Manually Creating initrd / initramfs to Boot Linux

Initrd and initramfs files are used together with a Linux kernel and a bootloader (such as GRUB and syslinux) to start up Linux and boot it in many different ways. Many Linux distributions provide prepackaged kernels and convenient tools for creating initramfs, but I like to build my own kernel and initramfs because it allows me to customize the boot process. For instance, one can choose to boot from the local disk or network-mounted disk with the help of initramfs and some user-defined boot parameters. Here's a list of things that I'd like to achieve with my custom initramfs.

1. Boot from a live CD
   
2. Boot from a read-only filesystem image, compressed with SquashFS
   
3. Copy the filesystem image to a RAM disk and run Linux entirely on memory
   
4. Boot Linux from a USB flash drive or a Firewire disk
   
5. Boot Linux from a local disk partition
   
6. Boot Linux from a network drive
   
7. Run a rescue shell without booting Linux
   
8. Start a kdrive X-server and run gparted, partclone or partimage
   

In this post, I show how I normally create initrd / initramfs files. For this tutorial, the following Debian/Ubuntu packages are needed:

• busybox
  
  provides small essential utilities for booting and rescue shell.
  
• cpio
  
  is used to create the actual initramfs format.
  
• dash
  
  provides a minimalist shell for running the init script and the rescue shell.
  
• e2fsprogs, jfsutils, etc.
  
  provides fsck to check Linux filesystems and replay any stale journal
  
• lzma or xz-utils
  
  compresses the initramfs
  
• module-init-tools
  
  is used to load kernel modules necessary to activate hardware and mount the filesystem
  
• pciutils
  
  is used to detect PCI hardware
  
• unionfs-fuse
  
  makes it possible to use read-only Linux systems, such as live CD and filesystem images
  
• unzip
  
  is used to apply customized settings and changes to the unionfs boot mode
  
• v86d
  
  is used to set the screen resolution for the framebuffer screen
  

First, create a text file with a list of files to put in initramfs. The following is an example of such file:

bin/busybox
bin/dash
bin/mount
etc/filesystems
etc/fuse.conf
etc/group
etc/modprobe.d
lib/ld-linux.so.2
lib/libacl.so.1
lib/libattr.so.1
lib/libblkid.so.1
lib/libbz2.so.1.0
lib/libcom_err.so.2
lib/libc.so.6
lib/libdl.so.2
lib/libe2p.so.2
lib/libext2fs.so.2
lib/libfuse.so.2
lib/libkmod.so.2
lib/libmount.so.1
lib/libm.so.6
lib/libpci.so.3
lib/libpthread.so.0
lib/libresolv.so.2
lib/librt.so.1
lib/libselinux.so.1
lib/libsepol.so.1
lib/libuuid.so.1
lib/libx86.so.1
lib/libz.so.1
lib/modules/3.5.3/kernel/drivers/ata
lib/modules/3.5.3/kernel/drivers/block/loop.ko
lib/modules/3.5.3/kernel/drivers/cdrom/cdrom.ko
lib/modules/3.5.3/kernel/drivers/connector/cn.ko
lib/modules/3.5.3/kernel/drivers/firewire/firewire-core.ko
lib/modules/3.5.3/kernel/drivers/firewire/firewire-ohci.ko
lib/modules/3.5.3/kernel/drivers/firewire/firewire-sbp2.ko
lib/modules/3.5.3/kernel/drivers/i2c/algos/i2c-algo-bit.ko
lib/modules/3.5.3/kernel/drivers/scsi/sd_mod.ko
lib/modules/3.5.3/kernel/drivers/scsi/sr_mod.ko
lib/modules/3.5.3/kernel/drivers/usb/host/ehci-hcd.ko
lib/modules/3.5.3/kernel/drivers/usb/host/ohci-hcd.ko
lib/modules/3.5.3/kernel/drivers/usb/host/uhci-hcd.ko
lib/modules/3.5.3/kernel/drivers/usb/storage/usb-storage.ko
lib/modules/3.5.3/kernel/drivers/video
lib/modules/3.5.3/kernel/fs/ext3/ext3.ko
lib/modules/3.5.3/kernel/fs/ext4/ext4.ko
lib/modules/3.5.3/kernel/fs/fat/fat.ko
lib/modules/3.5.3/kernel/fs/fat/vfat.ko
lib/modules/3.5.3/kernel/fs/fuse/fuse.ko
lib/modules/3.5.3/kernel/fs/isofs/isofs.ko
lib/modules/3.5.3/kernel/fs/jbd2/jbd2.ko
lib/modules/3.5.3/kernel/fs/jbd/jbd.ko
lib/modules/3.5.3/kernel/fs/jfs/jfs.ko
lib/modules/3.5.3/kernel/fs/nls/nls_cp437.ko
lib/modules/3.5.3/kernel/fs/nls/nls_iso8859-1.ko
lib/modules/3.5.3/kernel/fs/nls/nls_utf8.ko
lib/modules/3.5.3/kernel/fs/reiserfs/reiserfs.ko
lib/modules/3.5.3/kernel/fs/squashfs/squashfs.ko
lib/modules/3.5.3/kernel/fs/xfs/xfs.ko
lib/modules/3.5.3/kernel/lib/crc16.ko
lib/modules/3.5.3/kernel/lib/crc-t10dif.ko
lib/modules/3.5.3/modules.dep
lib/modules/3.5.3/modules.dep.bin
lib/modules/3.5.3/modules.pcimap
sbin/blkid
sbin/e2fsck
sbin/jfs_fsck
sbin/modprobe
sbin/v86d
usr/bin/pcimodules
usr/bin/unionfs-fuse
usr/bin/unzip

