Re: [gentoo-user] Building an initramfs into the kernel

[Neil Bothwick]

On Wed, 26 Dec 2012 07:58:34 -0800, Mark Knecht wrote:

>    OK, it's the day after Christmas and this little kid wants to play
> with the new toys Uncle Neil gave us yesterday - a copy of his well
> worn setup file for building an initramfs into the kernel - a copy of
> which I place here:
>
> [QUOTE]
>
> This is the file I use on a system that has / on a LUKS filesystem on
> top of LVM. The format is documented in the kernel docs at
> Documentation/filesystems/ramfs-rootfs-initramfs.txt
>
>
> dir /bin 755 0 0
> file /bin/busybox /bin/busybox 755 0 0
> slink /bin/sh busybox 777 0 0
>
> dir /realroot 755 0 0
> dir /etc 755 0 0
> dir /proc 755 0 0
> dir /sys 755 0 0
>
> dir /sbin 755 0 0
> file /sbin/lvm.static /sbin/lvm.static 755 0 0
> #file /sbin/mdadm /sbin/mdadm 755 0 0
> file /sbin/cryptsetup /sbin/cryptsetup 755 0 0
>
> file /sbin/e2fsck /sbin/e2fsck 755 0 0
> dir /lib 755 0 0
> file /lib/libext2fs.so /usr/lib64/libext2fs.so 755 0 0
>
> dir /dev 755 0 0
> nod /dev/console 600 0 0 c 5 1
> nod /dev/null 666 0 0 c 1 3
> nod /dev/tty 666 0 0 c 5 0
> nod /dev/urandom 666 0 0 c 1 9
>
> file /init /usr/src/init.sh 755 0 0
>
> [/QUOTE]
>
>
>    OK, so reading through this it seems moderately straight forward. My
> reading:
>
> a) Create some directories
> b) Populate them with some executables
> c) Make some nodes
> d) Execute a script
>
>
> I do have a few questions:
>
> 1) dir /realroot 755 0 0
>
> Is this something required to make the machine boot? Or is it possibly
> a mount point in case of problems and just used inside the initramfs
> if trouble arises? Something else? Google didn't point me toward
> anything meaningful.

It's where init.sh mounts the real root filesystem before running
switchroot.

> 2) Contained executables, as I understand them, either need to be
> built with the static flag or you have to include all the libraries.
> Static seems simpler so (in my case) should I rebuild mdadm &
> e2fsprogs with +static? (I don't currently use lvm or any crypt stuff)

Yes.

> 3) My system uses RAID today. Is there any significant risk in
> rebuilding mdadm with static support, rebooting the existing kernel
> without an initramfs and then mdadm having trouble?

Static mdadm has given me no problems whatsoever, I guarantee that you
will have no fewer problems than me :)

> 4) What's in /usr/src/init.sh ? From the Gentoo initramfs wiki I find
> this as an example:

I use one based on the wiki example


#!/bin/busybox sh

rescue_shell() {
	busybox --install -s
	echo $1
	exec /bin/sh
	}

### Mount the /proc and /sys filesystems.
mount -t proc none /proc
grep -q initdebug </proc/cmdline && set -x && DEBUG=1
mount -t sysfs none /sys
mount -t devtmpfs none /dev

### Get root and init parameters from cmdline
REAL_ROOT="$(sed 's/.*root=\(\S*\).*/\1/' <proc/cmdline)"
if grep -q init= </proc/cmdline; then
	REAL_INIT="$(sed 's/.*init=\(\S*\).*/\1/' <proc/cmdline)"
else
	REAL_INIT="/sbin/init"
	fi

### Assemble RAID
[[ -f /sbin/mdadm ]] && mdadm --assemble --scan

### Initialise LVM
if [[ -f /sbin/lvm.static ]]; then
	lvm.static vgchange -a y
	lvm.static vgscan --mknodes
	[[ -n "$DEBUG" ]] && sleep 10
	fi

### Mount the root filesystem.
if [[ -f /sbin/cryptsetup ]] && cryptsetup isLuks $REAL_ROOT; then
	busybox echo -e "\e[0;32m\n=====================\nEncrypted root device\n=====================\n\e[m"
    cryptsetup luksOpen $REAL_ROOT cryptroot
    REAL_ROOT="/dev/mapper/cryptroot"
    [[ -n "$DEBUG" ]] && echo $REAL_ROOT && sleep 10
    fi
mount -o ro $REAL_ROOT /realroot || rescue_shell "Could not mount REAL_ROOT"
[[ -n "$DEBUG" ]] && df -h /realroot && sleep 10

### Mount /usr if separate
if grep -qE '^[^#].*\s\/usr\s' /realroot/etc/fstab; then
    #mount $(awk '/\s\/usr\s/ {print $1, "/realroot"$2, "-t", $3, "-o", $4 ",ro"}' /realroot/etc/fstab)
    mount $(awk '/^[^#].*\s\/usr\s/ {print $1, "/realroot"$2, "-t", $3, "-o", $4 ",ro"}' /realroot/etc/fstab)
    [[ -n "$DEBUG" ]] && df -h /realroot//usr && sleep 10
    fi

grep -q waitforkey </proc/cmdline && WAITFORKEY=1
grep -q rescue </proc/cmdline && rescue_shell "...because you asked for it."

### Clean up.
umount /dev
umount /sys
umount /proc

### Boot the real thing
[[ -n "$DEBUG" ]] && echo "Ready to switch" && sleep 10
[[ -n "$WAITFORKEY" ]] && busybox showkey -a
exec busybox switch_root /realroot $REAL_INIT || rescue_shell "Failed to switch_root"

Most of it is debug stuff that I haven't used since I first started using
it.


-- 
Neil Bothwick

For security reasons, all text in this mail is double-rot13 encrypted.