PoC

build.sh

@@ -0,0 +1,84 @@
+#!/bin/bash
+
+OS_CODENAME=focal
+DEV=/dev/nbd0
+
+#sudo apt -yq install syslinux syslinux-common syslinux-efi extlinux
+
+mkdir -p build mnt out
+
+echo "Downloading"
+#wget -O build/initrd.img https://cloud-images.ubuntu.com/$OS_CODENAME/current/unpacked/${OS_CODENAME}-server-cloudimg-amd64-initrd-generic
+#wget -O build/vmlinuz https://cloud-images.ubuntu.com/${OS_CODENAME}/current/unpacked/${OS_CODENAME}-server-cloudimg-amd64-vmlinuz-generic
+#wget -O build/rootfs.tar.xz https://cloud-images.ubuntu.com/${OS_CODENAME}/current/${OS_CODENAME}-server-cloudimg-amd64-root.tar.xz
+
+KERNEL_VERSION="$(file build/vmlinuz |cut -d, -f2|cut -d\  -f3)"
+echo "Detected kernel ${KERNEL_VERSION}"
+
+echo "Creating empty image"
+qemu-img create -f qcow2 build/ubuntu-${OS_CODENAME}.qcow2 2G || exit 1
+
+echo "attaching image"
+sudo modprobe nbd
+sudo qemu-nbd -c "${DEV}" build/ubuntu-${OS_CODENAME}.qcow2 || exit 1
+
+echo "partitioning"
+sudo parted -s "${DEV}" -- \
+        mklabel gpt \
+        unit MiB \
+        mkpart '"Ubuntu"' btrfs 1 100% \
+        set 1 legacy_boot on
+until [ -b "${DEV}1" ] || [ -b "${DEV}p1" ]; do
+   sleep 1
+done
+
+echo "installing hybrid bootloader"
+sudo dd bs=440 count=1 conv=notrunc \
+   if=/usr/lib/syslinux/mbr/gptmbr.bin of="$DEV" 2> /dev/null
+sync
+
+echo "mkfs.btrfs"
+sudo mkfs.btrfs -L cloudimg-rootfs "${DEV}p1" || exit 1
+sync
+
+echo "mounting root"
+sudo mount -t btrfs "${DEV}p1" mnt || exit 1
+
+echo "unpacking root fs"
+sudo tar xJf build/rootfs.tar.xz -C mnt || exit 1
+
+echo "installing kernel and inintd"
+sudo mkdir -p mnt/boot
+sudo cp -ar build/initrd.img "mnt/boot/initrd.img-${KERNEL_VERSION}"
+sudo cp -ar build/vmlinuz "mnt/boot/vmlinuz-${KERNEL_VERSION}"
+sudo ln -s "vmlinuz-${KERNEL_VERSION}" mnt/boot/vmlinuz
+sudo ln -s "initrd.img-${KERNEL_VERSION}" mnt/boot/initrd.img
+
+echo "installing legacy syslinux files"
+sudo mkdir -p mnt/boot/syslinux
+sudo cp /usr/lib/syslinux/modules/bios/*.c32 mnt/boot/syslinux/
+sudo cp syslinux/{mainmenu,syslinux}.cfg mnt/boot/syslinux
+if [ -n "$1" ]; then
+    echo "Patching UUID $1"
+    sudo sed -i -e "s/?/?uuid=$1\&/g" mnt/boot/syslinux/mainmenu.cfg
+    cat mnt/boot/syslinux/mainmenu.cfg
+fi
+
+echo "here is the fstab"
+echo "LABEL=cloudimg-rootfs / btrfs defaults 0 1" | sudo tee mnt/etc/fstab
+
+echo "installing syslinux bootloader"
+sudo extlinux -i mnt/boot/syslinux
+
+echo "umounting"
+sync
+sudo umount mnt
+
+echo "detaching"
+sudo qemu-nbd -d "${DEV}"
+
+echo "compressing"
+qemu-img convert -cpO qcow2 build/ubuntu-${OS_CODENAME}.qcow2 out/ubuntu-${OS_CODENAME}.qcow2
+# rm -f out/ubuntu-${OS_CODENAME}.qcow2
+
+echo "done"

syslinux/mainmenu.cfg

@@ -0,0 +1,12 @@
+TIMEOUT 50
+TOTALTIMEOUT 3000
+
+LABEL Current
+  KERNEL /boot/vmlinuz
+  INITRD /boot/initrd.img
+  APPEND root=LABEL=cloudimg-rootfs ds=nocloud-net;s=https://cloud-init.sysdeploy.org/apiv2?filetype=
+
+LABEL Previous
+  KERNEL /boot/vmlinuz.old
+  INITRD /boot/initrd.img.old
+  APPEND root=LABEL=cloudimg-rootfs ds=nocloud-net;s=https://cloud-init.sysdeploy.org/apiv2?filetype=

syslinux/syslinux.cfg

@@ -0,0 +1,4 @@
+MENU TITLE Legacy boot
+UI vesamenu.c32
+
+INCLUDE /boot/syslinux/mainmenu.cfg

syslinux/syslx64.cfg

@@ -0,0 +1,4 @@
+MENU TITLE UEFI boot
+UI vesamenu.c32
+
+INCLUDE /syslinux/mainmenu.cfg