Immutable root with atomic upgrades

=== What? ===
This article provides a basic guide to setting up a read-only-root-based Alpine Linux system with several boot environments and atomic upgrades using a modern bootloader and btrfs.
=== Why? ===
Read-only root and atomic upgrades with the ability to easily rollback or boot previous configurations is a concept that has been gaining popularity recently. Distributions providing and promoting such features, for example, are Fedora Silverblue, Opensuse MicroOS, NixOS and GNU Guix.
While Alpine Linux has its killer features, it lacks the ones mentioned above on default setup. This is a proof of concept that it's possible to implement them in a minimal way on a minimal system.
= Preparation =
You should have bootable Alpine media. The process to obtain it is described on the installation page.
= Partitioning disks =
In this guide, it's assumed that you have a fresh UEFI system without an OS and have just booted into a live Alpine system using a USB flash drive or CD.
The first step is creating partition table on your HDD/SSD target device (/dev/sda here):
# apk add gptfdisk
# gdisk /dev/sda
> o ↵
> y ↵
> w ↵
> y ↵
Now we can define the partitions:
# cgdisk /dev/sda
Partition creation process consists of several steps:
# Start sector - you can safely use default value by pressing ↵
# Size
# Type (as hex code) - EFI is ef00, Linux filesystem is 8300, Swap is 8200.
Result table:
Part. # Size Partition Type Partition Name
----------------------------------------------------------------
1 200.0 MiB EFI System EFI
2 200.0 GiB Linux filesystem ROOT
3 32.0 GiB Linux swap SWAP
ROOT partition name will later be used in rEFInd configuration to identify boot volume.
Next step is creating filesystems:
# mkfs.vfat -F32 /dev/sda1
# mkfs.btrfs /dev/sda2
# mkswap /dev/sda3
Now we can mount our root volume:
# mount -t btrfs /dev/sda2 /mnt
= File system structure =
Now we should create the file structure that would provide reliable atomic system upgrades.
Start with following directories:
# mkdir /mnt/next
Stores next current link, is necessary due to how busybox mv does atomic link replacement.
# mkdir /mnt/commons
Stores common non-snapshotting subvolumes.
We may populate it right away:
# btrfs subvolume create /mnt/commons/@var
# btrfs subvolume create /mnt/commons/@home
Next, most important directories:
# mkdir /mnt/snapshots
Stores directories containing snapshots belonging to one generation.
# mkdir /mnt/links 
Stores generations of directories containing links to snapshot generations.
Let's create first generation and populate it with one OS root snapshot @:
# NEWSNAPSHOTS="$(date -u +"%Y%m%d%H%M%S")$(cat /dev/urandom | tr -dc 'a-zA-Z' | fold -w 8 | head -n 1)"
# mkdir "/mnt/snapshots/$NEWSNAPSHOTS"
# btrfs subvolume create /mnt/snapshots/$NEWSNAPSHOTS/@
Populate links:
# NEWLINKS="$(date -u +"%Y%m%d%H%M%S")$(cat /dev/urandom | tr -dc 'a-zA-Z' | fold -w 8 | head -n 1)"
# mkdir "/mnt/links/$NEWLINKS"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/0"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/1"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/2"
# ln -s "../../snapshots/$NEWSNAPSHOTS" "/mnt/links/$NEWLINKS/3"
You can have as many links as you like, just apply changes to rEFInd config and upgrade scripts described below accordingly.
Link that will point to latest links generation:
# ln -s "./links/$NEWLINKS" /mnt/current
This setup allows us to just have static rEFInd config that points to to /current/0/@, /current/1/@, etc. while the actual underlying boot environment will change with each upgrade.
But how will fs mounting services know which snapshot generation is currently loaded?
The answer is common fstab in the btrfs root.
Get UUIDs of the partitions first:
# blkid > /mnt/fstab
Now edit fstab accordingly:
# vi /mnt/fstab
Example:
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 / btrfs subvol=CURRENT_SNAPSHOTS_PATH/@,ro,noatime 0 0
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 /var btrfs subvol=/commons/@var,rw,noatime 0 0
UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 /home btrfs subvol=/commons/@home,rw,noatime 0 0
# UUID=2FE6-837A /boot/efi vfat rw,noatime,discard 0 2
tmpfs /tmp tmpfs mode=1777,noatime,nosuid,nodev,size=2G 0 0
UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 swap swap rw,noatime,discard 0 0
CURRENT_SNAPSHOTS_PATH will be replaced by scripts with, for example, /snapshots/20210411212549sdBXyLxg, and the result will be piped into /etc/fstab of a created @ snapshot during new generation preparations.
