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  • Editing grub/EFI config on (net) installer ISO for serial install

    From Andy Smith@21:1/5 to All on Mon Jul 29 12:00:01 2024
    Hi,

    I am used to installing Debian by PXE boot and serial console. For
    that purpose I'm familiar with editing the isolinux config files to
    have the kernel serial settings (console=ttyS… etc) in
    isolinux/txt.cfg.

    Now for the first time I am trying to install a system that has a
    management controller that adds virtual media from ISOs, but I would
    still like to see that install over the IPMI serial.

    Currently when I add the Debian 12 netinst ISO as a virtual media it
    EFI boots grub, not isolinux, so the output of grub only goes to the
    graphical terminal (a web interface of the management controller in
    this case).

    I can then force the install to proceed over serial by editing the
    "Install" grub option and adding the familiar:

    --- console=ttyS1,115200n8

    pn the end of the kernel line, and that works, so I am nearly there.

    I'd really like for this thing to by default boot grub in text mode
    on the serial console though.

    I guess I need to find the grub configuration that is in use from
    the ISO and add the usual

    serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1
    terminal_input serial
    terminal_output serial

    Should I just edit that into $iso_root/boot/grub/grub.cfg and repack
    the ISO?

    Is there any documentation page about this? Everything I found so
    far just covers the isolinux bit, which doesn't appear to be
    relevant here.

    Thanks,
    Andy

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  • From Thomas Schmitt@21:1/5 to Andy Smith on Mon Jul 29 12:50:01 2024
    Hi,

    Andy Smith wrote:
    Currently when I add the Debian 12 netinst ISO as a virtual media it
    EFI boots grub, not isolinux,

    That's because Debian ISOs advertise a EFI System Partition with GRUB
    initial boot equipment:

    $ xorriso -indev debian-12.2.0-amd64-netinst.iso \
    -report_el_torito plain -report_system_area plain
    ...
    El Torito images : N Pltf B Emul Ld_seg Hdpt Ldsiz LBA
    El Torito boot img : 1 BIOS y none 0x0000 0x00 4 5863
    El Torito boot img : 2 UEFI y none 0x0000 0x00 18976 1119
    El Torito img path : 1 /isolinux/isolinux.bin
    El Torito img opts : 1 boot-info-table isohybrid-suitable
    El Torito img path : 2 /boot/grub/efi.img
    ...
    MBR partition table: N Status Type Start Blocks
    MBR partition : 1 0x80 0x00 0 1286144
    MBR partition : 2 0x00 0xef 4476 18976
    MBR partition path : 2 /boot/grub/efi.img

    The file /boot/grub/efi.img is a FAT filesystem image.
    One could mount it read-write

    mount -o offset=2291712 /dvdbuffer/debian-12.2.0-amd64-netinst.iso /mnt/fat

    and manipulate it inside the ISO. (2291712 = 4476 * 512)
    But new SYSLINUX content would be an expert task and would not work from optical media, because SYSLINUX EFI from CDROM is broken and will hardly
    ever be fixed.


    I guess I need to find the grub configuration that is in use from
    the ISO and add the usual
    serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1
    terminal_input serial
    terminal_output serial

    Such stuff is in the /boot/grub/ directory of the ISO. The sparse config
    in the EFI partition /mnt/fat/efi/debian/grub.cfg

    search --file --set=root /.disk/id/1af76032-4f8c-416b-90c5-76b1833daf0a
    set prefix=($root)/boot/grub
    source $prefix/${grub_cpu}-efi/grub.cfg

    loads one of the ISO files /boot/grub/*-efi/grub.cfg which both load /boot/grub/grub.cfg :

    source /boot/grub/grub.cfg


    Problem is that the FAT filesystem is tightly wrapped around its content:

    $ df /mnt/fat
    Filesystem 1K-blocks Used Available Use% Mounted on
    /dev/loop0 9450 9446 4 100% /mnt/fat

    If you can omit 32-bit EFI, then there would be plenty of room to recover
    by deleting

    -rwxr-xr-x 1 root root 758552 Oct 7 2023 /mnt/fat/efi/boot/bootia32.efi
    -rwxr-xr-x 1 root root 3753408 Oct 7 2023 /mnt/fat/efi/boot/grubia32.efi


    Should I just edit that into $iso_root/boot/grub/grub.cfg and repack
    the ISO?

    If altering the EFI partition is not viable, then surely: Yes.
    (The debian-cd people might already object manipulations of their
    carefully composed EFI partition.)


    Is there any documentation page about this?

    Repacking ISOs:
    https://wiki.debian.org/RepackBootableISO
    Especially look at this example:
    https://wiki.debian.org/RepackBootableISO#In_xorriso_load_ISO_tree_and_write_modified_new_ISO

    (Beware of bug with -boot_image "any" "replay" in xorriso <= 1.5.6:
    Do not overwrite the BIOS boot image /isolinux/isolinux.bin by the
    manipulation commands like -map.)


    Have a nice day :)

    Thomas

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  • From Thomas Schmitt@21:1/5 to Andy Smith on Mon Jul 29 15:00:01 2024
    Hi,

    Andy Smith wrote:
    Is there some advantage in me editing one of the files in the EFI
    partition as opposed to just putting the grub serial directives in /boot/grub/grub.cfg of the ISO?

    None that i know of.

    Editing /efi/debian/grub.cfg of the EFI partition filesystem would just
    happen inside the ISO data storage without imposing the need for making
    a new ISO.

    The tight size of the Debian ISO EFI partition is not very inviting for
    this. It also would have to be explored whether modifying the grub.cfg
    file in the EFI partition can cause trouble with Secure Boot.


    Have a nice day :)

    Thomas

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  • From Andy Smith@21:1/5 to Thomas Schmitt on Mon Jul 29 14:30:01 2024
    Hi,

    On Mon, Jul 29, 2024 at 12:42:05PM +0200, Thomas Schmitt wrote:
    Andy Smith wrote:
    Should I just edit that into $iso_root/boot/grub/grub.cfg and repack
    the ISO?

    If altering the EFI partition is not viable, then surely: Yes.

    Is there some advantage in me editing one of the files in the EFI
    partition as opposed to just putting the grub serial directives in /boot/grub/grub.cfg of the ISO?

    Is there any documentation page about this?

    Repacking ISOs:
    https://wiki.debian.org/RepackBootableISO
    Especially look at this example:
    https://wiki.debian.org/RepackBootableISO#In_xorriso_load_ISO_tree_and_write_modified_new_ISO

    Thanks - I am okay with the actual repacking of the ISO though. I
    was just wondering about the grub layout on the ISO and where my
    changes should be done for this.

    Thanks,
    Andy

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