Then, create an empty directory, for example, /tmp/initrd and copy the files listed in the above-mentioned text file (called rd354.txt) to the new directory.

mkdir /tmp/initrd
cd /
tar cvhf - -T rd354.txt | (cd /tmp/initrd; tar xf -)

Then, create the necessary directory structure under /tmp/initrd:

cd /tmp/initrd
mkdir -p dev media mnt proc root sys tmp

Since I chose to use busybox, I need to create symbolic links to busybox in /bin. Busybox provides incomplete functionality for modprobe, mount, sh and unzip, so I removed their symbolic links.

cd /tmp/initrd/bin
for f in $(./busybox --list); do [ -e $f ] || ln -s busybox $f ; done
rm modprobe unzip

Then, create essential device nodes in /tmp/initrd/dev:

cd /tmp/initrd/dev
MAKEDEV std fb0 fd0 sda sdb scd
mknod console c 5 1
mknod fuse c 10 229
chmod 666 fuse

Now, create an init script. The contents of this init script is crucial for customization of the boot process. The following is the script I use. This script is capable of booting a live CD and running Linux within memory in addition to booting Linux from hard drives and USB flash.

#!/bin/dash

# Define a function to parse kernel command line options.
get_opt() {
  echo $@ | cut -d "=" -f 2
}

# Define a function to load drivers.
loadmod() {
  for i in $@ ; do
    for j in $(grep $i /tmp/pcimodules.txt); do
      modprobe $j
    done
  done
}

# Define a function to guess the partition type.
gpart() {
  for i in $(blkid | grep $1); do
    case $i in
      *\=*)
        eval $i
        ;;
      *)
        true
        ;;
    esac
  done
}

# Define a function for mounting the root partition.
mountr() {
  if [ $uuid ]; then
    if [ $# = 2 ]; then
      mount -r -U $uuid $2
    elif [ $# = 1 ]; then
      mount -r -U $uuid $1
    else mount -r -U $uuid /mnt
    fi
  elif [ $label ]; then
    if [ $# = 2 ]; then
      mount -r -L $label $2
    elif [ $# = 1 ]; then
      mount -r -L $label $1
    else mount -r -L $label /mnt
    fi
  else
    gpart $1
    case $TYPE in
      ext*)
        e2fsck -p $1
        [ $# = 2 ] && mount $1 $2 || mount $1 /mnt
        ;;
      jfs)
        jfs_fsck $1
        if [ $# = 2 ]; then
          mount -t jfs -o ro,iocharset=utf8 $1 $2
        else mount -t jfs -o ro,iocharset=utf8 $1 /mnt
        fi
        ;;
      vfat)
        if [ $# = 2 ]; then
          mount -t vfat -o ro,gid=100,dmask=2,fmask=113 $1 $2
        else mount -t vfat -o ro,gid=100,dmask=2,fmask=113 $1 /mnt
        fi
        ;;
      *)
        [ $# = 2 ] && mount -r $1 $2 || mount -r $1 /mnt
        ;;
    esac
  fi
}

