Custom rescue/ghost initramfs

Introduction

Initramfs is just the newer way for Linux to build an initrd. It uses cpio instead of a simple ext2 filesystem image. Here's how to build an LVM2 capable initramfs. Two flavors are described :

- Busybox and Slackware init

- Busybox and Redhat init

Note. the Busybox preparation may be done as user but you'll have to be root to extract the device files from the cpio archive. So might be convenient to stay as root all along.

See http://pbraun.nethence.com/sysutils_linux/busybox.html on how to prepare a Busybox system.

Busybox and Slackware init

Note. you need to be root to extract the device files from the cpio archive

Make sure you've got XZ (http://tukaani.org/xz/) first. It's available on recent Slackware distros. It's also available as package with Redhat/CentOS,

rpm -q xz

Prerequesties,

wget ftp://ftp.slackware.at/slackware-13.1/isolinux/initrd.img

mv -f initrd.img initrd.dist

wget ftp://ftp.slackware.at/slackware-13.1/slackware/a/lvm2-2.02.64-i486-1.txz

xz -d lvm2-2.02.64-i486-1.txz

don't forget initrd's relative kernel image, it might help to build the PXE or memstick later on !

wget ftp://ftp.slackware.at/slackware-13.1/kernels/hugesmp.s/bzImage

Extract a sample initramfs,

rm -rf sample/

mkdir sample/

cd sample/

gzip -dc ../initrd.dist | cpio -id

cd ..

note. "-id" or "--extract --make-directories"

Prepare the rescue initramfs,

rm -rf tmp/

mkdir -p tmp/etc/ tmp/lib/ tmp/_keymap/

cp -a sample/dev/ tmp/

cp -a sample/lib/modules/ tmp/lib/

cp -a sample/etc/group tmp/etc/

cp -a sample/etc/passwd tmp/etc/

tar xzf sample/etc/keymaps.tar.gz -C tmp/_keymap/

sed 's/mknod --mode=600/mknod/' sample/dev/devmap_mknod.sh > tmp/dev/devmap_mknod.sh

chmod +x tmp/dev/devmap_mknod.sh

Eventually enable PCMCIA,

#mkdir -p tmp/etc/rc.d

#cp -a sample/etc/rc.d/rc.pcmcia tmp/etc/rc.d

Note. for rc.pcmcia to work you also need pcmciautils package

Enable the network connectivity,

cp -a sample/scripts/network.sh tmp/sbin/

chmod +x tmp/sbin/network.sh

Make it LVM2 capable,

rm -rf addons/

mkdir addons/

mkdir -p tmp/sbin/

tar xf lvm2-2.02.64-i486-1.tar -C addons/

cp addons/sbin/lvm.static tmp/sbin/lvm

Note. this isn't needed,

#wget ftp://ftp.slackware.at/slackware-12.2/slackware/a/device-mapper-1.02.28-i486-1.tgz

#tar xzf device-mapper-1.02.28-i486-1.tgz -C addons

Note. and there's no udev.static binary provided by Slackware.

Create the initialization script,

cd tmp/

cat > init <<EOF9

#!/bin/sh

/bin/rm -f /etc/mtab* /etc/nologin 1> /dev/null 2> /dev/null

mkdir -p /proc /sys

mount -t proc none /proc

mount -t sysfs none /sys

/dev/makedevs.sh

/dev/devmap_mknod.sh

lvm vgscan --mknodes --ignorelockingfailure

lvm vgchange -ay --ignorelockingfailure

#blkid (floppy error)

loadkmap < /_keymap/fr-latin1.bmap

echo

echo "run network.sh to enable network cards"

echo

exec /linuxrc

EOF9

chmod 700 init

cd ..

Note. change the keymap accordingly

Configure linuxrc,

cd tmp/

mkdir -p etc/init.d/

touch etc/init.d/rcS

chmod +x etc/init.d/rcS

cd ..

Tune your environment,

cd tmp/

cat > .profile <<EOF9

alias ll='ls -alkF'

EOF9

cd ..

Finish up

Generate the initramfs and clean up,

cd tmp/

find . | cpio -co | gzip -9 > ../initrd.img

cd ..

#mkdir -p /tftpboot/rescue32/

#cp bzImage /tftpboot/rescue32/

#cp initrd.img /tftpboot/rescue32/

Busybox and Redhat init

Add udev support and userspace LVM2 tools,

cp /sbin/udevd.static sbin/udevd

cp /sbin/lvm.static sbin/lvm

#cp /sbin/dmsetup.static sbin/dmsetup

#cp /sbin/kpartx.static sbin/kpartx

Note. not using dmsetup, "vgscan --mknodes" does the job

Note. not using "kpartx", udev does the job

Note. the "lvm" binary itself is enought, no need for "/etc/lvm/" conf & subdirs.

