How to create a Bootable Live CD/DVD from your Linux From Scratch (LFS) build

Subject of Hint: How to create a Bootable Live CD/DVD from your Linux From Scratch (LFS) build

Good day from Singapore,

I am Mr. Turritopsis Dohrnii Teo En Ming, based in Singapore.

I have recently *successfully* created my own custom Linux distribution called Teo En Ming Linux 2020.03 (FINAL), which is based on
Linux From Scratch 20200302-systemd book and Linux Kernel 5.5.7, on 12 March 2020.

You may refer to my redundant blog posts for the announcement.

Blogger: Announcing Teo En Ming Linux 2020.03 (FINAL)
Link: https://tdtemcerts.blogspot.com/2020/03/announcing-teo-en-ming-linux-202003.html

Wordpress: Announcing Teo En Ming Linux 2020.03 (FINAL)
Link: https://tdtemcerts.wordpress.com/2020/03/12/announcing-teo-en-ming-linux-2020-03-final/

Jimmy Anderson's guide/hint was written on 20 Jan 2013, which is more than 7 years ago. Today is 14 March 2020 Saturday. Therefore, his guide is definitely outdated
and needs some tweaking to adapt to the *latest* LFS system at the time of this writing.

Jimmy Anderson's guide/hint was verified with LFS version 7.2. I have made *extensive* modifications to his guide/hint to work with my LFS 20200302-systemd build.

You can download Teo En Ming Linux 2020.03 (FINAL) Bootable Live CD/DVD ISOs from the following github.com link.

Link: https://github.com/teo-en-ming/teo-en-ming-linux/blob/master/iso%20download%20links.txt

Note: Please note that my Live CD/DVD boots with a Kernel Panic. I hope that Linux experts will be able to download my ISO, examine its contents and find out what's wrong.
I would like to thank all of you in advance for your kind assistance and advise.

==============================================================================================================================================================================

The attached file is a new hint which describes how to turn your LFS build
into your own Live CD/DVD.
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ORIGINAL AUTHOR: Jimmy Anderson (jimmy.anderson1057 at gmail.com)
ORIGINAL DATE: 2013-01-20
Link to original hint: http://lists.linuxfromscratch.org/pipermail/hints/2013-March/003307.html

EDITED BY: Turritopsis Dohrnii Teo En Ming, Singapore
EDIT DATE: 14 MARCH 2020, SATURDAY

LICENSE:

Portions of the instructions in this hint were derived from scripts
in the LFScript project.  These scripts are covered by the following
license:

# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.

This hint is also provided under the above license terms.


SYNOPSIS: How to create a bootable Live CD/DVD from your LFS build.

DESCRIPTION:

   This hint describes how to create a live CD/DVD from your LFS build.  The
resulting live CD/DVD can be used as an LFS/CLFS build host or for whatever
purpose you desire.

   Briefly, you will build your LFS system and a "squashed" version
of it will be created.   A busybox based ramdisk will be created
which sets up a union mount (either aufs or unionfs) of the squashed
LFS system with a ram based file system and then the system will be
started with the union filesystem as root.

   unionfs and aufs are not part of the standard kernel code.   To build
a live cd/dvd you must patch your kernel to add support for one or the other.
This hint will still work without aufs or unionfs support but the result
will not be a live CD/DVD but instead be a bootable CD/DVD (useful but
with lesser ability than a live cd/dvd). Teo En Ming *did not* patch Linux kernel 5.5.7
to add support for aufs or unionfs filesystems because he did not have the knowledge to tweak
the patches. 

   The original hint was verified with LFS 7.2 x86 (32 and 64 bit).  It may
(likely will) work with other LFS versions also but it only was
specifically tested with 7.2 and you are on your own to work out how
to make it work with other LFS versions. Teo En Ming has modifed this guide/hint to allow it
to work with the latest LFS 20200302-systemd development book.

   A description of each step and cut and paste for that
step is provided.  Cut and pasting should make it easier to build a live cd/dvd
but it is important that you understand what the purpose of each step
is.  Blindly cutting and pasting from this hint will probably not
result in success.

   Following this hint, like doing pretty much anything in LFS, has the
potential of damaging your system.   Before you begin, make sure
you've saved a copy of any data that is important to you.  It's also suggested
to follow this procedure on a system that is used only by you so that any
errors will not affect others.


