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README.md

Introduction

asciicast

zfsbootmenu is implemented as a Dracut module to provide an experience similar to FreeBSD's bootloader, for Linux distributions. In broad strokes, it works as follows:

  • Via GRUB, direct EFI booting, etc, boot a Linux kernel along with an initramfs containing ZFS Boot Menu
  • Look for zfsbootmenu on the kernel command line
    • Optionally specify a default pool (if multiple are present)
  • Find all healthy pools and then import them
  • If a specific pool was set, look for the bootfs pool value. Prefer this boot environment.
    • If no pool was defined on the command line, use the bootfs value on the first-found pool
    • If a bootfs value is defined, start a 10 second countdown to boot that environment with the highest kernel found in /boot
    • If no bootfs value is defined, find every filesystem that mounts to / with a /boot directory, and find every kernel. Prompt via fzf.
      • If needed, prompt for encryption passphrases
  • Once the count down has been reached for the bootfs-selected environment, prompt to encryption keys if they're needed
    • Mount the filesystem and find the highest-numbered kernel in /boot in the boot environment.
  • Load the kernel, initramfs and the kernel command line defined in /etc/default/grub into memory via kexec
  • Unmount all ZFS filesystems and export all pools
  • Boot the final kernel and initramfs

At this point, you'll be booting into your OS-managed kernel and initramfs, along with any arguments needed to correctly boot your system.

This tool makes uses of the following additional software:

  • fzf
  • kexec-tools
  • Linux Kernel
  • ZFS on Linux (currently 0.8.2 built on Void Linux).

Binary releases for x86_64 and ppc64le are built on Void Linux hosts.

System prereqs

To ensure the boot menu can find your kernels, you'll need to ensure /boot resides on your ZFS file system. An example filesystem layout is as follows:

NAME                           USED  AVAIL     REFER  MOUNTPOINT
zroot                          278G   582G       96K  none
zroot/ROOT                    10.9G   582G       96K  none
zroot/ROOT/void.2019.10.04    1.20M   582G     7.17G  /
zroot/ROOT/void.2019.11.01    10.9G   582G     7.17G  /
zroot/home                     120G   582G     11.8G  /home

There are two boot environments created, identified by mounting to /. The environment that this system will boot into is defined by the bootfs value set on the zroot zpool.

NAME   PROPERTY  VALUE                       SOURCE
zroot  bootfs    zroot/ROOT/void.2019.11.01  local

On start, ZFS Boot Menu will find the highest versioned kernel in zroot/ROOT/void.2019.11.01/boot, confirm that a matching initramfs is present, and default to booting the OS with that.

Installation

In the boot environment, the file /etc/default/grub will need to be created with the variable GRUB_CMDLINE_LINUX_DEFAULT defined. These are the kernel arguments passed to the kernel in your boot environment. Do not set any root= or any other pool-related options here. This value will be filled in when a boot environment is selected.

For example, I have the following set:

GRUB_CMDLINE_LINUX_DEFAULT="zfs.zfs_arc_max=8589934592 elevator=noop"

EFI

ZFS Boot Menu integrates nicely with an EFI system. There will be two key things to configure here.

  • The mountpoint of the EFI partition and it's contents
  • The mountpoint of the boot environment /boot and it's contents

Each boot environment should have /boot live on the ZFS filesystem. Using the above example, zroot/ROOT/void.2019.11.01 would contain /boot with any kernel/initramfs pairs.

# ls /boot
config-5.3.18_1
config-5.4.6_1
efi
initramfs-5.3.18_1.img
initramfs-5.4.6_1.img
System.map-5.3.18_1
System.map-5.4.6_1
vmlinuz-5.3.18_1
vmlinuz-5.4.6_1

Once /boot is backed by ZFS in a boot environment, you'll need to install the boot menu files. Typically, EFI partitions are mounted to /boot/efi, and contain a number of sub-directories. In this example, /boot/efi/EFI/void holds the ZFS Boot Menu kernel and initramfs.

# lsblk -f /dev/sda
NAME   FSTYPE LABEL UUID                                 FSAVAIL FSUSE% MOUNTPOINT
sdg                                                                     
├─sda1 vfat         AFC2-35EE                               7.9G     1% /boot/efi
└─sda2 swap         412401b6-4aec-4452-a6bd-6fc20fbdc2a5                [SWAP]

# ls /boot/efi/EFI/void/
initramfs-0.7.4.img
initramfs-0.7.5.img
vmlinuz-0.7.4
vmlinuz-0.7.5

With this layout, you'll now need a way to boot the kernel and initramfs via EFI. This can be done via a manual entry set via efibootmgr, or it can be done with rEFInd.

If you are using a pre-built kernel and initramfs downloaded from the releases page, you'll need to identify the following additional details:

  • Your system's hostid (hostid). It's important that this command is executed as root, to ensure that it returns the correct value.
  • Your boot pool name, if you have multiple.
  • The disk path and partition index of your EFI partition. (/dev/sda, part 1)

efibootmgr

efibootmgr --disk /dev/sda \
  --part 1 \
  --create \
  --label "ZFS Boot Menu" \
  --loader /vmlinuz-0.7.5 \
  --unicode 'root=zfsbootmenu:POOL=zroot ro initrd=\EFI\void\initramfs-0.7.5.img quiet spl_hostid=a8c0a2a8' \
  --verbose

Take note to adjust zfsbootmenu:POOL=, spl_hostid=, --disk and --part to match your system configuration.