# Create a union filesystem
union() {
  mount -t tmpfs none /opt/tmp
  modinfo unionfs > /dev/null 2>&1 &&
  mount -t unionfs -o dirs=/opt/tmp=rw:/opt=ro none /mnt ||
  ( mkdir /opt/tmp/.change
    modprobe fuse
    unionfs-fuse -o allow_other,use_ino,suid,dev,nonempty,kernel_cache \
     -o cow,chroot=/opt,max_files=32768 /tmp/.change=RW:/=RO /mnt )
}

# Mount proc and sysfs.
mount -t proc none /proc
mount -t sysfs none /sys

# Find the available PCI hardware
mount -t tmpfs none /tmp
pcimodules > /tmp/pcimodules.txt

# Populate /dev (Needs kernel >= 2.6.32)
mount -t devtmpfs none /dev
mkdir -m 755 /dev/pts
mount -t devpts -o gid=5,mode=620 none /dev/pts

# Set default values
boot=ata
root=/dev/sda6

# Find the root=, label=, uuid= and boot= values on kernel command line.
for i in $(cat /proc/cmdline); do
  case $i in
    root\=*)
        root=$(get_opt $i)
        case $root in
          /dev/cdr* | /dev/dvd* | /dev/sr* | /dev/scd*)
            boot=cdrom
            ;;
          0x200)
            root=/dev/fd0
            ;;
        esac
        ;;
    label\=* | uuid\=* | boot\=* | vmode\=* )
        eval $i
        ;;
    single)
        RUNLEVEL=single
        ;;
    nox)
        RUNLEVEL=2
        ;;
  esac
done

# Activate framebuffer display devices.
if [ $vmode ]; then
  if [ $boot = cdrom ]; then
    modprobe uvesafb scroll=ywrap mode_option=$vmode-16
  else for i in $(grep fb /tmp/pcimodules.txt); do
    case $i in
      atyfb)
        modprobe $i mode=$vmode-16
        ;;
      nvidiafb | rivafb)
        modprobe nvidiafb mode_option=$vmode bpp=16 hwcur=1
        ;;
      radeonfb | savagefb)
        modprobe $i mode_option=$vmode-16
        ;;
      sisfb)
        modprobe $i mode=$vmodex16 mem=12288 font=SUN12x22
        ;;
      viafb | vt8623fb)
        modprobe viafb viafb_mode=$vmode viafb_bpp=16
        ;;
      *)
        modprobe $i
        ;;
    esac
  done
    if grep -q i915 /tmp/pcimodules.txt; then true
    else [ -c /dev/fb0 ] || modprobe uvesafb scroll=ywrap mode_option=$vmode-16
    fi
  fi
fi

