About initramfs

The only purpose of an initramfs is to mount the root filesystem. The initramfs is a complete set of directories that you would find on a normal root filesystem. It is bundled into a single cpio archive and compressed with one of several compression algorithms.

At boot time, the boot loader loads the kernel and the initramfs image into memory and starts the kernel. The kernel checks for the presence of the initramfs and, if found, mounts it as / and runs /init. The init program is typically a shell script. Note that the boot process takes longer, possibly significantly longer, if an initramfs is used.

For most distributions, kernel modules are the biggest reason to have an initramfs. In a general distribution, there are many unknowns such as file system types and disk layouts. In a way, this is the opposite of LFS where the system capabilities and layout are known and a custom kernel is normally built. In this situation, an initramfs is rarely needed.

There are only four primary reasons to have an initramfs in the LFS environment: loading the rootfs from a network, loading it from an LVM logical volume, having an encrypted rootfs where a password is required, or for the convenience of specifying the rootfs as a LABEL or UUID. Anything else usually means that the kernel was not configured properly.

Building an initramfs

If you do decide to build an initramfs, the following scripts will provide a basis to do it. The scripts will allow specifying a rootfs via partition UUID or partition LABEL or a rootfs on an LVM logical volume. They do not support an encrypted root file system or mounting the rootfs over a network card. For a more complete capability see the LFS Hints or dracut.

To install these scripts, run the following commands as the root user:

cat > /sbin/mkinitramfs << "EOF"
#!/bin/bash
# This file based in part on the mkinitrafms script for the LFS LiveCD
# written by Alexander E. Patrakov and Jeremy Huntwork.

copy()
{
  local file

  if [ "$2" == "lib" ]; then
    file=$(PATH=/lib:/usr/lib type -p $1)
  else
    file=$(type -p $1)
  fi

  if [ -n $file ] ; then
    cp $file $WDIR/$2
  else
    echo "Missing required file: $1 for directory $2"
    rm -rf $WDIR
    exit 1
  fi
}

if [ -z $1 ] ; then
  INITRAMFS_FILE=initrd.img-no-kmods
else
  KERNEL_VERSION=$1
  INITRAMFS_FILE=initrd.img-$KERNEL_VERSION
fi

if [ -n "$KERNEL_VERSION" ] && [ ! -d "/lib/modules/$1" ] ; then
  echo "No modules directory named $1"
  exit 1
fi

printf "Creating $INITRAMFS_FILE... "

binfiles="sh cat cp dd killall ls mkdir mknod mount "
binfiles="$binfiles umount sed sleep ln rm uname"

sbinfiles="udevadm modprobe blkid switch_root"

#Optional files and locations
for f in mdadm udevd; do
  if [ -x /sbin/$f ] ; then sbinfiles="$sbinfiles $f"; fi
done

unsorted=$(mktemp /tmp/unsorted.XXXXXXXXXX)

DATADIR=/usr/share/mkinitramfs
INITIN=init.in

# Create a temporrary working directory
WDIR=$(mktemp -d /tmp/initrd-work.XXXXXXXXXX)

# Create base directory structure
mkdir -p $WDIR/{bin,dev,lib/firmware,run,sbin,sys,proc}
mkdir -p $WDIR/etc/{modprobe.d,udev/rules.d}
touch $WDIR/etc/modprobe.d/modprobe.conf
ln -s lib $WDIR/lib64

# Create necessary device nodes
mknod -m 640 $WDIR/dev/console c 5 1
mknod -m 664 $WDIR/dev/null    c 1 3

# Install the udev configuration files
if [ -f /etc/udev/udev.conf ]; then
  cp /etc/udev/udev.conf $WDIR/etc/udev/udev.conf
fi

for file in $(find /etc/udev/rules.d/ -type f) ; do
  cp $file $WDIR/etc/udev/rules.d
done

# Install any firmware present
cp -a /lib/firmware $WDIR/lib

# Copy the RAID configureation file if present
if [ -f /etc/mdadm.conf ] ; then
  cp /etc/mdadm.conf $WDIR/etc
fi

# Install the init file
install -m0755 $DATADIR/$INITIN $WDIR/init

if [  -n "$KERNEL_VERSION" ] ; then
  if [ -x /bin/kmod ] ; then
    binfiles="$binfiles kmod"
  else
    binfiles="$binfiles lsmod"
    sbinfiles="$sbinfiles insmod"
  fi
fi