Each time the bootmenu is updated, a new EFI entry will need to be manually added.

rEFInd

rEFInd is considerably easier to install and manage. Refer to your distributions packages for installation. Once rEFIndhas been installed, you can createrefind_linux.conf in the directory holding the ZFS Boot Menu files (/boot/efi/EFI/void` in our example):

"Boot Default BE" "ro quiet loglevel=0 timeout=0 zfsbootmenu:POOL= spl_hostid="
"Select BE" "ro quiet loglevel=0 timeout=-1 zfsbootmenu:POOL= spl_hostid="

As with the efibootmgr section, the zfsbootmenu:POOL= and spl_hostid= options need to be configured to match your environment.

This file will configure rEFInd to create two entries for each kernel and initrams pair it finds. The first will directly boot into the environment set via the bootfs pool property. The second will force ZFS Boot Menu to display an environment / kernel / snapshot selection menu, allowing you to boot alternate environments, kernels and snapshots.

Command line options

ZFS Boot Menu currently understands the following options:

  • spl_hostid= used to set the system hostid if you are using a pre-built initramfs from the release page.
  • force_import=0|1 set to 1 to attempt to force import all pools on the system. Use with caution!
  • read_write=0|1 set to 1 to enable writes to pools when importing. Pools are imported read-only by default as a safety precaution.
  • timeout=
  • A value of 0 will result in the system immediately booting from the pool configured in the bootfs pool property
  • A value of -1 will result in the system displaying a boot menu.
  • Any other positive value will show a countdown timer to boot the environment configured under bootfs, otherwise it will default to a boot menu.
  • ""|zfsbootmenu|zfsbootmenu:|zfsbootmenu:POOL=zroot The first three values are functionally identical. The fourth can be used to prefer a pool if multiple are present in the system.

initramfs creation

bin/generate-zbm can be used to create an initramfs on your system. It ships with void-specific defaults in etc/config.ini. To create an initramfs, the following additional tools/libraries will need to be available on your system:

If you want to create an unified EFI file (kernel, initramfs, command line), the following additional tools will be needed:

  • gnu objcopy (typically packaged as binutils)
  • linuxx64.efi.stub (typically packaged as gummiboot)

Your distribution should have packages for these already.

config.ini

/etc/zfsbootmenu/config.ini is used to control the behavior of generate-zbm. An example is documented below.

[Global]
ManageImages=0
DracutConfDir=/etc/zfsbootmenu/dracut.conf.d

[Kernel]
CommandLine="ro quiet loglevel=0"

[Components]
ImageDir=/boot/efi/EFI/void
Versioned=1
Copies=3

[EFI]
ImageDir=/boot/efi/EFI/void
Versioned=1
Copies=0

Global

  • ManageImages Set this to 1 to allow generate-zbm to perform any actions (creation, removal of old files, etc)
  • DracutConfDir Set this to the location of the dracut configuration director for ZFS Boot Menu. This CAN NOT be the same location as the system dracut.conf.d, as the configuration files there directly conflict with the creation of the bootmenu initramfs.

Kernel

  • CommandLine If you're making a unified EFI file, this is the command line passed to the module. Refer to Command line options.

Components

  • ImageDir This is the destination directory for the initramfs and kernel.
  • Versioned Set to 1 to create versioned files. Set to 0 to disable a version suffix, which is useful if you have static bootloader entries pointing to ZFS Boot Menu.
  • Copies This controls the number of copies to keep, in addition to the file that is currently being created.

EFI

  • ImageDir This is the destination directory for the unified EFI file.
  • Versioned Set to 1 to create versioned files. Set to 0 to disable a version suffix, which is useful if you have static bootloader entries pointing to ZFS Boot Menu.
  • Copies This controls the number of copies to keep, in addition to the file that is currently being created.

Native encryption

ZFS Boot Menu can import pools or filesystems with native encryption enabled. If your boot environments are not encrypted but say /home is, you will not receive a decryption prompt. To ensure that you can decrypt your pool to load the kernel and initramfs, you'll need to you have the filesystem parameters configured correctly.

zfs get all zroot | egrep '(encryption|keylocation|keyformat)' 
zroot  encryption            aes-256-gcm                -
zroot  keylocation           file:///etc/zfs/zroot.key  local
zroot  keyformat             passphrase                 -
zroot  encryptionroot        zroot                      -

It's critical that keyformat is set to passphrase, otherwise you'll be unable to enter the correct value in the boot loader. ZoL currently only supports one key, but it does have a behavior that we can exploit. If you configure the keylocation value to a file on disk, put your passphrase in that, and then put that file into the FINAL initramfs, you won't receive a second password prompt on boot. You'll then still receive a password prompt in the boot loader, since we can force a prompt for passphrase input.

For Dracut-based systems, this can be done by creating /etc/dracut.conf.d/zol.conf with the following contents:

install_items+=" /etc/zfs/zroot.key "

It's critical that you do not put this key file into the ZFS Boot Menu initramfs, since that file exists on an unencrypted volume - leaving your pool essentially wide-open.

Limitations

Currently, the kernel command line for the boot environment is read from /etc/default/grub. I'd like to support multiple locations determined by probing the OS installed in the boot environment.

When building a kernel command line to pass to the kexec'd kernel, the command line generated is always created for Dracut's ZFS module. Again, this will need to be modified based on the detected OS in the boot environment.

Both of the above issues are readily resolved by hopefully reading /etc/os-release from the boot environment and acting based on that.

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