HOWTO Custom Initramfs From Scratch
From Gentoo Linux Wiki
This article will show you how to construct a custom initramfs from the ground up, to do whatever you want. This article will focus on creating an initramfs for lvm2 root.
Warning: While I have done this quite a number of times, this article is currently written completely from memory. Errors and omissions are likely, proceed at your own risk. ALWAYS have another way to boot into your system available - if ash doesn't like the script, things can get tricky.
Note: Most of this process should work fine in a user's home directory except mounting stuff into the chroot and copying the new kernel to /boot
Contents |
[edit] Working Directories
First, we set up some working directories:
| Code: Set up working directories |
cd /usr/src
mkdir -p initramfs/{usr/portage,image/{dev,proc,newroot}}
|
[edit] Chroot
Then, we download and unpack a stage3 to initramfs-chroot:
Note: This is to keep uclibc out of your main system
| Code: Create minimal gentoo in chroot |
cd initramfs wget http://your-favourite-mirror/pub/gentoo/releases/x86/current/stage3-i686-2007.0.tar.bz2 tar xjvpf stage3-i686-2007.0.tar.bz2 |
We pull in some things from our host to keep things easy:
Warning: PAY ATTENTION to where the slashes are - half these directories are inside the chroot, half outside. You do not want to get them mixed up
| Code: Link stuff from host to save space and time |
mount -o bind /dev dev/ mount -o bind /dev/pts dev/pts mount -t sysfs sysfs-chroot sys mount -t proc proc-chroot proc mount -o bind /usr/portage usr/portage cp /etc/resolv.conf etc/ |
Then we chroot:
| Code: Chroot |
chroot . bash env-update source /etc/profile |
[edit] Making Uclibc
We make ourselves a minimal c library, in this case uclibc:
Note: TODO: pare uclibc down to the minimum we need to save space
| Code: Emerge uclibc |
emerge -avt uclibc |
[edit] Static busybox and LVM2
We make ourselves a static busybox and lvm2:
Warning: Placing USE flags on the command line like this is a BAD IDEA - however, in this specific case it does exactly what we want.
Note: TODO: pare busybox down to the minimum we need to save space - just ash, mdev, mount, umount and switch_root
Note: The ROOT directive here tells emerge to put the resultant files somewhere other than /
| Code: Emerge static busybox and lvm2 |
USE="static" ROOT="/image" emerge -avt busybox lvm2 |
[edit] Finishing the Image
We remove all the extraneous cruft that got installed, and pull in a few things that were missing:
Warning: PAY ATTENTION to where the slashes are - we are doing stuff to the image pointed to by ROOT directive above, NOT your chroot or main filesystem
| Code: Cleanup |
cd /image
rm -r usr/{doc,share,man,info}
cp -av /dev/{console,null} dev/
|
[edit] Boot Script
We write a script that will set up our lvm and then hop into the new root:
Note: TODO: test and refine this script - It's presented here purely from memory
| Code: Boot Script |
cat <<END > /image/linuxrc
#!/bin/ash
fail() {
echo
echo "hrm, something is broke, have a shell"
exec ash
}
echo -n Mounting /proc...
mount -t proc proc /proc || fail
echo " OK"
echo -n Mounting tmpfs over /dev...
mount -t tmpfs dev-tmpfs /dev
echo " OK"
echo -n Starting Mdev...
mdev -s
echo " OK"
echo -n Starting LVM2
vgchange -ay || fail
echo " OK"
echo -n Creating Nodes
vgmknodes || fail
echo " OK"
echo -n Mounting rootfs...
mount -t YOUR-ROOTFS-TYPE -o ro,OTHER-OPTIONS /dev/YOUR-VG/YOUR-ROOT-LV /newroot
echo " OK"
echo Umounting intermediaries...
umount /proc
umount /dev
echo " OK"
echo Switching root, hold on!
switch_root /newroot /sbin/init
END
|
[edit] Kernel Setup
We set our kernel up to use the new image:
| Linux Kernel Configuration: Initramfs Options |
General setup ---> [*] Initial RAM filesystem and RAM disk (initramfs/initrd) support (/usr/src/initramfs/image) Initramfs source file(s) |
Now make and install your kernel as usual:
Warning: This is how I do my kernels. Your process is probably different
Note: TODO: figure out how to make this wrap without the wiki clobbering it
| Code: Install new kernel |
cd /usr/src/linux make O=../linux-build bzImage modules modules_install && \ mount /boot && \ cp ../linux-build/arch/i386/boot/bzImage /boot/kernel-$(( head -n5 Makefile | sed 's/ //g'; echo 'echo $VERSION.$PATCHLEVEL.$SUBLEVEL$EXTRAVERSION'; ) | bash) && \ mv /boot/new /boot/current && \ ln -s kernel-$(( head -n5 Makefile | sed 's/ //g'; echo 'echo $VERSION.$PATCHLEVEL.$SUBLEVEL$EXTRAVERSION'; ) | bash) /boot/new && \ umount /boot |
[edit] Final Step
Reboot and see how you went!
[edit] See Also
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