# Install basic binaries
for f in $binfiles ; do
  ldd /bin/$f | sed "s/\t//" | cut -d " " -f1 >> $unsorted
  copy $f bin
done

# Add lvm if present
if [ -x /sbin/lvm ] ; then sbinfiles="$sbinfiles lvm dmsetup"; fi

for f in $sbinfiles ; do
  ldd /sbin/$f | sed "s/\t//" | cut -d " " -f1 >> $unsorted
  copy $f sbin
done

# Add udevd libraries if not in /sbin
if [ -x /lib/udev/udevd ] ; then
  ldd /lib/udev/udevd | sed "s/\t//" | cut -d " " -f1 >> $unsorted
fi

# Add module symlinks if appropriate
if [ -n "$KERNEL_VERSION" ] && [ -x /bin/kmod ] ; then
  ln -s kmod $WDIR/bin/lsmod
  ln -s kmod $WDIR/bin/insmod
fi

# Add lvm symlinks if appropriate
# Also copy the lvm.conf file
if  [ -x /sbin/lvm ] ; then
  ln -s lvm $WDIR/sbin/lvchange
  ln -s lvm $WDIR/sbin/lvrename
  ln -s lvm $WDIR/sbin/lvextend
  ln -s lvm $WDIR/sbin/lvcreate
  ln -s lvm $WDIR/sbin/lvdisplay
  ln -s lvm $WDIR/sbin/lvscan

  ln -s lvm $WDIR/sbin/pvchange
  ln -s lvm $WDIR/sbin/pvck
  ln -s lvm $WDIR/sbin/pvcreate
  ln -s lvm $WDIR/sbin/pvdisplay
  ln -s lvm $WDIR/sbin/pvscan

  ln -s lvm $WDIR/sbin/vgchange
  ln -s lvm $WDIR/sbin/vgcreate
  ln -s lvm $WDIR/sbin/vgscan
  ln -s lvm $WDIR/sbin/vgrename
  ln -s lvm $WDIR/sbin/vgck
  # Conf file(s)
  cp -a /etc/lvm $WDIR/etc
fi

# Install libraries
sort $unsorted | uniq | while read library ; do
  if [ "$library" == "linux-vdso.so.1" ] ||
     [ "$library" == "linux-gate.so.1" ]; then
    continue
  fi

  copy $library lib
done

cp -a /lib/udev $WDIR/lib

# Install the kernel modules if requested
if [ -n "$KERNEL_VERSION" ]; then
  find                                                                        \
     /lib/modules/$KERNEL_VERSION/kernel/{crypto,fs,lib}                      \
     /lib/modules/$KERNEL_VERSION/kernel/drivers/{block,ata,md,firewire}      \
     /lib/modules/$KERNEL_VERSION/kernel/drivers/{scsi,message,pcmcia,virtio} \
     /lib/modules/$KERNEL_VERSION/kernel/drivers/usb/{host,storage}           \
     -type f 2> /dev/null | cpio --make-directories -p --quiet $WDIR

  cp /lib/modules/$KERNEL_VERSION/modules.{builtin,order}                     \
            $WDIR/lib/modules/$KERNEL_VERSION

  depmod -b $WDIR $KERNEL_VERSION
fi

( cd $WDIR ; find . | cpio -o -H newc --quiet | gzip -9 ) > $INITRAMFS_FILE

# Remove the temporary directory and file
rm -rf $WDIR $unsorted
printf "done.\n"

EOF

chmod 0755 /sbin/mkinitramfs
mkdir -p /usr/share/mkinitramfs &&
cat > /usr/share/mkinitramfs/init.in << "EOF"
#!/bin/sh

