Initramfs - make your own

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Make Yourself an Initramfs

Why

There are several tools to build an initramfs. Dracut and genkernel come to mind. They can generate a working initramfs too but users have no idea what is inside. They are usually, but don't need to be, tied to the kernel and gcc version too.

The only reason for following this guide is because you want to be in control.

What

The initramfs is a root filesystem in a file. It can contain whatever is required to boot the system and anything extra too.

Originally the root filesystem in a file was called the initrd. The difference is in the internal structure, which is not a concern at the level of this page. The two terms are frequently used interchangeably.

Overview

This page describes how to build an initramfs that is free of kernel modules. That means that its not tied to kernel versions, it's a once in the lifetime of the equipment thing, rather like firmware. The authors April 2009 initramfs still works.

The worked example covers root in LVM on top of mdadm raid. LUKS could be added or any of the bits illustrated that are not required can be removed.

It assumes that the kernel is built to include everything needed to mount root configured an built in. Kernel modules could be included in the initramfs but that is not described.

To enable the initramfs to be reassembled, should the embedded init script need to change, the binaries to build the initramfs will be built separately. This has the advantage that the initramfs can be built with its own USE flags.

Tip
Unless user space tools are needed to mount root, no initramfs is required at all
Important
The kernel must include everything required to mount root as built in

Method

The kernel provides /usr/src/linux/usr/gen_init_cpio which assembles the initramfs from a list of files. It's input is a file containing a list of binary files to copy to the initramfs together with the destinations in the initramfs.

One of these files will be the init script, which will need to be written.

Initramfs Design

Don't skip this part. What does the initramfs need to do?

  • It must include all the binaries to do whatever is needed to mount root.
  • It must include an init script that controls what will be done

Build Location

The author likes /root/initramfs and that is used in this example.

Eventually it will contain bins/, init and initramfs_list. bins/ is the location of the binaries used to assemble the initramfs. init is the init script to get started and initramfs_list is the what goes where list to feed to gen_init_cpio

Important
The example in this page is just that. It will need to be adjusted to suit the individual install

Example List of Requirements

  • Assemble and start mdadm RAID.
  • Activate Logical Volumes
  • Mount root and possibly other filesystems from inside their Logical Volumes
  • A filesystem checker for non root filesystem mounted in the initramfs
  • Interactive Shell for debug
  • Init script to control everything.
  • Other things to suit the install at hand


Now to discover all the binaries required to meet the explicit requirements. They must be included in the initramfs_list.

Assemble and start mdadm RAID

This requires Multiple Device support built into the kernel and the mdadm user space tool.

That's /sbin/mdadm and all the libraries that it depends on

user $lddtree /sbin/mdadm
|/sbin/mdadm (interpreter => /lib64/ld-linux-x86-64.so.2)
|    libc.so.6 => /lib64/libc.so.6
Tip
Different USE settings will produce different lists

Activate Logical Volumes

This requires Multiple Device and Logical Volume Manager support built into the kernel and the lvm userspace tool.

user $lddtree /sbin/lvm
/sbin/lvm (interpreter => /lib64/ld-linux-x86-64.so.2)
    libdevmapper-event.so.1.02 => /lib64/libdevmapper-event.so.1.02
        libdevmapper.so.1.02 => /lib64/libdevmapper.so.1.02
            libm.so.6 => /lib64/libm.so.6
    libreadline.so.8 => /lib64/libreadline.so.8
        libtinfow.so.6 => /lib64/libtinfow.so.6
    libblkid.so.1 => /lib64/libblkid.so.1
    libaio.so.1 => /lib64/libaio.so.1
    libc.so.6 => /lib64/libc.so.6

It has a bigger list of dependencies, also including /lib64/libc.so.6. Duplicates need only be provided once.

Note
lvm2 takes USE=static, so a monolithic build can be used in the initramfs and a dynamic build used in the man install

Mount root and possibly other filesystems

Most users will want to use mount by filesystem UUID. Not all filesystem are on partitions, so PARTUUID cannot be used.