Add the missing modules,

tmp=`ls -1t /lib/modules/ | grep xen$ | head -1`

cp -f \

$tmp/kernel/fs/jbd/jbd.ko \

$tmp/kernel/fs/ext3/ext3.ko \

$tmp/kernel/drivers/scsi/scsi_mod.ko \

$tmp/kernel/drivers/scsi/sd_mod.ko \

$tmp/kernel/drivers/md/dm-mod.ko \

$tmp/kernel/drivers/md/dm-log.ko \

$tmp/kernel/drivers/md/dm-mem-cache.ko \

$tmp/kernel/drivers/md/dm-region_hash.ko \

$tmp/kernel/drivers/md/dm-message.ko \

lib

unset tmp

chmod 600 lib/*.ko

Note. to make a XEN guest capable initramfs, add,

#$tmp/kernel/drivers/xen/blkfront/xenblk.ko \

Create the "init" file,

cat > init <<EOF9

#!/bin/sh

mkdir -p /proc /sys /dev/pts /dev/shm /dev/mapper

mount -t proc none /proc

mount -t sysfs none /sys

mount -t devpts none /dev/pts

mknod /dev/null c 1 3

mknod /dev/zero c 1 5

mknod /dev/systty c 4 0

mknod /dev/tty c 5 0

# mknod /dev/console c 5 1

mknod /dev/ptmx c 5 2

mknod /dev/rtc c 10 135

mknod /dev/tty0 c 4 0

mknod /dev/tty1 c 4 1

mknod /dev/tty2 c 4 2

mknod /dev/tty3 c 4 3

mknod /dev/tty4 c 4 4

mknod /dev/tty5 c 4 5

mknod /dev/tty6 c 4 6

mknod /dev/tty7 c 4 7

mknod /dev/tty8 c 4 8

mknod /dev/tty9 c 4 9

mknod /dev/tty10 c 4 10

mknod /dev/tty11 c 4 11

mknod /dev/tty12 c 4 12

mknod /dev/ttyS0 c 4 64

mknod /dev/ttyS1 c 4 65

mknod /dev/ttyS2 c 4 66

mknod /dev/ttyS3 c 4 67

udevd -d

insmod /lib/xenblk.ko

insmod /lib/jbd.ko

insmod /lib/ext3.ko

insmod /lib/scsi_mod.ko

insmod /lib/sd_mod.ko

insmod /lib/dm-mod.ko

insmod /lib/dm-log.ko

insmod /lib/dm-mem-cache.ko

insmod /lib/dm-region_hash.ko

insmod /lib/dm-message.ko

lvm vgscan --mknodes --ignorelockingfailure

#lvm vgchange -ay --ignorelockingfailure

blkid

exec /linuxrc

EOF9

chmod 700 init

Note. "/dev/console" is created by the kernel.

Footnotes

We could also add KSH93 (http://www.research.att.com/~gsf/cgi-bin/download.cgi?action=list&name=ksh) into "bin/",

cp ../ksh.2009-05-05.linux.i386 bin/ksh

and install its dependencies (nonsegneg when applicable),

ldd ksh

cp /lib/libdl.so.2 lib

cp /lib/i686/nosegneg/libm.so.6 lib

cp /lib/i686/nosegneg/libc.so.6 lib

cp /lib/ld-linux.so.2 lib

Note. "linux-gate.so.1" is provided by the kernel.

Using the nonsegneg (when applicable) libraries should even prevent from disabling TLS,

#echo 'hwcap 0 nosegneg' >> etc/ld.so.conf

RHEL/XEN footnotes

Use the bare domU kernel,

cp `ls -1t /boot/vmlinuz-*xen | head -1` vmlinuz

note. the RHEL XEN kernel has both, dom0 and domU support.

and temporarly change the guest configuration,

...

kernel = "/data/guest1/vmlinuz"

ramdisk = "/data/guest1/initrd.img"

#bootloader = "/usr/bin/pygrub"

...

A simple way to add the "xenblk" module to the default initrd,

mkinitrd --fstab=/data/guest3/loop/etc/fstab \

--omit-lvm-modules --omit-raid-modules --omit-scsi-modules \

--with=xenblk --with=jbd --with=ext3 \

initrd.img 2.6.18-128.1.10.el5xen

We could also add USB support into the initramfs, adding this to its init,

echo "Loading ehci-hcd.ko module"

insmod /lib/ehci-hcd.ko

echo "Loading ohci-hcd.ko module"

insmod /lib/ohci-hcd.ko

echo "Loading uhci-hcd.ko module"

insmod /lib/uhci-hcd.ko

mount -t usbfs /proc/bus/usb /proc/bus/usb

Without udev, we would have to make the block device nodes. As for XEN tap:aio devices,

mknod /dev/xvda b 202 0

mknod /dev/xvdb b 202 16

mknod /dev/xvdc b 202 32

mknod /dev/xvdd b 202 48

mknod /dev/xvde b 202 64

mknod /dev/xvdf b 202 80

mknod /dev/xvdg b 202 96

mknod /dev/xvdh b 202 112

and make the partition devices nodes with "partprobe" or "kpartx",

kpartx -a /dev/xvda

References

Busybox : http://www.busybox.net/

Info on the 'in memory' filesystems : http://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt

Ash (Almquist Shell) Variants : http://www.in-ulm.de/~mascheck/various/ash/

ttylinux downloads : http://minimalinux.org/ttylinux/download.html

Buildroot: making Embedded Linux easy : http://buildroot.uclibc.org/

XEN + LVM : http://frlinux.net/?section=virtualisation&article=207

BusyBox simplifies embedded Linux systems : http://www.ibm.com/developerworks/library/l-busybox/

Coreutils - busybox : http://siso.sourceforge.net/kernel-busybox.html