Creating an Initramfs

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 fs is formatted reiserfs

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Dependencies

Required

• Cpio
• Busybox

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

 -f|--rootfstype       - use filesystem type for root device

Example:
EOT
}
usage(){
  cat << EOT
 $(basename ${0}) -c -k 2.6.30.2 -m ext3 -r /dev/hda3 -f ext3
EOT
}
vrbmsg(){
  if [ "${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
     ;;
    -f|--rootfstype)
     initramfs_use_rootfstype="${2}"
     shift
     ;;
    -h|--help)
     helpme
     usage
     exit 0
     ;;
    -u|--usage)
     usage
     exit 0
     ;;
  esac
  shift
done
for mkinitramfs_tool in /bin/busybox /usr/bin/cpio /bin/gzip; do
  if [ ! -x "${mkinitramfs_tool}" ]; then
    killme "Required tool ${mkinitramfs_tool} missing!"
  fi
done
if [ ! -d "$(dirname ${initramfs_output})" ]; then
  killme "Can not create ${initramfs_output}"
fi
if [ ! -d '/boot/initramfs-tree' ]; then
  mkdir /boot/initramfs-tree
  mkdir /boot/initramfs-tree/{bin,sbin,lib,etc,proc}
  mkdir /boot/initramfs-tree/{lib/modules,root,sys,mnt}
  ln -s . /boot/initramfs-tree/usr
fi
if [ -f /etc/sysconfig/initramfs.modules ]; then
  cat /etc/sysconfig/initramfs.modules | while read line; do
    if [ -n "${line}" -a "$(head -c1 <<<${line})" != "#" ]; then
      module="$(fnd_mod ${line})"
      if [ -n "${module}" ]; then
        vrbmsg "Found ${module}"
        case "${module}" in
          *.ko)
           cat ${module} > /boot/initramfs-tree/lib/modules/$(basename ${module})
           ;;
          *.ko.gz)
           gzip -dc ${module} > /boot/initramfs-tree/lib/modules/$(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 ${module} > /boot/initramfs-tree/lib/modules/$(basename ${module})
         echo "$(basename ${module} .ko)" >> /boot/initramfs-tree/etc/modules
         ;;
        *.ko.gz)
         gzip -dc ${module} > /boot/initramfs-tree/lib/modules/$(basename ${module} .gz)
         echo "$(basename ${module} .ko.gz)" >> /boot/initramfs-tree/etc/modules
         ;;
      esac
    else
      errmsg "Couldn't find ${line}"
    fi
  done
fi
cat /bin/busybox > /boot/initramfs-tree/bin/busybox
chmod 755 /boot/initramfs-tree/bin/busybox
cat > /boot/initramfs-tree/init << EOT
#!/bin/busybox sh
# Mount things which are needed by this script
/bin/busybox mount -t proc proc /proc
/bin/busybox mount -t sysfs sysfs /sys
# Disable kernel messages from popping up on
# the screen
/bin/busybox echo 0 > /proc/sys/kernel/printk
# Clear the screen
/bin/busybox clear
/bin/busybox cat << END
 This system is uses Community Beyond Linux From Scratch
 <http://cblfs.cross-lfs.org/>
END
# Create the symlinks to busybox
/bin/busybox --install -s
# Create device nodes
/bin/busybox mknod /dev/null c 1 3
/bin/busybox mknod /dev/tty c 5 0
# Check for modules
if [ -f /etc/modules ]; then
  /bin/busybox depmod /lib/modules
  /bin/busybox cat /etc/modules | while read line; do
    if [ -n "\${line}" ]; then
      /bin/busybox modprobe \${line/.ko/}
    fi
  done
fi
# Sleep for devices to connect, like USB
/bin/busybox sleep 5
/bin/busybox mdev -s
# Function for parsing command line options with
# '=' in them. Ex. get_opt("init=/sbin/init")
# would return "/sbin/init"
get_opt() {
	echo "\$@" | /bin/busybox cut -d "=" -f 2
}
# Defaults
init="/sbin/init"
root="${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 -t ${initramfs_use_rootfstype} "\${root}" /root
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
EOF
chmod 755 /usr/sbin/mkinitramfs