Mount by filesystem UUID requires the user space mount command

root #lddtree /bin/mount
/bin/mount (interpreter => /lib64/ld-linux-x86-64.so.2)
    libmount.so.1 => /lib64/libmount.so.1
        libblkid.so.1 => /lib64/libblkid.so.1
    libc.so.6 => /lib64/libc.so.6

Interactive Shell for Debug

That will be busybox. Everyone uses busybox. Its small and has lots of utilities too.

user $lddtree /bin/busybox
/bin/busybox (interpreter => None)

My example busybox is statically linked. That's a lifesaver when almost nothing works.

Design Implementation

Init script to control everything

This is the hard bit. Its all the commands that need to be entered at a root shell, using the initramfs to get started.

If the initramfs only contained busybox, what would need to be entered to boot?

Some error handing is a good idea too, so that debug is possible.

Elements of the Init Script

Its a shell script so it must start with the shebang line. Its not a comment.

#!/bin/busybox sh

The error handler is a function which will be called when something goes wrong. It takes one parameter, which is a text string to be printed when it is invoked.

Comments are good for maintenance later.

rescue_shell() {
    echo "$@"
    echo "Something went wrong. Dropping you to a shell."
    # The symlinks are not required any longer
    # but it helps tab completion
    /bin/busybox --install -s
    exec /bin/sh
}

Parse the root filesystem out of the kernel command line and mount it.

# allow the use of UUIDs or filesystem lables
uuidlabel_root() {
    for cmd in $(/bin/cat /proc/cmdline) ; do
        case $cmd in
        root=*)
            type=$(echo $cmd | /bin/cut -d= -f2)
            echo "Mounting rootfs"
            if [ $type == "LABEL" ] || [ $type == "UUID" ] ; then
                uuid=$(echo $cmd | /bin/cut -d= -f3)
                /bin/mount -o ro $(/bin/findfs "$type"="$uuid") /mnt/root
            else
                /bin/mount -o ro $(echo $cmd | /bin/cut -d= -f2) /mnt/root
            fi
            ;;
	esac
    done
}

We only do that once, so it need not be a function but it makes the main flow of the script easier to read.

When things are mounted inside the initramfs, its good to be able check the filesystems first. The localmount service cannot check mounted filesystems.

# We need this for things that are mounted before localmount runs
# like /usr and possibly /var
check_filesystem() {
    # most of code coming from /etc/init.d/fsck

    local fsck_opts= check_extra= RC_UNAME=$(uname -s)

    # FIXME : get_bootparam forcefsck
    if [ -e /forcefsck ]; then
        fsck_opts="$fsck_opts -f"
        check_extra="(check forced)"
    fi

    echo "Checking local filesystem $check_extra : $1"

    if [ "$RC_UNAME" = Linux ]; then
        fsck_opts="$fsck_opts -C0 -T"
    fi

    trap : INT QUIT

    # using our own fsck, not the builtin one from busybox
    /sbin/fsck -p $fsck_opts $1

    ret_val=$?
    case $ret_val in
        0)	return 0;;
        1)	echo "Filesystem repaired"; return 0;;
        2|3)    if [ "$RC_UNAME" = Linux ]; then
                        echo "Filesystem repaired, but reboot needed"
                        /sbin/reboot -f
                else                       rescue_shell "Filesystem still have errors; manual fsck required"
                fi;;
        4)	if [ "$RC_UNAME" = Linux ]; then
                        rescue_shell "Fileystem errors left uncorrected, aborting"
                else
                    	echo "Filesystem repaired, but reboot needed"
                        /sbin/reboot
                fi;;
        8)	echo "Operational error"; return 0;;
        16)     echo "Use or Syntax Error"; return 16;;
        32)     echo "fsck interrupted";;
        127)    echo "Shared Library Error"; sleep 20; return 0;;
        *)	echo $ret_val; echo "Some random fsck error - continuing anyway"; sleep 20; return 0;;
    esac

# rescue_shell can't find tty so its broken
    rescue_shell
}

With those functions in support, we can do what's needed.

