source: postlfs/filesystems/initramfs.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
  <!ENTITY % general-entities SYSTEM "../../general.ent">
  %general-entities;
]>

<sect1 id="initramfs">
  <?dbhtml filename="initramfs.html"?>


  <title>About initramfs</title>

  <para>
    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.
  </para>

  <para>
    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.
  </para>

  <para>
    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.
  </para>

  <para>
    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.
  </para>

  <sect2 id="initramfs-build">
    <title>Building an initramfs</title>

    <para>
      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
      <ulink url="https://www.linuxfromscratch.org/hints/read.html"> the LFS
      Hints</ulink> or <ulink
      url="https://fedoraproject.org/wiki/Dracut">dracut</ulink>.
    </para>

    <para>
      To install these scripts, run the following commands as the
      <systemitem class="username">root</systemitem> user:
    </para>

    <screen role="root"><userinput>cat &gt; /usr/sbin/mkinitramfs &lt;&lt; "EOF"
<literal>#!/bin/bash
# This file based in part on the mkinitramfs script for the LFS LiveCD
# written by Alexander E. Patrakov and Jeremy Huntwork.

copy()
{
  local file

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

  if [ -n "$file" ] ; then
    cp $file $WDIR/usr/$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" ] &amp;&amp; [ ! -d "/usr/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="$binfiles umount sed sleep ln rm uname"
binfiles="$binfiles readlink basename"

# Systemd installs udevadm in /bin. Other udev implementations have it in /sbin
if [ -x /usr/bin/udevadm ] ; then binfiles="$binfiles udevadm"; fi

sbinfiles="modprobe blkid switch_root"

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

# Add lvm if present (cannot be done with the others because it
# also needs dmsetup
if [ -x /usr/sbin/lvm ] ; then sbinfiles="$sbinfiles lvm dmsetup"; fi

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

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

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

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

# Copy the RAID configuration 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 "$KERNEL_VERSION" ] ; then
  if [ -x /usr/bin/kmod ] ; then
    binfiles="$binfiles kmod"
  else
    binfiles="$binfiles lsmod"
    sbinfiles="$sbinfiles insmod"
  fi
fi

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

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

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

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

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

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

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

# Install libraries
sort $unsorted | uniq | while read library ; do
# linux-vdso and linux-gate are pseudo libraries and do not correspond to a file
# libsystemd-shared is in /lib/systemd, so it is not found by copy, and
# it is copied below anyway
  if [[ "$library" == linux-vdso.so.1 ]] ||
     [[ "$library" == linux-gate.so.1 ]] ||
     [[ "$library" == libsystemd-shared* ]]; then
    continue
  fi

  copy $library lib
done

if [ -d /usr/lib/udev ]; then
  cp -a /usr/lib/udev $WDIR/usr/lib
fi
if [ -d /usr/lib/systemd ]; then
  cp -a /usr/lib/systemd $WDIR/usr/lib
fi
if [ -d /usr/lib/elogind ]; then
  cp -a /usr/lib/elogind $WDIR/usr/lib
fi

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

  cp /usr/lib/modules/$KERNEL_VERSION/modules.{builtin,order} \
            $WDIR/usr/lib/modules/$KERNEL_VERSION
  if [ -f /usr/lib/modules/$KERNEL_VERSION/modules.builtin.modinfo ]; then
    cp /usr/lib/modules/$KERNEL_VERSION/modules.builtin.modinfo \
            $WDIR/usr/lib/modules/$KERNEL_VERSION
  fi

  depmod -b $WDIR $KERNEL_VERSION
fi

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

# Prepare early loading of microcode if available
if ls /usr/lib/firmware/intel-ucode/* &gt;/dev/null 2&gt;&amp;1 ||
   ls /usr/lib/firmware/amd-ucode/*   &gt;/dev/null 2&gt;&amp;1; then

# first empty WDIR to reuse it
  rm -r $WDIR/*

  DSTDIR=$WDIR/kernel/x86/microcode
  mkdir -p $DSTDIR

  if [ -d /usr/lib/firmware/amd-ucode ]; then
    cat /usr/lib/firmware/amd-ucode/microcode_amd*.bin &gt; $DSTDIR/AuthenticAMD.bin
  fi

  if [ -d /usr/lib/firmware/intel-ucode ]; then
    cat /usr/lib/firmware/intel-ucode/* &gt; $DSTDIR/GenuineIntel.bin
  fi