case $boot in
    cdrom)
        # Boot Linux from a live CD.
        loadmod ata_ ahci pdc_adma ^.hci-hcd
        modprobe usb-storage &&
        modprobe sr_mod &&
        sleep 7
        modprobe isofs
        mount -t iso9660 /dev/sr0 /media
        [ -d /media/isolinux -o -d /media/boot/isolinux ] || 
          mount -t iso9660 /dev/sr1 /media
        if [ -f /media/*.[Ss][Qq]* ]; then
          SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
          if [ $root = /dev/ram ]; then 
               echo "Please wait until the RAM disk is ready."
               dd if=$SQF of=/dev/ram1 bs=2048 &&
               mount -t squashfs /dev/ram1 /opt
          else modprobe loop
               mount -t squashfs -o loop $SQF /opt
          fi
        else
          mount --move /media /opt
        fi
        union
        ;;
    loop)
        # Boot Linux from an image file.
        loadmod ata_ ahci pdc_adma ^.hci-hcd
        modprobe usb-storage &&
        modprobe sd_mod &&
        sleep 7
        mountr $root /media
        modprobe loop
        if [ -f /media/*.[Ss][Qq]* ]; then
          SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
          mount -t squashfs -o loop $SQF /opt
        elif [ -f /media/*.[Ii][Ss][Oo] ]; then
          ISO=$(ls -t /media/*.[Ii][Ss][Oo] | head -n 1)
          modprobe isofs
          mount -t iso9660 -o loop $ISO /opt
        fi
        union
        ;;
    ram)
        # Boot Linux from ramdisk.
        loadmod ata_ ahci pdc_adma ^.hci-hcd
        modprobe usb-storage &&
        modprobe sd_mod &&
        sleep 7
        mountr $root /media
        echo "Please wait until the RAM disk is ready."
        if [ -f /media/*.[Ss][Qq]* ]; then
          SQF=$(ls -t /media/*.[Ss][Qq]* | head -n 1)
          dd if=$SQF of=/dev/ram1 &&
          mount -t squashfs /dev/ram1 /opt
        elif [ -f /media/*.[Ii][Ss][Oo] ]; then
          ISO=$(ls -t /media/*.[Ii][Ss][Oo] | head -n 1)
          dd if=$ISO of=/dev/ram1 bs=2048 &&
          modprobe isofs
          mount -t iso9660 /dev/ram1 /opt
        fi
        union
        ;;
    usb*)
        # Boot Linux from a USB drive.
        loadmod ^.hci-hcd
        modprobe usb-storage &&
        modprobe sd_mod &&
        sleep 7
        mountr $root
        ;;
    ata*)
        loadmod ata_ ahci pdc_adma &&
        modprobe sd_mod &&
        mountr $root
        ;;
esac

# Make sure that init exists and is executable.
if [ -x /mnt/sbin/init ]; then
  mount --move /dev /mnt/dev
  mount --move /proc /mnt/proc
  mount --move /sys /mnt/sys
  umount /tmp

# Start init from the root filesystem.
  cd /mnt
  [ -f /media/updates.zip ] && unzip -o /media/updates.zip
  case $boot in
    cdrom)
      [ $root = /dev/ram ] && umount /media
      [ $RUNLEVEL ] || RUNLEVEL=3
      ;;
    loop | ram)
      umount /media
      [ $RUNLEVEL ] || RUNLEVEL=3
      ;;
    *)
      [ $RUNLEVEL ] || RUNLEVEL=5
      ;;
  esac
  [ -d initrd ] && pivot_root . initrd
  exec chroot . /sbin/init $RUNLEVEL
fi

# Start a shell as a last resort.
echo "Error booting from the root filesystem. Starting a shell."
exec /bin/dash

Copy the init script to the initrd folder (Assuming its filename is init.sh).

cp init.sh /tmp/initrd/init
cp init.sh /tmp/initrd/etc
chmod 775 /tmp/initrd/init

Now, use cpio and lzma to create an initramfs file. This assumes that the kernel was compiled with CONFIG_RD_LZMA option enabled.

cd /tmp/initrd
find . | cpio -H newc -o | lzma -c > ../initram.lzm

A file named initram.lzm will be created. If you want to create an old-style initrd file instead, use the mkcramfs command. This assumes your kernel has built-in cramfs support (CONFIG_CRAMFS):

mkcramfs /tmp/initrd /boot/initrd.bin

Copy the kernel and the initramfs file to your boot directory (whereever it is). I use SYSLINUX which I installed at /dev/sda1 partition. Finally, edit the boot configuration file. The following is the contents of a sample syslinux.cfg:

LABEL debian
KERNEL 314.lnx
INITRD /initrd/initram.lzm
APPEND root=/dev/sda7

LABEL sid
KERNEL 314.lnx
INITRD /initrd/initram.lzm
APPEND boot=usb label=SID edd=off vmode=800x600

LABEL bfile
KERNEL 314.lnx
INITRD /initrd/initram.lzm
APPEND boot=loop root=/dev/sda1

LABEL ram
KERNEL 314.lnx
INITRD /initrd/initram.lzm
APPEND boot=ram ramdisk_size=524288 root=/dev/sda1