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.
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" "$@" EOF
User Notes: http://wiki.linuxfromscratch.org/blfs/wiki/initramfs
To build an initramfs, run the following as the root
user:
mkinitramfs [KERNEL VERSION]
The optional argument is the directory where the appropriate kernel
modules are located. This must be a subdirectory of /lib/modules
. If no modules are specified, then
the initramfs is named initrd.img-no-kmods. If a kernel version
is specified, the initrd is named initrd.img-$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 2013-11-02 17:12:46 -0700