Tip
Notice the comments to make hard coding things easier
PATH="/sbin:/bin"
# start for real here
# temporarily mount proc,sys and dev
mount -t proc proc /proc
mount -t sysfs sysfs /sys
mount -t devtmpfs devtmpfs /dev

#mdam arrays to assemble
#boot  UUID : a25b05eb:3db18cbe:afb9312b:d1d97546
#host  UUID : de8f2cbc:17ca3275:0b69db3c:b9f91a6b
#kvm   UUID : a3aab047:413ed52d:b15158fc:cdb637ef

# boot
/sbin/mdadm --assemble /dev/md0 --uuid=a25b05eb-3db18cbe-afb9312b-d1d97546 || echo "boot failed to assemble"
/sbin/mdadm --assemble /dev/md1 --uuid=de8f2cbc-17ca3275-0b69db3c-b9f91a6b || rescue_shell "The host RAID set failed to assemble"
/sbin/mdadm --assemble /dev/md2 --uuid=a3aab047:413ed52d:b15158fc:cdb637ef || echo "THE KVM space did not assemble"

Use /sbin/mdadm --assemble --run to start the raid set with missing members, if possible.

Important
root in LVM on RAID on USB requires a sleep to allow USB HDD to be available before mdadm --assemble runs

Its left as an exercise for the reader to parse the RAID UUID(s) out of the kernel command line.

If boot failed to assemble, it does not impact the boot process as both BIOS and UEFI are not raid aware. Indeed, the boot loader has to read boot to load initramfs to work out that /boot did not assemble. There is no need to call rescue_shell here.

Boot on RAID requires a RAID level and on disk RAID data layout leaves the filesystem untouched. RAID 1 and a raid metadata that lives at the end of the volume works.

Being lazy, start all the logical volumes and call the rescue shell if any one fails. In practice, only the one housing root is required. That's a local design decision.

# Then start LVM
vgchange -ay || rescue_shell "Some/All Volume Groups failed to start"

Now mount other filesystems if needed. Typically /usr and /var

# space separated list of mountpoints that ...
mountpoints="/usr"
# /var"
# ... we want to find in /etc/fstab ...
/bin/ln -s /mnt/root/etc/fstab /etc/fstab

# loop through the list of mountpoints
for m in $mountpoints ; do

#echo $m

    check_filesystem $m

    echo "Mounting $m"
    # mount the device and ...
    mount $m || rescue_shell "Error while mounting $m"

    # ... move the tree to its final location
    mount --move $m "/mnt/root"$m || rescue_shell "Error while moving $m"
done
Tip
Set noauto in /etc/fstab for filesystems mounted here
# That's put all the pieces together, now tidy up

echo "All done. Switching to real root."

# clean up. The init process will remount proc sys and dev later
umount /proc
umount /sys
umount /dev

# switch to the real root and execute init
exec /sbin/switch_root /mnt/root /sbin/init

That final exec call never returns so nervous readers could add rescue_shell "Fell off the end of init" as the very last line.

Warning
Its a horrible script and has grown to its present state over 20 years or more

Building the Binaries

To avoid using binaries from the live filesystem, the initramfs binaries will be installed in /root/initramfs/bins. This allows trivial changes to the init script in years to come without, tracking down all the changed dependencies on the live filesystem. Both ways work. Its a design decision.

The down side of a separate build is that all the dependencies that will not go into the initramfs will be built too.

root #emerge -av --root=/root/initramfs/bins <list_of_packages>

Set the USE flags to your liking and build your binaries. This authors preference is to build everything that supports static linking with USE=static.

<list_of_packages> depends on what is required of the initramfs. Like the rest of Gentoo its easy to add to if needed.

