Early Userspace Mounting

This article will detail [[Article description::how to build a custom minimal initramfs that checks the /usr filesystem and pre-mounts /usr.]] This has become necessary for affected configurations because of various changes in udev (see bug #364235).

In this article we'll be working with the following:

• Busybox
• An initramfs content list
• The gen_init_cpio and gen_initramfs_list.sh utilities, provided by the kernel itself.

The initramfs also contains the required libraries and binaries to run an ext4 fsck. Most of the code to run the fsck is coming from the /etc/init.d/fsck script.

If you are using any other filesystem than ext4, you will need to add the required binaries / libraries into the initramfs list.

Basically, the init script is doing following actions:

1. Mounts the root partition on /mnt/root as read-only.
2. Symlinks the fstab from the root partition to the initramfs environment.
3. Checks the filesystem of our /usr device using the embedded /sbin/fsck binary.
4. Mounts /usr, then moves it to /mnt/root/usr using the --move mount parameter.
5. Switches to real root and executes init.

The article also assumes we are working in /usr/src/initramfs, so for the sake of ease, begin with creating this directory.

Requirements

The most important package here is sys-apps/busybox as it provides utilities suitable for an initramfs. It is also critical that you emerge it with static USE flag enabled:

The kernel sources provide the gen_init_cpio and gen_initramfs_list.sh utilities. The gen_init_cpio utility does not come prepackaged and needs to be built:

Make sure that these two are executable:

Make sure that your running kernel is built with the devtmpfs option enabled. It is required by the init script below and udev:

Device Drivers  --->
    Generic Driver Options  --->
        [*] Maintain a devtmpfs filesystem to mount at /dev

Next up is the initramfs_list file which will tell gen_initramfs_list.sh how to construct the initramfs:

# directory structure
dir /proc       755 0 0
dir /usr        755 0 0
dir /bin        755 0 0
dir /sys        755 0 0
dir /var        755 0 0
dir /lib        755 0 0
dir /sbin       755 0 0
#dir /lib64      755 0 0
#dir /lib32      755 0 0
dir /mnt        755 0 0
dir /mnt/root   755 0 0
dir /etc        755 0 0
dir /root       700 0 0
dir /dev        755 0 0

# busybox
file /bin/busybox /bin/busybox 755 0 0

# libraries required by /sbin/fsck.ext4 and /sbin/fsck
file    /lib/ld-linux.so.2      /lib/ld-linux.so.2                  755 0 0
file    /lib/libext2fs.so.2     /lib/libext2fs.so.2                 755 0 0
file    /lib/libcom_err.so.2    /lib/libcom_err.so.2                755 0 0
file    /lib/libpthread.so.0    /lib/libpthread.so.0                755 0 0
file    /lib/libblkid.so.1      /lib/libblkid.so.1                  755 0 0
file    /lib/libmount.so.1      /lib/libmount.so.1                  755 0 0
file    /lib/libuuid.so.1       /lib/libuuid.so.1                   755 0 0
file    /lib/libe2p.so.2        /lib/libe2p.so.2                    755 0 0
file    /lib/libc.so.6          /lib/libc.so.6                      755 0 0
file    /lib/librt.so.1         /lib/librt.so.1                     755 0 0
file    /lib/libdl.so.2         /lib/libdl.so.2                     755 0 0

file    /sbin/fsck              /sbin/fsck                          755 0 0
file    /sbin/fsck.ext4         /sbin/fsck.ext4                     755 0 0

# our init script
file    /init                   /usr/src/initramfs/init             755 0 0

# libraries required by /sbin/fsck.ext4 and /sbin/fsck
file    /lib64/ld-linux-x86-64.so.2      /lib64/ld-linux-x86-64.so.2                  755 0 0
file    /lib64/libext2fs.so.2   /lib64/libext2fs.so.2               755 0 0

Copy and save the contents of the above to /usr/src/initramfs/initramfs_list after adjusting for your architecutre.

