkdir -p /mnt
sudo mount -o loop,offset=$((227328 * 512)) focal-server-cloudimg-amd64.raw /mnt

Set up DNS

The next step describes changing the root directory to the rootfs contained by the cloud image. For DNS to work in the root directory, you will need to first bind-mount the host /etc/resolv.conf onto the mounted linux partition of the cloud image.

sudo mount -o bind /etc/resolv.conf /mnt/etc/resolv.conf

Change root directory

Changing the root directory will allow us to install new packages to the rootfs contained by the cloud image.

sudo chroot /mnt
mount -t proc proc /proc
mount -t devpts devpts /dev/pts

Install needed packages

In the context Cloud Hypervisor's integration tests, we need several utilities. Here is the way to install them for a Ubuntu image. This step is specific to Ubuntu distributions.

apt update
apt install fio iperf iperf3 socat stress

Remove counterproductive packages

• snapd:

This prevents snapd from trying to mount squashfs filesystem when the kernel might not support it. This might be the case when the image is used with direct kernel boot. This step is specific to Ubuntu distributions.

• pollinate:

Remove this package which can fail and lead to the SSH daemon failing to start. See #2113 for details.

apt remove --purge snapd pollinate

Cleanup the image

Leave no trace in the image before unmounting its content.

umount /dev/pts
umount /proc
history -c
exit
umount /mnt/etc/resolv.conf
umount /mnt

Rename the image

Renaming is important to identify this is a modified image.

mv focal-server-cloudimg-amd64.raw focal-server-cloudimg-amd64-custom-$(date "+%Y%m%d")-0.raw

The -0 is the revision and is only necessary to change if multiple images are updated on the same day.

Create QCOW2 from RAW

Last step is to create the QCOW2 image back from the modified image.

qemu-img convert -p -f raw -O qcow2 focal-server-cloudimg-amd64-custom-$(date "+%Y%m%d")-0.raw focal-server-cloudimg-amd64-custom-$(date "+%Y%m%d")-0.qcow2

Switch CI to use the new image

Upload to Azure storage

The next step is to update both images (QCOW2 and RAW) stored as part of the Azure storage account, replacing them with the newly created ones. This will make these new images available from the integration tests. This is usually achieved through the web interface.

Update integration tests

Last step is about updating the integration tests to work with this new image. The key point is to identify where the Linux filesystem partition is located, as we might need to update the direct kernel boot command line, replacing /dev/vda1 with the appropriate partition number.

Update all references to the previous image name to the new one.