Root btrfs volume structure mounted on /mnt:
|--mnt
| |--commons
| | |--@var
| | |--@home
| |--current
| |--fstab
| |--links
| | |--20210411213742qwrXAJBz
| | | |--0
| | | |--1
| | | |--2
| | | |--3
| |--next
| |--snapshots
| | |--20210411212549sdBXyLxg
| | | |--@
= Base system install =
With the directory structure prepared, we can begin installing a basic Alpine Linux system.
Considering that installation is done from Alpine system, we only need following parts of the process:
# apk -X https://dl-cdn.alpinelinux.org/alpine/latest-stable/main -U --allow-untrusted -p /mnt/snapshots/20210411212549sdBXyLxg/@ --initdb add alpine-base
Now we can setup basic chroot to complete the installation process:
# export SNP="/mnt/snapshots/20210411212549sdBXyLxg/@"
# mount -o bind /dev $SNP/dev
# mount -t proc none $SNP/proc
# mount -t sysfs sys $SNP/sys
# sed "s#CURRENT_SNAPSHOTS_PATH#/snapshots/20210411212549sdBXyLxg#g" /mnt/fstab > "$SNP/etc/fstab"
# cp -L /etc/resolv.conf "$SNP/etc/"
# chroot "$SNP" /bin/sh
# mount -a
# mv /etc/resolv.conf /tmp/
# ln -s /tmp/resolv.conf /etc/resolv.conf
As soon as you're in chroot, define repositories:
# echo "https://dl-cdn.alpinelinux.org/alpine/latest-stable/main" > /etc/apk/repositories
This example shows only main, but you should also add testing and community if you need any packages in those.
Now it's time for the firmware, kernel, and btrfs packages:
# apk add -U linux-firmware linux-lts btrfs-progs
You may want to change linux-firmware to a custom set of firmware packages suitable for your system, for example, linux-firmware-amd linux-firmware-amd-ucode linux-firmware-amdgpu linux-firmware-ath10k linux-firmware-qca for a typical AMD laptop.
It's also important to add the btrfs feature to mkinitfs.conf and run mkinitfs manually:
# vi /etc/mkinitfs/mkinitfs.conf
# mkinitfs
These steps prepare the kernel and generate the initramfs, which will be used later to boot from our first snapshot.
After that, you should install any package you may need on first boot.
Now, configure the system, start with setting a password for the root:
# passwd root
Don't forget to add essential services to their respective runlevels:
rc-update add devfs sysinit
rc-update add dmesg sysinit
rc-update add mdev sysinit
rc-update add hwdrivers sysinit
rc-update add hwclock boot
rc-update add modules boot
rc-update add sysctl boot
rc-update add hostname boot
rc-update add bootmisc boot
rc-update add syslog boot
rc-update add mount-ro shutdown
rc-update add killprocs shutdown
rc-update add savecache shutdown
With the snapshot prepared and configured, we can chroot out of it and unmount everything:
# umount -a
# exit
Finish editing the snapshot by setting the ro flag and unmounting the root volume:
# btrfs property set -ts "/mnt/snapshots/20210411212549sdBXyLxg/@" ro true
# umount /mnt
= Bootloader installation =
Mount the EFI partition:
# mount -t vfat /dev/sda1 /mnt
# mkdir /mnt/EFI
== Bootloader configuration ==
There are 2 options as examples of the bootloader installation: rEFInd and GRUB.
Sometimes one of them will refuse to work on a system for no particular reason, in this case try the other one.
=== rEFInd ===
Check the latest version number of the refind package:
# apk info -X https://dl-cdn.alpinelinux.org/alpine/edge/testing -U refind
Download the latest version (replace 0.13.2-r3 in the example below) of the refind package:
# wget https://dl-cdn.alpinelinux.org/alpine/edge/testing/x86_64/refind-0.13.2-r3.apk
Unpack the prepared rEFInd archive and copy relevant files to /mnt/EFI/
# tar -xzf refind-0.13.2-r3.apk
# cp -r usr/share/refind /mnt/EFI/
# cd /mnt/EFI/refind
Rename config file and edit it:
# mv refind.conf-sample refind.conf
# vi refind.conf
And append following to the end of the file, remember to replace example UUIDs with your own for root (btrfs partition) and resume (swap partition). Keep in mind that if you named the btrfs volume other than ROOT during "Partitioning disks" stage, you have to change the volume field below accordingly.