ATTACHMENTS:
None

PREREQUISITES:

This hint assumes the reader is familiar with Linux From Scratch (LFS).

Much of this hint assumes that the LFS variable is properly set. So,
when following this hint, at all times make sure LFS is set properly:

export LFS=/mnt/lfs   (or as needed for your setup)


HINT:

Step 1) Download additional needed packages.

   This step is done on the HOST (not in LFS chroot).

   You will need some additional packages.   Download the squashfs tools,
busybox and syslinux.   The squashfs tools will be needed on the host. 


#------------ Cut and Paste start
cd /home/teo-en-ming/Downloads
wget -4 https://sourceforge.net/projects/squashfs/files/squashfs/squashfs4.4/squashfs4.4.tar.gz/download
wget -4 https://busybox.net/downloads/busybox-1.31.1.tar.bz2
wget -4 http://www.kernel.org/pub/linux/utils/boot/syslinux/syslinux-4.06.tar.xz
#------------ Cut and Paste end

   Additionally, the addendum of this hint contains a script that is
required for the live cd/dvd.   Save the addendum of this hint as:

$LFS/sources/init.sh

   Note: You can skip the aufs/unionfs kernel patch altogether if a
more simple bootable CD/DVD is adequate for your needs.


Step 2) Add squashfs tools to your host.   (Do this on your HOST)

Teo En Ming's host operating system is Ubuntu 18.04.3 LTS Desktop Edition.

   If you are running a host distribution which provides prepackaged
squashfs tools, you may be able to use them.   However they must be
at least version 4.1 to work properly with this hint.

   It is recommended however, that you build squashfs tools 4.4 from
the source. This hint was tested with squashfs tools 4.4 and better
(xz) compression is obtained with 4.4 (compared with 4.1).  To build the
squashfs tools do:

#------------ Cut and Paste start
cd /home/teo-en-ming/Downloads
tar xvfz squashfs4.4.tar.gz
cd squashfs4.4/squashfs-tools
Edit Makefile as follows:
XZ_SUPPORT = 1
COMP_DEFAULT = xz
sudo apt install liblzma-dev
make
sudo make install
mksquashfs -version
#------------ Cut and Paste end

The version string displayed should be 4.4.   If not, you may need to
adjust your PATH so that $HOME/bin appears first in it.

If you are on a single user system, or if no other version of
squashfs tools exists, squashfs tools could be installed in /usr/bin
or /usr/local/bin rather than $HOME/bin.

If the lzma libraries are not installed, the build of squashfs-tools
will fail.   To resolve it, either install the lzma libraries or
rebuild squashfs without xz support.   squashfs (and this hint) can operate
properly without xz support but without xz support, the resulting
live CD/DVD iso size will be somewhat larger.


Step 3) Build your LFS system and kernel.

   Follow the instructions in the LFS/BLFS book and build your LFS system
as you normally would.

When you configure the kernel, make sure the following are enabled
as builtins (not as modules):

   SQUASHFS support (and support for SQUASHFS XZ compressed file systems).
   UNIONFS or AUFS support (Teo En Ming *did not* patch Linux Kernel 5.5.7 for AUFS or UNIONFS support).
   CDROM support (ISO9660).
   DEVTMPFS support.

   Then build and install as normal:
#------------ Cut and Paste start
make
make modules_install
cp arch/x86/boot/bzImage /boot/vmlinuz-5.5.7-turritopsis.dohrnii.teo.en.ming
cp .config /boot/config-5.5.7
#------------ Cut and Paste end


   Additionally, /etc/fstab needs some tweaks for a live cd/dvd. 
Edit /etc/fstab and comment out all hard drive mounts.
But leave the virtual filesystem mounts (/proc and such) in it:
#------------ Cut and Paste start
# Teo En Ming's favorite text editor in Linux is nano.
nano /etc/fstab
>> comment out any hard drive mounts <<
#------------ Cut and Paste end

   Build and configure the remainder of your LFS system as normal.


Step 4) Build busybox in your LFS.

