Creating an Initramfs
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Introduction to Creating an Initramfs
The mkinitramfs script creates a gzip compressed cpio archive using busybox and a simple shell init script, which can be used in place of an initrd to do such things as load kernel modules before the system boots, in order to access functionality that is not compiled into the kernel, and therefore must be loaded, such as usb disks, scsi disks, etc. This is different than loading modules automatically at boot time, because those are loaded after the root disk is mounted, fsck'd etc, and typically an initrd contains modules which are loaded into the kernel before anything else, to fascilitate mounting the root fs. IE kernel doesn't have reiserfs support compiled in, but your root disk is formatted reiserfs
Project Homepage: Unknown
Dependencies
Required
Note
The init script created for the initramfs will drop to the shell in busybox if there is an error, while only a few things are actually needed for the init to function, while compiling busybox you might want to keep in mind that it may need additional functionality to be able to work properly if the initramfs drops to the shell. Also this assumes that busybox is compiled statically linked, if that is not the case, additional libraries will be required to be added to the initramfs-tree. To test this run 'file /bin/busybox'. If file reports it as dynamically linked, running 'ldd /bin/busybox' should help you determine what libraries may be needed.
Creating the mkinitramfs script
cat > /usr/sbin/mkinitramfs << "EOF"
#!/bin/bash
errmsg(){
cat << EOT
[ERROR] $@
EOT
}
killme(){
errmsg "$@"
errmsg "Exiting ..."
exit 1
}
helpme(){
cat << EOT
$(basename ${0}) is a script to automagically make
a gzip compressed cpio archive containsing busybox
which can act as an initrd.
-c - remove initramfs-tree before creating the
initramfs
-v|--verbose - use verbose messages
-k <version> - use modules from kernel <version>
-m <arg1,arg2> - comma separated list of additional modules
besides the ones contained in the file
/etc/sysconfig/mkinitramfs.modules to add
to the initramfs archive
-o <arg> - output finished archive to <arg> instead
of /boot/initramfs.igz
-r|--rootfs - use device as root device
Example:
EOT
}
usage(){
cat << EOT
$(basename ${0}) -c -k 2.6.30.2 -m ext3 -r /dev/hda3 -o /boot/initramfs.igz-2.6.30.2
EOT
}
vrbmsg(){
if [ "${initramfs_use_verbose}" ]; then
cat << EOT
$@
EOT
fi
}
fnd_mod(){
module="$(find /lib/modules/${initramfs_use_kernel} -name ${1}.ko)"
if [ ! -n "${module}" ]; then
module="$(find /lib/modules/${initramfs_use_kernel} -name ${1}.ko.gz)"
fi
cat << EOT
${module}
EOT
unset module
}
initramfs_use_kernel="$(uname -r)"
initramfs_output="/boot/initramfs.igz"
initramfs_use_rootfs="/dev/hda2"
initramfs_use_rootfstype="ext3"
until [ -z "${1}" ]; do
case "${1}" in
-c)
rm -rf /boot/initramfs-tree
;;
-v|--verbose)
initramfs_use_verbose='true'
;;
-k)
initramfs_use_kernel="${2}"
shift
;;
-m)
initramfs_use_modules="${2}"
shift
;;
-o)
initramfs_output="${2}"
shift
;;
-r|--rootfs)
initramfs_use_rootfs="${2}"
shift
;;
-h|--help)
helpme
usage
exit 0
;;
-u|--usage)
usage
exit 0
;;
esac
shift
done
for mkinitramfs_tool in /bin/busybox /usr/bin/cpio /bin/gzip; do
if [ ! -x "${mkinitramfs_tool}" ]; then
killme "Required tool ${mkinitramfs_tool} missing!"
