Comment 4 for bug 1813126

Further investigation revealed that a change to the semantics of 'cryptsetup' tool is responsible for the specific manual build I did.

On 18.04 'cryptsetup luksFormat --type=luks ...' would create a LUKS v1 header. On 19.04 it creates a LUKS v2 header. GRUB only supports LUKS v1 headers currently.

To resolve this on 19.04 we need to use "cryptsetup luksFormat --type=luks1 ...".

If the do-release-upgrade process is breaking existing bootable GRUB file-systems this is likely because there is some LUKSv1 to LUKSv2 conversion being done - which the OS is fine with. If this is correct, presumably the conversion code doesn't check if GRUB is also using the LUKS device and in converting it to v2 breaks GRUB.

My workaround from a liveISO to switch back to LUKS v1 was to do the following (as UID 0 root). I've tried to make this portable but there may be some typing mistakes so please proof-read carefully!

# set the following variables for your target system

# device where OS is installed (usually the same as GRUB_DEV)
DEV_INSTALL="sdZ"
# partition number for /boot/ partition (e.g. from /dev/sdZ3)
PART_BOOT="3"
# partition for root file-system (e.g. from /dev/sdZ4)
PART_ROOT="4"
# device-mapper name of unencrypted /boot/ device
LUKS_BOOT="LUKS_BOOT"
# LVM names of root file-system
VG="ubuntu"
LV="rootfs"

# shouldn't need to edit any variables from here onwards

# device to install GRUB on
export DEV_GRUB="/dev/$INSTALL_DEV"

# device-mapper name of the unencrypted /boot device
FS_BOOT="/dev/mapper/$LUKS_BOOT"
# encrypted /boot device
DEV_BOOT="/dev/${DEV_INSTALL}${PART_BOOT}"

# device-mapper name of the rootfs device
FS_ROOT="/dev/mapper/$VG-$LV"
LUKS_ROOT="${INSTALL_DEV}${PART_ROOT}_crypt"
# encrypted rootfs device
DEV_ROOT="/dev/${INSTALL_DEV}${PART_ROOT}"

# mountpoint for root file-system
TARGET="/target"
# boot directory
BOOT="$TARGET/boot"
# back-up directory
BAK="/tmp/boot.bak"

mkdir $TARGET $BAK
cryptdisk_start $LUKS_ROOT
lvm vgchange -ay
mount $FS_ROOT $TARGET
cryptdisk_start $LUKS_BOOT
mount $FS_BOOT $BOOT
cp -a $BOOT/* $BAK/
umount $BOOT
cryptdisk_stop $LUKS_BOOT
cryptsetup luksFormat --type=luks1 $DEV_BOOT
# use the existing UUID from crypttab
cryptsetup luksUUID --uuid $( sed -n '/'$LUKS_BOOT'/ s/.*UUID=\([^ ]*\).*/\1/ p' $TARGET/etc/crypttab ) $DEV_BOOT
cryptdisk_start $LUKS_BOOT
mkfs.ext -L /boot $FS_BOOT
mount $FS_BOOT $BOOT
cp -a $BAK/* $BOOT/

for n in proc sys dev etc/resolv.conf; do mount --rbind /$n $TARGET/$n; done
chroot $TARGET

# inside the chroot of the broken OS
mount -a
update-grub
grep -o 'cryptodisk' || echo "Eror: grub.cfg has no LUKS support"
grub-install -v $DEV_GRUB >& /tmp/grub-install.log
grep 'grub-mkimage' /tmp/grub-install.log | grep -o cryptodisk || echo "Error: GRUB has no LUKS support installed"

# if all OK...
exit

# clean up
for n in etc/resolv.conf dev sys proc $BOOT $TARGET; do umount $TARGET/$n; done
cryptdisk_stop $LUKS_BOOT
cryptdisk_stop $LUKS_ROOT

# finished
exit

# reboot and test