   While inside the chroot environment add busybox to your
   LFS system:

ERROR compiling busybox with glibc-2.31: "sys-apps/busybox-1.31.1 fails to compile with sys-devel/glibc-2.31"

Software Bug Link: https://bugs.gentoo.org/708350

Link to patch file: https://708350.bugs.gentoo.org/attachment.cgi?id=611780

===Start File busybox-glibc-2.31.patch===

diff --git a/coreutils/date.c b/coreutils/date.c
index 3414d38ae..4ade6abb4 100644
--- a/coreutils/date.c
+++ b/coreutils/date.c
@@ -279,6 +279,9 @@ int date_main(int argc UNUSED_PARAM, char **argv)
  time(&ts.tv_sec);
 #endif
  }
+#if !ENABLE_FEATURE_DATE_NANO
+ ts.tv_nsec = 0;
+#endif
  localtime_r(&ts.tv_sec, &tm_time);

  /* If date string is given, update tm_time, and maybe set date */
@@ -301,9 +304,10 @@ int date_main(int argc UNUSED_PARAM, char **argv)
  if (date_str[0] != '@')
  tm_time.tm_isdst = -1;
  ts.tv_sec = validate_tm_time(date_str, &tm_time);
+ ts.tv_nsec = 0;

  /* if setting time, set it */
- if ((opt & OPT_SET) && stime(&ts.tv_sec) < 0) {
+ if ((opt & OPT_SET) && clock_settime(CLOCK_REALTIME, &ts) < 0) {
  bb_perror_msg("can't set date");
  }
  }
diff --git a/libbb/missing_syscalls.c b/libbb/missing_syscalls.c
index 87cf59b3d..dc40d9155 100644
--- a/libbb/missing_syscalls.c
+++ b/libbb/missing_syscalls.c
@@ -15,14 +15,6 @@ pid_t getsid(pid_t pid)
  return syscall(__NR_getsid, pid);
 }

-int stime(const time_t *t)
-{
- struct timeval tv;
- tv.tv_sec = *t;
- tv.tv_usec = 0;
- return settimeofday(&tv, NULL);
-}
-
 int sethostname(const char *name, size_t len)
 {
  return syscall(__NR_sethostname, name, len);
diff --git a/util-linux/rdate.c b/util-linux/rdate.c
index 70f829e7f..878375d78 100644
--- a/util-linux/rdate.c
+++ b/util-linux/rdate.c
@@ -95,9 +95,13 @@ int rdate_main(int argc UNUSED_PARAM, char **argv)
  if (!(flags & 2)) { /* no -p (-s may be present) */
  if (time(NULL) == remote_time)
  bb_error_msg("current time matches remote time");
- else
- if (stime(&remote_time) < 0)
+ else {
+ struct timespec ts;
+ ts.tv_sec = remote_time;
+ ts.tv_nsec = 0;
+ if (clock_settime(CLOCK_REALTIME, &ts) < 0)
  bb_perror_msg_and_die("can't set time of day");
+ }
  }

  if (flags != 1) /* not lone -s */

===End File busybox-glibc-2.31.patch===

#------------ Cut and Paste start
cd /sources
tar xvf busybox-1.31.1.tar.bz2
cd busybox-1.31.1
cat ../busybox-glibc-2.31.patch | patch -Np1
make defconfig
sed 's/# CONFIG_STATIC is not set/CONFIG_STATIC=y/' -i .config

sed 's/CONFIG_FEATURE_HAVE_RPC=y/# CONFIG_FEATURE_HAVE_RPC is not set/' \
        -i .config
sed 's/CONFIG_FEATURE_MOUNT_NFS=y/# CONFIG_FEATURE_MOUNT_NFS is not set/' \
        -i .config
sed 's/CONFIG_FEATURE_INETD_RPC=y/# CONFIG_FEATURE_INETD_RPC is not set/' \
        -i .config
make
cp -v busybox /bin
#------------ Cut and Paste end


Step 5) Create ramdisk.

Do this step from inside the LFS chroot environment.   This step
creates a busybox based ramdisk.   The ramdisk does the work of
creating and mounting the union filesystem and switching to it
for the root file system.

#------------ Cut and Paste start
cd /sources
echo "Teo En Ming Linux 2020.03 (FINAL) Bootable Live CD/DVD" >id_label
mkdir -pv mnt_init/{bin,boot}
cp -v id_label mnt_init/boot
cp -v /bin/busybox mnt_init/bin
cp init.sh mnt_init/init
sed -i "s/<ARCH>/$(uname -m)/g" mnt_init/init
chmod +x mnt_init/init

pushd mnt_init
find . | ./bin/busybox cpio -o -H newc -F ../initramfs.cpio
popd
gzip -9 initramfs.cpio
rm -rf mnt_init
mv initramfs.cpio.gz /boot/initram.fs
mv id_label /boot
#------------ Cut and Paste end


Step 6) Make a squashed version of your LFS.