fi
done
if [ ! -d "$(dirname ${initramfs_output})" ]; then
killme "Can not create ${initramfs_output}"
fi
if [ ! -d '/boot/initramfs-tree' ]; then
mkdir /boot/initramfs-tree
mkdir /boot/initramfs-tree/{bin,sbin,lib,etc,proc,root,sys,mnt}
mkdir /boot/initramfs-tree/lib/modules/${initramfs_use_kernel}
ln -s . /boot/initramfs-tree/usr
fi
if [ -f /etc/sysconfig/initramfs.modules ]; then
cat /etc/sysconfig/initramfs.modules | while read line; do
if [ -n "${line}" -a "$(head -c1 <<<${line})" != "#" ]; then
module="$(fnd_mod ${line})"
if [ -n "${module}" ]; then
vrbmsg "Found ${module}"
case "${module}" in
*.ko)
cat ${module} > /boot/initramfs-tree/lib/modules/${initramfs_use_kernel}/$(basename ${module})
;;
*.ko.gz)
gzip -dc ${module} > /boot/initramfs-tree/lib/modules${initramfs_use_kernel}/$(basename ${module} .gz)
:;
esac
else
errmsg "Couldn't find ${line}"
fi
fi
done
fi
if [ -n "${initramfs_use_modules}" ]; then
for line in ${initramfs_use_modules//,/ }; do
module="$(fnd_mod ${line})"
if [ -n "${module}" ]; then
vrbmsg "Found ${module}"
case "${module}" in
*.ko)
cat ${module} > /boot/initramfs-tree/lib/modules/${initramfs_use_kernel}/$(basename ${module})
echo "$(basename ${module} .ko)" >> /boot/initramfs-tree/etc/modules
;;
*.ko.gz)
gzip -dc ${module} > /boot/initramfs-tree/lib/modules/${initramfs_use_kernel}/$(basename ${module} .gz)
echo "$(basename ${module} .ko.gz)" >> /boot/initramfs-tree/etc/modules
;;
esac
else
errmsg "Couldn't find ${line}"
fi
done
fi
cat /bin/busybox > /boot/initramfs-tree/bin/busybox
chmod 755 /boot/initramfs-tree/bin/busybox
cat > /boot/initramfs-tree/init << EOT
#!/bin/busybox sh
# Mount things which are needed by this script
/bin/busybox mount -t proc proc /proc
/bin/busybox mount -t sysfs sysfs /sys
# Disable kernel messages from popping up on
# the screen
/bin/busybox echo 0 > /proc/sys/kernel/printk
# Clear the screen
/bin/busybox clear
/bin/busybox cat << END
This system uses Community Beyond Linux From Scratch
<http://cblfs.cross-lfs.org/>
END
# Create the symlinks to busybox
/bin/busybox --install -s
# Create device nodes
/bin/busybox mknod /dev/null c 1 3
/bin/busybox mknod /dev/tty c 5 0
# Check for modules
if [ -f /etc/modules ]; then
/bin/busybox depmod /lib/modules
/bin/busybox cat /etc/modules | while read line; do
if [ -n "\${line}" ]; then
/bin/busybox modprobe \${line/.ko/}
fi
done
fi
# Sleep for devices to connect, like USB
/bin/busybox sleep 5
/bin/busybox mdev -s
# Function for parsing command line options with
# '=' in them. Ex. get_opt("init=/sbin/init")
# would return "/sbin/init"
get_opt() {
echo "\$@" | /bin/busybox cut -d "=" -f 2
}
# Defaults
init="/sbin/init"
root="${initramfs_use_rootfs}"
runlevel="3"
for i in \$(cat /proc/cmdline); do
case \${i} in
root\=*)
root=\$(get_opt \${i})
;;
init\=*)
init=\$(get_opt \${i})
;;
runlevel\=*)
runlevel=\$(get_opt \${i})
;;
esac
done
# Mount the root device
mount -o ro "\${root}" /root
# Below is an example of how to use the initramfs
# to load clfs or lfs from a disk image
#if [ -f /root/lfs-root.img ]; then
# BNROOT=\$(/bin/busybox basename "\${root}")
# umount root
# mkdir -p /mnt/"\${BNROOT}" &&
# mount "\${root}" /mnt/"\${BNROOT}" &&
# mount -o loop /mnt/"\${BNROOT}"/lfs-root.img /root &&
# mkdir -p /root/mnt/"\${BNROOT}" &&
# mount /mnt/"\${BNROOT}" /root/mnt/"\${BNROOT}"
# unset BNROOT
#fi
if [ -x "/root/\${init}" ]; then
# Unmount all other mounts so that the ram used by
# the initramfs can be cleared after switch_root
umount /sys /proc
# Switch to the new root and execute the init at
# the specified runlevel
exec switch_root /root "\${init}" "\${runlevel}"
fi
echo "Failed 'exec switch_root /root \\"\${init}\\" \\"\${runlevel}\\"'"
exec sh
EOT
chmod 755 /boot/initramfs-tree/init
( cd /boot/initramfs-tree
find . | cpio -H newc -o | gzip -9c > ${initramfs_output}
) || exit 1
EOF
chmod 755 /usr/sbin/mkinitramfs