It looks a bit dated now but the bins package list on the example install is

user $ls bins/var/db/pkg/*/*/*.ebuild
app-arch/bzip2-1.0.8-r1/bzip2-1.0.8-r1.ebuild                  sys-block/thin-provisioning-tools-0.9.0-r1/thin-provisioning-tools-0.9.0-r1.ebuild
app-arch/gzip-1.11/gzip-1.11.ebuild                            sys-fs/e2fsprogs-1.46.4/e2fsprogs-1.46.4.ebuild
dev-libs/expat-2.4.3/expat-2.4.3.ebuild                        sys-fs/lvm2-2.02.188-r2/lvm2-2.02.188-r2.ebuild
dev-libs/libaio-0.3.112/libaio-0.3.112.ebuild                  sys-fs/mdadm-4.2-r1/mdadm-4.2-r1.ebuild
dev-libs/libpcre-8.45/libpcre-8.45.ebuild                      sys-libs/e2fsprogs-libs-1.46.4-r1/e2fsprogs-libs-1.46.4-r1.ebuild
dev-libs/libpcre2-10.39/libpcre2-10.39.ebuild                  sys-libs/glibc-2.33-r7/glibc-2.33-r7.ebuild
dev-libs/libunistring-0.9.10-r1/libunistring-0.9.10-r1.ebuild  sys-libs/libcap-2.62/libcap-2.62.ebuild
net-dns/libidn2-2.3.2/libidn2-2.3.2.ebuild                     sys-libs/libxcrypt-4.4.25-r1/libxcrypt-4.4.25-r1.ebuild
sys-apps/acl-2.3.1/acl-2.3.1.ebuild                            sys-libs/ncurses-6.2_p20210619/ncurses-6.2_p20210619.ebuild
sys-apps/attr-2.5.1/attr-2.5.1.ebuild                          sys-libs/pam-1.5.1_p20210622-r1/pam-1.5.1_p20210622-r1.ebuild
sys-apps/baselayout-2.7-r3/baselayout-2.7-r3.ebuild            sys-libs/readline-8.1_p1-r1/readline-8.1_p1-r1.ebuild
sys-apps/busybox-1.34.1/busybox-1.34.1.ebuild                  sys-libs/timezone-data-2021a-r1/timezone-data-2021a-r1.ebuild
sys-apps/gentoo-functions-0.14/gentoo-functions-0.14.ebuild    sys-libs/zlib-1.2.11-r4/zlib-1.2.11-r4.ebuild
sys-apps/grep-3.7/grep-3.7.ebuild                              virtual/awk-1/awk-1.ebuild
sys-apps/systemd-tmpfiles-249.9/systemd-tmpfiles-249.9.ebuild  virtual/libcrypt-2/libcrypt-2.ebuild
sys-apps/util-linux-2.37.2-r1/util-linux-2.37.2-r1.ebuild      virtual/libiconv-0-r2/libiconv-0-r2.ebuild
sys-auth/pambase-20210201.1/pambase-20210201.1.ebuild          virtual/libintl-0-r2/libintl-0-r2.ebuild
sys-auth/passwdqc-2.0.2-r1/passwdqc-2.0.2-r1.ebuild            virtual/tmpfiles-0-r1/tmpfiles-0-r1.ebuild

Putting the Pieces Together

Thats what the initramfs_list file is for.

Important
All the files discovered to be required during design, using lddtree, must be included

Describe the directory structure for the initramfs this example is from an arm64 system. amd64/x86 may differ.

# directory structure
dir /proc	755 0 0
dir /usr        755 0 0
dir /bin        755 0 0
dir /sys        755 0 0
dir /var        755 0 0
#dir /lib        755 0 0
dir /lib64	755 0 0
dir /sbin	755 0 0
dir /mnt        755 0 0
dir /mnt/root   755 0 0
dir /etc        755 0 0
dir /root	700 0 0
dir /dev        755 0 0

Make a few critical device nodes

nod /dev/null   666 0 0 c 1 3
nod /dev/tty    666 0 0 c 5 0
nod /dev/console        600 0 0 c 5 1

They are probably not required with modern DEVTMPFS in the kernel.