   This step should be done on the HOST (not inside chroot).

   Do any needed cleanup and final tweaking of your LFS system prior to
continuing.  Strip any binaries you want stripped, remove any build cruft,
remove $LFS/tools, etc.   Add any additional data that you would like
to be present on the live CD/DVD, etc.  A base LFS system (without source) turns into only
a 267 megabyte (or thereabouts) size iso, so there should be plenty of
space available for extras.

   Exit the chroot and unmount any filesystems that are associated with
the $LFS directory.   At this point, it is a good idea to do a system
shutdown and restart to ensure that the LFS directory is unused and
in a quiescent state.   Make sure LFS is set again after the reboot.


#------------ Cut and Paste start
# Set LFS value as needed...
export LFS=/mnt/lfs
cd /home/teo-en-ming
echo $LFS
rm -f root.sfs
sudo mksquashfs $LFS root.sfs -comp xz
#------------ Cut and Paste end

(omit the -comp xz if your squashfs tools don't support xz compression).



Step 7) Add syslinux and create CD/DVD image.

This step should be done on the HOST (not inside chroot).

Setup the isolinux configuration file:

#------------ Cut and Paste start
cd /home/teo-en-ming

cat > isolinux.cfg << EOF
DEFAULT menu.c32
PROMPT 0
MENU TITLE Select an option...
TIMEOUT 300

LABEL live
    MENU LABEL ^Boot Teo En Ming Linux 2020.03 (FINAL) Live CD/DVD ($(uname -m))
    MENU DEFAULT
    KERNEL /boot/$(uname -m)/vmlinuz
    APPEND initrd=/boot/$(uname -m)/initram.fs quiet

LABEL busybox
    MENU LABEL ^Boot busybox ($(uname -m))
    KERNEL /boot/$(uname -m)/vmlinuz
    APPEND initrd=/boot/$(uname -m)/initram.fs quiet busybox

LABEL harddisk
    MENU LABEL Boot from first ^Hard disk
    LOCALBOOT 0x80

EOF
#------------ Cut and Paste end

Package the isolinux bootloader, squashed rootfs and ramdisk
into a subdirectory:

#------------ Cut and Paste start
tar xf syslinux-4.06.tar.xz
rm -rf live
mkdir -p live/boot/{isolinux,$(uname -m)}
cp -v syslinux-4.06/core/isolinux.bin live/boot/isolinux
cp -v syslinux-4.06/com32/menu/menu.c32 live/boot/isolinux
mv -v /home/teo-en-ming/isolinux.cfg                 live/boot/isolinux

cp -v /home/teo-en-ming/root.sfs live/boot/$(uname -m)
cp -v $LFS/boot/vmlinuz-5.5.7-turritopsis.dohrnii.teo.en.ming   live/boot/$(uname -m)/vmlinuz
cp -v $LFS/boot/id_label live/boot/$(uname -m)
cp -v $LFS/boot/initram.fs live/boot/$(uname -m)/initram.fs

#------------ Cut and Paste end

Create the live CD/DVD iso image using whatever iso creation tool you
have.   In this example, it is 'genisoimage':

#------------ Cut and Paste start
genisoimage -o teo-en-ming-linux-2020.03-final-live-cd-dvd.iso     \
        -b boot/isolinux/isolinux.bin \
        -c boot.cat                   \
        -no-emul-boot                 \
        -boot-load-size 4             \
        -boot-info-table              \
        -joliet -l -R                 \
        live

rm -rf live ### OPTIONAL
#------------ Cut and Paste end

Step 8) Burn and boot.

Burn the iso using whatever cd/dvd writing tools you prefer.
Teo En Ming uses k3b. Reboot the
system with the CD/DVD inserted and it should boot into your live CD/DVD.

Step 9) Usage

The CD/DVD boot menu allows you to boot busybox or the live cd/dvd.   Busybox
may be useful if debugging or experimenting is needed.

