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.
- Boot from a live CD
- Boot from a read-only filesystem image, compressed with SquashFS
- Copy the filesystem image to a RAM disk and run Linux entirely on memory
- Boot Linux from a USB flash drive or a Firewire disk
- Boot Linux from a local disk partition
- Boot Linux from a network drive
- Run a rescue shell without booting Linux
- 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
