initramfs-tools

Ben Hutchings

What is the initramfs for?

• Huge variety of disk drivers, net drivers and filesystems that may be needed to mount root
• Generic kernel shouldn't have all these built-in
• Root might also need RAID, LVM, dm-crypt, which generally need to be configured by userland
• Kernel needs a way to load modules and run scripts but it can't read them from the root filesystem
• Solution: boot loader provides a bundle of files (initramfs image) to the kernel

How the kernel uses an initramfs

• Kernel always mounts an initial root filesystem using either ramfs or tmpfs
• If the boot loader provides an initramfs image, kernel unpacks it into this filesystem, then runs /init
• The initramfs init system is then responsible for mounting the real root filesystem and running the real init
  • Although it is not required to hand over at all - e.g. debian-installer

Overview

• Started in Ubuntu in 2005 and adopted in Debian later that year
• mkinitramfs builds an initramfs image using the included scripts, necessary kernel modules, udev, and utilities from klibc-utils or busybox
• Uses build-time and boot-time hook scripts for extensibility
  • Used by btrfs-tools, cryptsetup, lvm2, mdadm, etc.
• Supports dependencies within each set of scripts

The init system

• Mount devtmpfs, devpts, procfs, sysfs, tmpfs at /run
• Run init-top scripts - includes starting udev
• Load explicitly configured modules
• Run init-premount scripts
• Run local-top or nfs-top scripts - e.g. mdadm setup
• Run local-premount or nfs-premount scripts - e.g. btrfs scan, resume from disk
• Mount root filesystem

Problems (1)

Root filesystem is not enough. Depenency-based boot means there's no clean split between root and /usr. So initramfs should mount it (implemented in 0.117). However:

• initscripts and mountall won't remount it read-write if it's already mounted read-only
  • Current workaround: only do this for systemd
• fsck will refuse to operate on /usr if already mounted
  • So we need to fsck it from the initramfs
    • But e2fsck complains if the superblock time is newer than system time, and system time may not be correct yet
      • So we need to fix the system time in the initramfs
• Selecting the right fsck to install is harder than it seemed

Problems (2)

Device discovery by kernel is asynchronous. But that's why we have udev, right?

Unfortunately not everything needed for device setup is or can be in udev hooks (e.g. cryptsetup requires interaction)

Lots of hacks used (while ... do sleep... done, udevadm settle in too many places), and they still sometimes fail (#616689 and merged bugs)

Ubuntu has a somewhat better approach but we still haven't merged it back...

...in fact, Debian and Ubuntu branches have been diverging for about 5 years

More information

• Manual pages: initramfs-tools(8), update-initramfs(8), etc.
• Source docs: docs/example_hook, docs/example_script, docs/maintainer-notes.html
• Mailing list: initramfs-tools@packages.debian.org (really debian-kernel)

Questions?

Credits

• Linux 'Tux' logo © Larry Ewing, Simon Budig.
  • Modified by Ben to add Debian open-ND logo
• Debian open-ND logo © Software in the Public Interest, Inc.
• Xzibit promotional photo © Retna, used for purpose of parody