You can use the live cd/dvd to inject your LFS image onto the target computer (
rather than rebuilding from scratch..)   First, mount
/dev/cdrom somewhere.   In the boot subdirectory in the cd image, you will
find the root.sfs file.   root.sfs is your LFS system in a squashed format.
To install it into the target, either use the unsquashfs tools to
uncompress it onto the target computer system or loop mount the root.sfs
file and then use tar or some other directory copy tool to inject
it into the target computer.

 

ACKNOWLEDGEMENTS:
  * The LFS from script project (http://lfscript.org/wiki/index.php/Main_Page)
  * The LFS project (http://www.linuxfromscratch.org/lfs/)

CHANGELOG:
[2013-01-20]
  * Initial hint by Jimmy Anderson.
[2020-03-14]
  * Modifications by Turritopsis Dohrnii Teo En Ming, Singapore

ADDENDUM: v Cut and save the remainder of this file as $LFS/sources/init.sh v
#!/bin/busybox sh
#
# This file is a modified version of:
#
# Initramfs boot script 1.3.1 (2012-02-09)
# Copyright (c) 2010-2012   Marcel van den Boer
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to
# deal in the Software without restriction, including without limitation the
# rights to use, copy, modify, merge, publish, distribute, sublicense, and/or
# sell copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
# FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
# IN THE SOFTWARE.

# FS layout at the start of this script:
# - /boot/id_label
# - /bin/busybox
# - /dev/console (created by kernel)
# - /init (this file)

set -e

ARCH="<ARCH>"

###########################################
copyBindMount() { # COPY/BIND LIVECD MODE #
###########################################

# This function bind-mounts directories which are designed to be capable of
# read-only access and copies the remaining directories to a tmpfs.
#
# The downside of this method is that the resulting root filesystem is not
# fully writable. So, for example, installation of new programs will not be
# possible.
#
# However, this function can be used without any modification to the kernel and
# is therefore perfect for use as a fallback if other options are not available.

# Mount a tmpfs where the new rootfs will be.
mount -t tmpfs tmpfs ${ROOT} # Allows remounting root in the bootscripts

# Bind mount read-only filesystems, copy the rest
cd /mnt/system
for dir in $(ls -1); do
    case ${dir} in
        lost+found)
            ;;
        bin | boot | lib | opt | sbin | usr)
            mkdir ${ROOT}/${dir}
            mount --bind ${dir} ${ROOT}/${dir}
            ;;
        *)
            cp -R ${dir} ${ROOT}
            ;;
    esac
done
cd /

##############################################
}; unionMount() { # UNIONFS/AUFS LIVECD MODE #
##############################################

# A union mount takes one or more directories and combines them transparantly
# in a third. This function creates a writable directory in memory (tmpfs) and
# uses it to overlay the read-only system image, resulting in a fully writable
# root file system.
#
# The only downside to this method is that it requires a union type filesystem
# in the kernel, which can only be accomplished by patching the kernel as there
# is no such feature in a vanilla kernel.

mkdir -p /mnt/writable
mount -t tmpfs -o rw tmpfs /mnt/writable

UNIONFSOPT="/mnt/writable=rw:/mnt/system=ro"
AUFSOPT="/mnt/writable=rw:/mnt/system=ro"
mount -t unionfs -o dirs=${UNIONFSOPT} unionfs ${ROOT} 2> /dev/null || \
mount -t aufs -o br=${AUFSOPT} none ${ROOT} 2> /dev/null || \
{
    # If UnionFS fails, fall back to copy/bind mounting
    copyBindMount
}

######################
} # END OF FUNCTIONS #
######################

# Make required applets easier to access
for applet in cat chmod cp cut grep ls mkdir mknod mount umount switch_root \
    rm mv vi cpio tar mke2fs sync fdisk dd ; do
    /bin/busybox ln /bin/busybox /bin/${applet}
done

# Clear the screen
#clear # Don't! This will clear the Linux boot logo when using a framebuffer.
       # If you want to clear the screen on boot add the "clear" command to
       # '/usr/share/live/sec_init.sh' in the system image.