All the main commands

# busybox
# Output file name              Input file name
file /bin/busybox               /root/initramfs/bins/bin/busybox        755 0 0
# Need real mount as busybox did not support UUID
file /bin/mount                 /root/initramfs/bins/bin/mount          755 0 0

# for raid on lvm
# Output file name              Input file name
file /sbin/mdadm                /root/initramfs/bins/sbin/mdadm         755 0 0
file /sbin/lvm.static           /root/initramfs/bins/sbin/lvm.static    755 0 0

Add some symbolic links to make life easier.

slink /sbin/vgchange                    /sbin/lvm.static                777 0 0
slink /sbin/vgscan                      /sbin/lvm.static                777 0 0

slink /bin/cat                          /bin/busybox                    777 0 0
slink /bin/cut                          /bin/busybox                    777 0 0
slink /bin/findfs                       /bin/busybox                    777 0 0
slink /bin/ln                           /bin/busybox                    777 0 0
slink /sbin/switch_root                 /bin/busybox                    777 0 0

slink /lib64/libdl.so.2                 /lib64/libdl-2.33.so            777 0 0

# libraries required by /sbin/fsck.ext4 and /sbin/fsck
# The /lib -> /lib64 symlink is mostly harmless but its not right on arm64
slink   /lib                            /lib64                          777 0 0

The symlinks to /bin/busybox are probably not required as busybox assumes internal commands when a command is not found.

# libraries required by /sbin/fsck.ext4 and /sbin/fsck
# The /lib -> /lib64 symlink is mostly harmless but its not right on arm64
slink   /lib                            /lib64                          777 0 0

All the required libraries too

# Output file name                      Input file name
file    /lib/ld-linux-aarch64.so.1	/root/initramfs/bins/lib/ld-linux-aarch64.so.1      755 0 0
file    /lib64/libext2fs.so.2           /root/initramfs/bins/lib64/libext2fs.so.2           755 0 0
file    /lib64/libcom_err.so.2          /root/initramfs/bins/lib64/libcom_err.so.2          755 0 0
file    /lib64/libpthread.so.0          /root/initramfs/bins/lib64/libpthread.so.0          755 0 0
file    /lib64/libblkid.so.1            /root/initramfs/bins/lib64/libblkid.so.1            755 0 0
file    /lib64/libuuid.so.1             /root/initramfs/bins/lib64/libuuid.so.1             755 0 0
file    /lib64/libe2p.so.2              /root/initramfs/bins/lib64/libe2p.so.2              755 0 0
file    /lib64/libc.so.6                /root/initramfs/bins/lib64/libc.so.6                755 0 0
file    /lib64/libmount.so.1            /root/initramfs/bins/lib64/libmount.so.1            755 0 0
file    /lib64/libdl-2.33.so            /root/initramfs/bins/lib64/libdl-2.33.so            755 0 0


file    /sbin/fsck              /sbin/fsck                      755 0 0
file    /sbin/fsck.ext4         /sbin/fsck.ext4                 755 0 0

# our init script
file    /init                   /root/initramfs/init               755 0 0
Warning
This example works with ext4. Choose your /sbin/fsck.fs_type

Mount /boot if its not mounted.

root #/usr/src/linux/usr/gen_init_cpio /root/initramfs/initramfs_list > /boot/<initramfs_name>

Tell your boot loader about /boot/<initramfs_name> and reboot to test.

Ideas For Further Contributions

  • Cover LUKS
  • Cover root over NFS (Possible without an initlamfs too)
  • Bring up the network in the initramfs with ssh access
  • Other things


Warning
Networking in the initramfs is a security risk. The initramfs will need to be maintained