Failed to boot LiveCD iso from ext4 partition

[Dylan Thacker-Smith]

Binary package hint: lupin-casper

The use case is that I was trying to run the LiveCD for the Ubuntu Karmic beta (http://releases.ubuntu.com/releases/9.10/ubuntu-9.10-beta-desktop-i386.iso) without burning a CD. Instead I tried using grub2's loopback feature to load the iso image from an ext4 partition.

Instead my ext4 partition wasn't loaded, and I got dropped to the shell from the initramfs due to a panic in /scripts/casper-premount/20iso_scan (on line 35 along with the error message I get). I was able to continue loading the LiveCD with the following commands:
(initramfs) mount -t ext4 /dev/sda5 /isodevice
(initramfs) echo "LIVEMEDIA=/isodevice/home/dylan/Desktop/ubuntu-9.10-beta-desktop-i386.iso" >> /conf/param.conf
(initramfs) echo "LIVEMEDIA_OFFSET=0" >> /conf/param.conf
(initramfs) exit

# Grub2 menuentry for booting the iso image:
menuentry "ubuntu-9.10-beta.iso" {
    insmod ext2
    loopback loop (hd0,5)/home/dylan/Desktop/ubuntu-9.10-beta-desktop-i386.iso
    linux (loop)/casper/vmlinuz iso-scan/filename=/home/dylan/Desktop/ubuntu-9.10-beta-desktop-i386.iso file=/cdrom/preseed/ubuntu.seed boot=casper quiet
    initrd (loop)/casper/initrd.lz
}

I found the problem was in /scripts/lupin-helper, since it's is_supported_fs function doesn't list ext4 as supported, so my ext4 partitions get skipped when looking for the iso file, even though I had no problem mounting the ext4 filesystem manually. I fixed the problem with the following steps after booting normally (i.e. no LiveCD):

mount -o loop /home/dylan/Desktop/ubuntu-9.10-beta-desktop-i386.iso /mnt
mkdir ~/cpio && cd ~/cpio
lzcat < /mnt/casper/initrd.lz | cpio -id
vi ~/cpio/scripts/lupin-helper # See the attached patch for my changes.
find . -print | cpio -o -H newc | lzma -c > ~/initrd.lz
sudo vi /boot/grub/grub.cfg # Change the initrd line: initrd (hd0,5)/home/dylan/initrd.lz

The lupin-helper script appears to be part of the lupin-casper package, and my patch still applies to bazaar repository for the package (http://bazaar.launchpad.net/~ubuntu-installer/lupin/hardy).

lupin-casper version: 0.25

Let me know if anything isn't clear (e.g. I may have made typos in the commands for this post).

[Wolfgang Kufner (wolfgangkufner)]

Thank you Dylan for your detailed explanations. I have found them very useful in working on bug #520500 (booting iso from stick via grub2 loopback gets stuck on particular crypt-luks partition).

I have now been able to loopback boot from ext4 partitions for a long time. This issue seems to have been solved by http://bazaar.launchpad.net/~ubuntu-installer/lupin/hardy/revision/130#casper/scripts/lupin-helpers, starting from a duplicate (Bug #460740) of your report. I am setting your report to duplicate of this so that it inherits the "fix released" status.

Regards