menuentry "Alpine Linux" {
 icon /EFI/refind/icons/os_linux.png
 volume "ROOT"
 loader /current/0/@/boot/vmlinuz-lts
 initrd /current/0/@/boot/initramfs-lts
 options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
 submenuentry "Boot fallback 1" {
 loader /current/1/@/boot/vmlinuz-lts
 initrd /current/1/@/boot/initramfs-lts
 options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
 }
 submenuentry "Boot fallback 2" {
 loader /current/2/@/boot/vmlinuz-lts
 initrd /current/2/@/boot/initramfs-lts
 options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
 }
 submenuentry "Boot fallback 3" {
 loader /current/3/@/boot/vmlinuz-lts
 initrd /current/3/@/boot/initramfs-lts
 options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash"
 }
}
=== GRUB ===
# apk add grub-efi
GRUB requires two configuration files this time as we will use grub-mkstandalone.
The first configuration file is internal and should only point to the second file, where we store the menu:
# cd /tmp
# vi grub_internal.cfg
Set the contents to the following, but make sure to replace 2FE6-837A with your own EFI partition UUID:
insmod part_gpt
insmod fat
search --set efi --fs-uuid 2FE6-837A
configfile (${efi})/EFI/grub/grub.cfg
The second config file is the main config where we describe the entire boot menu.
# vi grub.cfg
Set to contain, but replace UUIDs:
set timeout=3
menuentry "Alpine Linux Current" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/0/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/0/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 1" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/1/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/1/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/1/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 2" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/2/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/2/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/2/@/boot/initramfs-edge
}
menuentry "Alpine Linux Snapshot 3" {
	search --set root --fs-uuid b9ff5e7b-e128-4e64-861a-2fdd794a9828
	linux /current/3/@/boot/vmlinuz-edge root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/3/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 quiet splash
	initrd /current/3/@/boot/initramfs-edge
}
Generate the grubx64.efi binary:
# grub-mkstandalone -O x86_64-efi -o grubx64.efi "boot/grub/grub.cfg=/tmp/grub_internal.cfg"
# mkdir /mnt/EFI/grub
# mv grubx64.efi /mnt/EFI/grub/
# mv grub.cfg /mnt/EFI/grub/
== Adding EFI boot entry ==
To add the chosen bootloader to UEFI, efibootmgr is a suitable tool. The following example is for rEFInd, but could be easily adjusted for GRUB:
# apk add efibootmgr
# efibootmgr --create --disk /dev/sda --part 1 --loader /EFI/refind/refind_x64.efi --label "rEFInd" --verbose
/dev/sda is our disk device and 1 is the number of the FAT32 partition containing the bootloader data.
= Updating or altering the system =
# touch /usr/sbin/sysmut
# chmod +x /usr/sbin/sysmut
# vi /usr/sbin/sysmut
Example script to mutate the the system:
#!/bin/execlineb -W
unshare --mount
importas -D 0 source 1
define mnt /media/root
if { mkdir -p ${mnt} }
if { mount -t btrfs -o rw,noatime UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 ${mnt} }
foreground {
	backtick -E dt {
		date -u +%Y%m%d%H%M%S
	}
	backtick -E rnd {
		pipeline { cat /dev/urandom }
		pipeline { tr -dc a-zA-Z }
		pipeline { fold -w 8 }
		head -n 1
	}
	define newsnap ${dt}${rnd}
	if { mkdir -p ${mnt}/snapshots/${newsnap} }
	if { btrfs subvolume snapshot ${mnt}/current/${source}/@ ${mnt}/snapshots/${newsnap}/@ }
	if {
		redirfd -w 1 ${mnt}/snapshots/${newsnap}/@/etc/fstab
			sed s#CURRENT_SNAPSHOTS_PATH#/snapshots/${newsnap}#g ${mnt}/fstab
	}
	if { mount -t proc none ${mnt}/snapshots/${newsnap}/@/proc }
	if { mount -t sysfs sys ${mnt}/snapshots/${newsnap}/@/sys }
	if { mount -o bind,ro /dev ${mnt}/snapshots/${newsnap}/@/dev }
	foreground {
		foreground { mount -o bind,ro /etc/resolv.conf ${mnt}/snapshots/${newsnap}/@/etc/resolv.conf }
		foreground {
			chroot ${mnt}/snapshots/${newsnap}/@
			foreground { mount -a }
			foreground { sh }
			importas apply ?