PATH=/bin:/usr/bin:/sbin:/usr/sbin
export PATH

problem()
{
   printf "Encountered a problem!\n\nDropping you to a shell.\n\n"
   sh
}

no_device()
{
   printf "The device %s, which is supposed to contain the\n" $1
   printf "root file system, does not exist.\n"
   printf "Please fix this problem and exit this shell.\n\n"
}

no_mount()
{
   printf "Could not mount device %s\n" $1
   printf "Sleeping forever. Please reboot and fix the kernel command line.\n\n"
   printf "Maybe the device is formatted with an unsupported file system?\n\n"
   printf "Or maybe filesystem type autodetection went wrong, in which case\n"
   printf "you should add the rootfstype=... parameter to the kernel command line.\n\n"
   printf "Available partitions:\n"
}

do_mount_root()
{
   mkdir /.root
   [ -n "$rootflags" ] && rootflags="$rootflags,"
   rootflags="$rootflags$ro"

   case "$root" in
      /dev/* ) device=$root ;;
      UUID=* ) eval $root; device="/dev/disk/by-uuid/$UUID"  ;;
      LABEL=*) eval $root; device="/dev/disk/by-label/$LABEL" ;;
      ""     ) echo "No root device specified." ; problem    ;;
   esac

   while [ ! -b "$device" ] ; do
       no_device $device
       problem
   done

   if ! mount -n -t "$rootfstype" -o "$rootflags" "$device" /.root ; then
       no_mount $device
       cat /proc/partitions
       while true ; do sleep 10000 ; done
   else
       echo "Successfully mounted device $root"
   fi
}

init=/sbin/init
root=
rootdelay=
rootfstype=auto
ro="ro"
rootflags=
device=

mount -n -t devtmpfs devtmpfs /dev
mount -n -t proc     proc     /proc
mount -n -t sysfs    sysfs    /sys
mount -n -t tmpfs    tmpfs    /run

read -r cmdline < /proc/cmdline

for param in $cmdline ; do
  case $param in
    init=*      ) init=${param#init=}             ;;
    root=*      ) root=${param#root=}             ;;
    rootdelay=* ) rootdelay=${param#rootdelay=}   ;;
    rootfstype=*) rootfstype=${param#rootfstype=} ;;
    rootflags=* ) rootflags=${param#rootflags=}   ;;
    ro          ) ro="ro"                         ;;
    rw          ) ro="rw"                         ;;
  esac
done

# udevd location depends on version
if [ -x /sbin/udevd ]; then
  UDEV_PATH=/sbin
else
  UDEV_PATH=/lib/udev
fi

${UDEV_PATH}/udevd --daemon --resolve-names=never
udevadm trigger
udevadm settle

if [ -f /etc/mdadm.conf ] ; then mdadm -As                       ; fi
if [ -x /sbin/vgchange  ] ; then /sbin/vgchange -a y > /dev/null ; fi
if [ -n "$rootdelay"    ] ; then sleep "$rootdelay"              ; fi

do_mount_root

killall -w ${UDEV_PATH}/udevd

exec switch_root /.root "$init" "$@"

EOF

Using an initramfs

Required Runtime Dependency

cpio-2.11

User Notes: http://wiki.linuxfromscratch.org/blfs/wiki/initramfs

To build an initramfs, run the following as the root user:

mkinitramfs [KERNEL VERSION]

The optional argument is the directory where the appropriate kernel modules are located. This must be a subdirectory of /lib/modules. If no modules are specified, then the initramfs is named initrd.img-no-kmods. If a kernel version is specified, the initrd is named initrd.img-$KERNEL_VERSION and is only appropriate for the specific kernel specified. The output file will be placed in the current directory.

After generating the initrd, copy it to the /boot directory.

Now edit /boot/grub/grub.cfg and add a new menuentry. Below are several examples.

# Generic initramfs and root fs identified by UUID
menuentry "LFS Dev (LFS-7.0-Feb14) initrd, Linux 3.0.4"
{
  linux  /vmlinuz-3.0.4-lfs-20120214 root=UUID=54b934a9-302d-415e-ac11-4988408eb0a8 ro
  initrd /initrd.img-no-kmods
}
# Generic initramfs and root fs on LVM partition
menuentry "LFS Dev (LFS-7.0-Feb18) initrd lvm, Linux 3.0.4"
{
  linux  /vmlinuz-3.0.4-lfs-20120218 root=/dev/mapper/myroot ro
  initrd /initrd.img-no-kmods
}
# Specific initramfs and root fs identified by LABEL
menuentry "LFS Dev (LFS-7.1-Feb20) initrd label, Linux 3.2.6"
{
  linux  /vmlinuz-3.2.6-lfs71-120220 root=LABEL=lfs71 ro
  initrd /initrd.img-3.2.6-lfs71-120220
}

Finally, reboot the system and select the desired system.

Last updated on 2013-11-03 00:12:46 +0000