Last up is the actual init file which will execute the initramfs:

<syntaxhighlight lang="bash">#!/bin/busybox sh

rescue_shell() {
    echo "$@"
    echo "Something went wrong. Dropping you to a shell."
    busybox --install -s
    exec /bin/sh
}

uuidlabel_root() {
    for cmd in $(cat /proc/cmdline) ; do
        case $cmd in
        root=*)
            type=$(echo $cmd | cut -d= -f2)
            echo "Mounting rootfs"
            if [ $type == "LABEL" ] || [ $type == "UUID" ] ; then
                uuid=$(echo $cmd | cut -d= -f3)
                mount -o ro $(findfs "$type"="$uuid") /mnt/root
            else
                mount -o ro $(echo $cmd | cut -d= -f2) /mnt/root
            fi
            ;;
        esac
    done
}

check_filesystem() {
    # most of code coming from /etc/init.d/fsck

    local fsck_opts= check_extra= RC_UNAME=$(uname -s)

    # FIXME : get_bootparam forcefsck
    if [ -e /forcefsck ]; then
        fsck_opts="$fsck_opts -f"
        check_extra="(check forced)"
    fi

    echo "Checking local filesystem $check_extra : $1"

    if [ "$RC_UNAME" = Linux ]; then
        fsck_opts="$fsck_opts -C0 -T"
    fi

    trap : INT QUIT

    # using our own fsck, not the builtin one from busybox
    /sbin/fsck -p $fsck_opts $1

    case $? in
        0)      return 0;;
        1)      echo "Filesystem repaired"; return 0;;
        2|3)    if [ "$RC_UNAME" = Linux ]; then
                        echo "Filesystem repaired, but reboot needed"
                        reboot -f
                else
                        rescue_shell "Filesystem still have errors; manual fsck required"
                fi;;
        4)      if [ "$RC_UNAME" = Linux ]; then
                        rescue_shell "Fileystem errors left uncorrected, aborting"
                else
                        echo "Filesystem repaired, but reboot needed"
                        reboot
                fi;;
        8)      echo "Operational error"; return 0;;
        12)     echo "fsck interrupted";;
        *)      echo "Filesystem couldn't be fixed";;
    esac
    rescue_shell
}

# temporarily mount proc and sys
mount -t proc none /proc
mount -t sysfs none /sys
mount -t devtmpfs none /dev

# disable kernel messages from popping onto the screen
echo 0 > /proc/sys/kernel/printk

# clear the screen
clear

# mounting rootfs on /mnt/root
uuidlabel_root || rescue_shell "Error with uuidlabel_root"

# space separated list of mountpoints that ...
mountpoints="/usr" #note: you can add more than just usr, but make sure they are declared in /usr/src/initramfs/initramfs_list

# ... we want to find in /etc/fstab ...
ln -s /mnt/root/etc/fstab /etc/fstab

# ... to check filesystems and mount our devices.
for m in $mountpoints ; do
    check_filesystem $m

    echo "Mounting $m"
    # mount the device and ...
    mount $m || rescue_shell "Error while mounting $m"

    # ... move the tree to its final location
    mount --move $m "/mnt/root"$m || rescue_shell "Error while moving $m"
done

echo "All done. Switching to real root."

# clean up. The init process will remount proc sys and dev later
umount /proc
umount /sys
umount /dev

# switch to the real root and execute init
exec switch_root /mnt/root /sbin/init</syntaxhighlight>

Copy and save the contents of the above to /usr/src/initramfs/init.

Generating the Initramfs

Run the gen_initramfs_list.sh script with the -o argument pointing to where we want the initramfs image to be placed followed by the path to our initramfs_list file:

System preparation

In fstab, we must set the sixth field for the /usr entry to 0, this will prevent the OpenRC fsck init script to try to check the filesystem for the already mounted /usr:

/dev/sdb3   /usr    ext4   noatime  0 0

Bootloader configuration

To use the initramfs, the bootloader needs to be configured as shown below for GRUB and LILO as examples.

title Gentoo Linux <version>
root (hd0,0)
kernel /boot/kernel-<version>-gentoo
initrd /boot/initrd.cpio.gz

image = /boot/vmlinuz-<version>-gentoo
  #root = /dev/sda4
  label = gentoo
  read-only
  append = "real_root=/dev/sda4"
  initrd = boot/initrd.cpio.gz

Result

When booting, the output looks like this:

Mounting rootfs
Checking local filesystem :  /usr
/dev/sdb3: clean, 285754/1640160 files, 1663220/6556528 blocks
Mounting /usr
(Potentially other fs checks on other partitions here...)
All done. Switching to real root.
INIT: version 2.88 booting

    OpenRC 0.9.8.4 is starting up Gentoo Linux (i686)

Press I to enter interactive boot mode
[...]

See also

• Custom Initramfs — the successor of initrd. It provides early userspace which can do things the kernel can't easily do by itself during the boot process.

External resources

• http://jootamam.net/howto-initramfs-image.htm
• http://whitehathouston.com/documentation/gentoo/initramfs_howto.htm Link seems dead
• http://www.landley.net/writing/rootfs-howto.html
• https://forums.gentoo.org/viewtopic-p-7038132.html