Ubuntu Live ISO – How to Remaster and Revise grub.cfg for Autoinstall

I'm working on Intel and M1 Mac's and running Ubuntu Server in VMWare Fusion VMs. I'm currently using the latest ubuntu-23.10-live-server-amd64.iso and ubuntu-23.10-live-server-arm64.iso. I can get everything to work as needed for Intel-based systems, but when using the arm64.iso, it doesn't contain a /boot.catalog and /boot/grub/i386-pc/eltorito.img. So, I'm unable to recreate a bootable ISO after my revisions to the grub.cfg.

I've pieced together bits of information from various sources, starting here: https://ubuntu.com/server/docs/install/autoinstall-quickstart, but haven't found or understood enough to fill in this last piece of the puzzle for my arm64 platform. Also, I was unable to use livefs-edit because it depends on 'losetup', which isn't available on macOS; however, xorriso solved the problem 100% on Intel and I imagine… mostly solves it on the M1s if I can solve the boot image issue.

For amd64.iso's this works:

1. Extract the original ISO
2. Create user-data and meta-data files
3. Revise the grub.cfg to add an autoinstall menuitem
4. Repackage the ISO with the following:

    xorriso -as mkisofs \
    --modification-date='2021101314195100' \
    --grub2-mbr \
    --interval:local_fs:0s-15s:zero_mbrpt,zero_gpt:"${ORIGINAL_ISO_PATH}" \
    --protective-msdos-label \
    -partition_cyl_align off \
    -partition_offset 16 \
    --mbr-force-bootable \
    -append_partition 2 28732ac11ff8d211ba4b00a0c93ec93b \
    --interval:local_fs:2470124d-2478587d::"${ORIGINAL_ISO_PATH}" \
    -part_like_isohybrid \
    -iso_mbr_part_type a2a0d0ebe5b9334487c068b6b72699c7 \
    -c '/boot.catalog' \
    -b '/boot/grub/i386-pc/eltorito.img' \
    -no-emul-boot \
    -boot-load-size 4 \
    -boot-info-table \
    --grub2-boot-info \
    -eltorito-alt-boot -e \
    '--interval:appended_partition_2_start_617531s_size_8464d:all::' \
    -no-emul-boot \
    -boot-load-size 8464 \
    -isohybrid-gpt-basdat \
    -o ubuntu-autoinstall.iso \
    -V 'Ubuntu autoinstall' ${EXTRACTED_ISO_PATH}

Update
Using Thomas Schmitt's advice, I queried for the proposal and revised to use:

xorriso -as mkisofs \
--modification-date='2023101104561500' \
-partition_cyl_align off \
-partition_offset 16 \
-append_partition 2 0xef --interval:local_fs:4615776d-4629311d::"${ORIGINAL_ISO_PATH}" \
-G --interval:local_fs:0s-15s:zero_mbrpt:"${ORIGINAL_ISO_PATH}" \
-iso_mbr_part_type 0xcd \
-c '/boot/boot.cat' \
-e '--interval:appended_partition_2_start_1153944s_size_13536d:all::' \
-no-emul-boot \
-boot-load-size 13536 \
-output ubuntu-autoinstall.iso \
extracted-iso/

The result was:

GNU xorriso 1.5.6 : RockRidge filesystem manipulator, libburnia project.
Drive current: -outdev 'stdio:ubuntu-autoinstall.iso'
Media current: stdio file, overwriteable
Media status : is blank
Media summary: 0 sessions, 0 data blocks, 0 data, 1415g free
Added to ISO image: directory '/'='/extracted-iso'
xorriso : UPDATE :     980 files added in 1 seconds
xorriso : UPDATE :     980 files added in 1 seconds
xorriso : NOTE : Copying to System Area: 32768 bytes from file '--interval:local_fs:0s-15s:zero_mbrpt:ubuntu-23.10-live-server-amd64.iso'
libisofs: NOTE : Automatically adjusted MBR geometry to 1020/159/32
xorriso : UPDATE :  3.18% done
xorriso : UPDATE :  54.15% done
xorriso : UPDATE :  87.88% done
ISO image produced: 1300424 sectors
Written to medium : 1300424 sectors at LBA 0
Writing to 'stdio:ubuntu-autoinstall.iso' completed successfully.

The only difference I can see upon inspecting the extracted ISOs of both the original, which is bootable, and the new, which is not:

And attempting to boot results in this:

I've also tried:

xorriso -as mkisofs \
--modification-date='2023101104561500' \
-partition_cyl_align off \
-partition_offset 16 \
-append_partition 2 0xef --interval:local_fs:4615776d-4629311d::"${ORIGINAL_ISO_PATH}" \
-iso_mbr_part_type 0xcd \
-c '/boot/boot.cat' \
-e '--interval:appended_partition_2_start_1153944s_size_13536d:all::' \
-no-emul-boot \
-output ubuntu-autoinstall.iso \
extracted-iso/

I'm pulling my hair out and grasping at straws. Much of this I don't fully understand, but I can see that the -c placed the boot catalog. So, to get the new ISO closer to the original, I used `c '/boot.catalog', which now gets me to one difference in an original file, that looks like this:

I suspect there is something NOT visible to me, perhaps in the actual MBR? Just speculating about things I'm not very familiar with while digging for more information.

Solved thanks to Thomas Shchmitt's help. Next steps would be to replace the ORIGINAL_EXTRACTED_ISO_DIR with a revised target, new cloud-init user-data, grub.cfg, etc.

case "$ARCHITECTURE" in
    "m1")
        echo "Running on M1 Mac."
        xorriso -as mkisofs \
        --modification-date='2023081005071100' \
        -partition_cyl_align off \
        -partition_offset 16 \
        -append_partition 2 0xef --interval:local_fs:4030464d-4042271d::"${ORIGINAL_ISO_FILE}" \
        -G --interval:local_fs:0s-15s:zero_mbrpt:"${ORIGINAL_ISO_FILE}" \
        -iso_mbr_part_type 0xcd \
        -c '/boot/boot.cat' \
        -e '--interval:appended_partition_2_start_1007616s_size_11808d:all::' \
        -no-emul-boot \
        -boot-load-size 11808 \
        -output ${AUTOINSTALL_ISO_FILE} \
        ${ORIGINAL_EXTRACTED_ISO_DIR}/
        ;;
    "intel")
        echo "Running on Intel Mac."
        xorriso -as mkisofs \
        --modification-date='2023081005062500' \
        --grub2-mbr --interval:local_fs:0s-15s:zero_mbrpt,zero_gpt:"${ORIGINAL_ISO_FILE}" \
        --protective-msdos-label \
        -partition_cyl_align off \
        -partition_offset 16 \
        --mbr-force-bootable \
        -append_partition 2 28732ac11ff8d211ba4b00a0c93ec93b --interval:local_fs:4156048d-4166115d::"${ORIGINAL_ISO_FILE}" \
        -appended_part_as_gpt \
        -iso_mbr_part_type a2a0d0ebe5b9334487c068b6b72699c7 \
        -c '/boot.catalog' \
        -b '/boot/grub/i386-pc/eltorito.img' \
        -no-emul-boot \
        -boot-load-size 4 \
        -boot-info-table \
        --grub2-boot-info \
        -eltorito-alt-boot \
        -e '--interval:appended_partition_2_start_1039012s_size_10068d:all::' \
        -no-emul-boot \
        -boot-load-size 10068 \
        -output ${AUTOINSTALL_ISO_FILE} \
        ${ORIGINAL_EXTRACTED_ISO_DIR}/
        ;;
    *)
        echo "Unsupported architecture: $ARCHITECTURE"
        exit 1
        ;;
esac