			foreground { umount -a }
			exit ${apply}
		}
		importas apply ?
		foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/etc/resolv.conf }
		ifelse { exit ${apply} } {
			if { btrfs property set -ts ${mnt}/snapshots/${newsnap}/@ ro true }
			define newlink ${dt}${rnd}
			if { mkdir -p ${mnt}/links/${newlink} }
			if { ln -s ../../snapshots/${newsnap} ${mnt}/links/${newlink}/0 }
			if { cp -P ${mnt}/current/0 ${mnt}/links/${newlink}/1 }
			if { cp -P ${mnt}/current/1 ${mnt}/links/${newlink}/2 }
			if { cp -P ${mnt}/current/2 ${mnt}/links/${newlink}/3 }
			if { mkdir -p ${mnt}/next }
			if { ln -sfn ./links/${newlink} ${mnt}/next/current }
			if { mv ${mnt}/next/current ${mnt}/ }
			echo "Changes applied"
		}
		echo "Changes discarded"
	}
	foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/proc }
	foreground { redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/sys }
	redirfd -w 2 /dev/null umount ${mnt}/snapshots/${newsnap}/@/dev
}
umount ${mnt}
It will get you into the root shell chrooted into the new snapshot, where you can apply any change you like. The origin of the new snapshot is defined by the first and only argument, in form of number. If no argument provided the 0 (current latest) is taken as origin.
If chroot shell exits with an error, there will be no switch to the new snapshots. This means you can manually discard changes while in the chroot by:
# exit 1
= Deleting unused snapshots =
Unused snapshots can be garbage-collected by:
# touch /usr/sbin/syscln
# chmod +x /usr/sbin/syscln
# vi /usr/sbin/syscln
#!/bin/execlineb -W
unshare --mount
define mnt /media/root
if { mkdir -p ${mnt} }
if { mount -t btrfs -o rw,noatime,compress=zstd:3 UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 ${mnt} }
foreground {
	foreground {
		pipeline {
			foreground {
				pipeline {
					find -H ${mnt}/snapshots/ -maxdepth 1 -mindepth 1 -print0
				}
				xargs -0 -r realpath
			}
			pipeline {
				find -H ${mnt}/current/ -maxdepth 1 -mindepth 1 -print0
			}
			xargs -0 -r realpath
		}
		pipeline { tr \\n \\0 }
		pipeline { sort -z }
		pipeline { uniq -u -z }
		pipeline { xargs -0 -r -n 1 -I [] find -H [] -maxdepth 1 -mindepth 1 -print0 }
		xargs -0 -r btrfs subvolume delete
	}
	foreground { find -H ${mnt}/snapshots/ -maxdepth 1 -mindepth 1 -empty -type d -delete }
	foreground {
		pipeline {
			foreground {
				pipeline {
					find -H ${mnt}/links/ -maxdepth 1 -mindepth 1 -print0
				}
				xargs -0 -r realpath
			}
			realpath ${mnt}/current
		}
		pipeline { tr \\n \\0 }
		pipeline { sort -z }
		pipeline { uniq -u -z }
		pipeline { xargs -0 -r -n 1 -I [] find -H [] -maxdepth 1 -mindepth 1 -print0 }
		xargs -0 -r -n 1 unlink
	}
	find -H ${mnt}/links/ -maxdepth 1 -mindepth 1 -empty -type d -delete
}
umount ${mnt}
= Allowing temporary runtime alterations =
You can use overlayfs with tmpfs built into Alpine's init script to allow changes in the rootfs which will be automatically reverted upon reboot.
To make use of this, add overlaytmpfs to the kernel boot options in refind.conf, e.g.:
...
 initrd /current/0/@/boot/initramfs-lts
 options "root=UUID=b9ff5e7b-e128-4e64-861a-2fdd794a9828 rootfstype=btrfs rootflags=subvol=/current/0/@,ro,noatime resume=UUID=f0239163-9d46-47c1-67a4-3ee1d63d0676 overlaytmpfs quiet splash"
 submenuentry "Boot fallback 1" {
...
Category:InstallationCategory:Filesystems