# Create device nodes required to run this script
# Note: /dev/console will already be available in the ramfs
mknod /dev/null c  1  3

mknod /dev/scd0 b 11  0  # +--------
mknod /dev/scd1 b 11  1  # |
mknod /dev/scd2 b 11  2  # |
mknod /dev/scd3 b 11  3  # |
                         # |
mknod /dev/hdc  b 22  0  # |
                         # |
mknod /dev/sda  b  8  0  # |
mknod /dev/sda1 b  8  1  # |
mknod /dev/sda2 b  8  2  # |
mknod /dev/sda3 b  8  3  # |
mknod /dev/sda4 b  8  4  # |
                         # |
mknod /dev/sdb  b  8 16  # |    <----
mknod /dev/sdb1 b  8 17  # |        Devices which could be or contain the
mknod /dev/sdb2 b  8 18  # |        boot medium...
mknod /dev/sdb3 b  8 19  # |
mknod /dev/sdb4 b  8 20  # |
                         # |
mknod /dev/sdc  b  8 32  # |
mknod /dev/sdc1 b  8 33  # |
mknod /dev/sdc2 b  8 34  # |
mknod /dev/sdc3 b  8 35  # |
mknod /dev/sdc4 b  8 36  # |
                         # |
mknod /dev/sdd  b  8 48  # |
mknod /dev/sdd1 b  8 49  # |
mknod /dev/sdd2 b  8 50  # |
mknod /dev/sdd3 b  8 51  # |
mknod /dev/sdd4 b  8 52  # +--------

# Create mount points for filesystems
mkdir -p /mnt/medium
mkdir -p /mnt/system
mkdir -p /mnt/rootfs

# Mount the /proc filesystem (enables filesystem detection for 'mount')
mkdir /proc
mount -t proc proc /proc

# Invoke busybox if requested via the kernel command line.
if [ `grep -c "busybox" /proc/cmdline` -ne "0" ]; then
    echo "Busybox requested";
    busybox sh
fi

# Search for, and mount the boot medium
LABEL="$(cat /boot/id_label)"
for device in $(ls /dev); do
    [ "${device}" == "console" ] && continue
    [ "${device}" == "null"    ] && continue

    mount -o ro /dev/${device} /mnt/medium 2> /dev/null && \
    if [ "$(cat /mnt/medium/boot/${ARCH}/id_label)" != "${LABEL}" ]; then
        umount /mnt/medium
    else
        DEVICE="${device}"
        break
    fi
done

if [ "${DEVICE}" == "" ]; then
    echo "FATAL: Boot medium not found."
    /bin/busybox sh
fi

# Mount the system image
mount -t squashfs -o ro,loop /mnt/medium/boot/${ARCH}/root.sfs /mnt/system || {
    echo "FATAL: Boot medium found, but system image is missing."
    /bin/busybox sh
}

# Define where the new root filesystem will be
ROOT="/mnt/rootfs" # Also needed for /usr/share/live/sec_init.sh

# Run in bootable cd mode if requested via the kernel command line.
if [ `grep -c "bootcd" /proc/cmdline` -ne "0" ]; then
    echo "Bootcd mode (bind mount) requested";
    copyBindMount
else
    # Select LiveCD mode
    unionMount # Might fall back to copyBindMount
fi

# Tell LFS to skip fsck during startup:
> $ROOT/fastboot

# Get rid of / manipulations in mountfs script:
grep -v ' \/ ' $ROOT/etc/rc.d/init.d/mountfs > $ROOT/tmp/mountfs
rm -f $ROOT/etc/rc.d/init.d/mountfs
mv $ROOT/tmp/mountfs $ROOT/etc/rc.d/init.d/mountfs
chmod +x $ROOT/etc/rc.d/init.d/mountfs

# Move current mounts to directories accessible in the new root
cd /mnt
for dir in $(ls -1); do
    if [ "${dir}" != "rootfs" ]; then
        mkdir -p ${ROOT}/mnt/.boot/${dir}
        mount --move /mnt/${dir} ${ROOT}/mnt/.boot/${dir}
    fi
done
cd /

# Run secondary initialization (if the system provides it)
if [ -x ${ROOT}/usr/share/live/sec_init.sh ]; then
    . ${ROOT}/usr/share/live/sec_init.sh
fi

# Clean up
umount /proc

# Switch to the new root and launch INIT!
exec switch_root -c /dev/console ${ROOT} /sbin/init





REFERENCES
==========

[6] https://lkml.org/lkml/2020/3/14/22

[7] http://lkml.iu.edu/hypermail/linux/kernel/2003.1/07820.html

[8] https://marc.info/?l=linux-kernel&m=158423592128029&w=2