  ( cd $WDIR; find . | cpio -o -H newc --quiet ) &gt; microcode.img
  cat microcode.img $INITRAMFS_FILE &gt; tmpfile
  mv tmpfile $INITRAMFS_FILE
  rm microcode.img
fi

# Remove the temporary directories and files
rm -rf $WDIR $unsorted
printf "done.\n"
</literal>
EOF

chmod 0755 /usr/sbin/mkinitramfs</userinput></screen>

    <screen role="root"><userinput>mkdir -p /usr/share/mkinitramfs &amp;&amp;
cat &gt; /usr/share/mkinitramfs/init.in &lt;&lt; "EOF"
<literal>#!/bin/sh

PATH=/usr/bin:/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 "$rootflags" ] &amp;&amp; rootflags="$rootflags,"
   rootflags="$rootflags$ro"

   case "$root" in
      /dev/*    ) device=$root ;;
      UUID=*    ) eval $root; device="/dev/disk/by-uuid/$UUID" ;;
      PARTUUID=*) eval $root; device="/dev/disk/by-partuuid/$PARTUUID" ;;
      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
}

do_try_resume()
{
   case "$resume" in
      UUID=* ) eval $resume; resume="/dev/disk/by-uuid/$UUID"  ;;
      LABEL=*) eval $resume; resume="/dev/disk/by-label/$LABEL" ;;
   esac

   if $noresume || ! [ -b "$resume" ]; then return; fi

   ls -lH "$resume" | ( read x x x x maj min x
       echo -n ${maj%,}:$min &gt; /sys/power/resume )
}

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

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 &lt; /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=}   ;;
    resume=*    ) resume=${param#resume=}         ;;
    noresume    ) noresume=true                   ;;
    ro          ) ro="ro"                         ;;
    rw          ) ro="rw"                         ;;
  esac
done

# udevd location depends on version
if [ -x /sbin/udevd ]; then
  UDEVD=/sbin/udevd
elif [ -x /lib/udev/udevd ]; then
  UDEVD=/lib/udev/udevd
elif [ -x /lib/systemd/systemd-udevd ]; then
  UDEVD=/lib/systemd/systemd-udevd
else
  echo "Cannot find udevd nor systemd-udevd"
  problem
fi

${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 &gt; /dev/null ; fi
if [ -n "$rootdelay"    ] ; then sleep "$rootdelay"              ; fi

do_try_resume # This function will not return if resuming from disk
do_mount_root

killall -w ${UDEVD##*/}

exec switch_root /.root "$init" "$@"
</literal>
EOF</userinput></screen>

  </sect2>

  <sect2 id="initramfs-install">
    <title>Using an initramfs</title>

    <bridgehead renderas="sect3">Required Runtime Dependency</bridgehead>

    <para role="required">
      <xref role="runtime" linkend="cpio"/>
    </para>

    <bridgehead renderas="sect3">Other Runtime Dependencies</bridgehead>

    <para role="optional">
      <xref role="runtime" linkend="lvm2"/> and/or
      <xref role="runtime" linkend="mdadm"/> must be installed before
      generating the initramfs, if the system partition uses them.
    </para>



    <para>
      To build an initramfs, run the following as the  <systemitem
      class="username">root</systemitem> user:
    </para>

    <screen role="nodump"><userinput>mkinitramfs [KERNEL VERSION]</userinput></screen>

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

    <para>
      If early loading of microcode is needed (see <xref
      linkend="cpu-microcode"/>), you can install the appropriate blob or
      container in <filename class="directory">/lib/firmware</filename>.
      It will be automatically added to the initrd when running
      <command>mkinitramfs</command>.
    </para>

    <para>
      After generating the initrd, copy it to the <filename
      class='directory'>/boot</filename> directory.
    </para>

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

     <screen role="nodump"><userinput># 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
}</userinput></screen>

     <screen role="nodump"><userinput># 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
}</userinput></screen>

     <screen role="nodump"><userinput># 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
}</userinput></screen>

    <para>
      Finally, reboot the system and select the desired system.
    </para>

  </sect2>

</sect1>