Bootloader to use

[MilkyDeveloper]

Current Options:

• Ventoy, GRUB, or GRUB4DOS (uses bootx64.efi, which is a PE EFI executable that cannot be kexeced)

[MilkyDeveloper]

Resources: https://unix.stackexchange.com/questions/360834/kexec-to-grub-or-to-syslinux-or-windows

[EMLommers]

Any bootloader that can be executed via MBR is theoretically easy to convert, convert the executable from MBR to ELF/MZ or load into memory (including BIOS routines) and jump to it. Ventoy is able to do a lot! and plugins are available.

[MilkyDeveloper]

MBR is a disk layout, whereas ELF is an executable format (I think :D). If you mount the Ventoy ISO, reverse-dd the 2M partition of it to a file, and then mount that file as MBR, you'll find a bootx64.efi image. This is a PE EFI executable that, IIRC, cannot be executed with kexec. Could you expand on how to convert this?

[EMLommers]

I do not currenty know what the development stage is for Ventoy, but u-boot//depthcharge/vboot is not (yet) supported yet I think. Every bootloader has its pros and cons.

[EMLommers]

The boot sector / MBR is read by the BIOS, (Non-UEFI), get it's name and other info like FAT, etc and pulls x86 code 'bootstrap' executable and runs it, often running a certain file or reading some sectors and executing. copying this code and having the bios calls available, pasting to the right address and executing.. or am I thinking to simplistic??

[MilkyDeveloper]

Respectfully, I have no idea what any of this (very low level) stuff means :D

[MilkyDeveloper]

I think I'm going to use this. You can kexec the grub.exe file.

[EMLommers]

Another idea.

I don't know if it is possible..
Original layout GPT for chromeOS.
changing partition 1 : mmcblk0p1 to entire linux system incl /boot
partition 2: ChromeOS kernel signed (KERNEL-A)
partition 3: change to bootloader or kexec partition 1 and run any kernel + rest of OS and sign this partition (ROOT-A)
partition 4: idem partition 2 (KERNEL-B)
partition 5: idem partition 3 (ROOT-B)
partition 7: id partition 1 is not working, partition linux here. (ROOT-C)

Return from dev to normal.

[MilkyDeveloper]

Nobody has ever done this before though. When you disable dev mode, you are forced to use a recovery image that needs some type of signing by Google.

[MilkyDeveloper]

Actually, I was wrong. Recovery images can be used to load custom software. See: https://www.chromium.org/chromium-os/chromiumos-design-docs/recovery-mode/

[EMLommers]

Also see: https://www.chromium.org/chromium-os/developer-information-for-chrome-os-devices/custom-firmware/

In developer mode, the RW firmware doesn't verify that the kernel is signed by Google, just that it looks like a correctly signed kernel image. It doesn't matter who signed it, bla blabla

Does it also count for normal mode...??? Just thinking out of the box.

Also ....
changing /sbin/init/ to bootloder or some kind of kexec to another partition... I guess you understand what I am thinking about..

[MilkyDeveloper]

No, in normal mode the kernel and rootfs are checked to be signed by Google. Recovery images also can’t be used to bypass this. I think the best solution would be to keep the GBB flags timeout and find some way to disable the dev mode beep and change the screen.

[EMLommers]

I have not checked whether is was signed by google or me. Have you?
Testing is knowing. Never trust the docs. Docs sometimes may mislead you! ;)

[MilkyDeveloper]

Whether what is signed? DM_Verity protects the rootfs and the kernel is definitely signed. I have experience with tinkering with this in dev mode.

[MilkyDeveloper]

Aha! I’ve got it.

config KERNEL_MULTIBOOT
--
  | bool "Allow alternative(multiboot) kernels"
  | default n

This should let us boot GRUB2 multi boot (core.img) files and we can now boot GRUB through Depthcharge (provided you recompile Depthcharge).

https://chromium.googlesource.com/chromiumos/platform/depthcharge/+/refs/heads/main/src/boot/Kconfig#34

[EMLommers]

Could be, but in this case you still need to flash firmware. No everyone likes to open their chromebook, disables battery, flash, or use suzyQCable, which is out of stock or build your own suzyQCable, with usb-C outbreak board, a 22 K & 56K resistor and fighting with soldering unit.

[EMLommers]

Also an Idea is to use mmcblk0p12
With alt-boot=1 , running isolinux/extlinux

What I meant with signed , for kernel it is clear,
for ROOFS, enable ROOFS_VERIFICATION

maybe only small kerrnel runner initiated by /sbin/init or modified /sbin/init
Also if you have kernel (KERNEL-A) setup, maybe put the Ventoy.iso somewhere and isoboot

[EMLommers]

Why not use --bootloader /lib64/bootstub/bootstub.efi with vbutil_kernel...

[MilkyDeveloper]

You can test it. I’m developing a UEFI solution.

[EMLommers]

This is a kernel as well as bootloadehr : https://www.plop.at/en/plopkexec/intro.html
I tried to pack but constantly after dd to /dev/sda1 it runs the kernel on mmcblk0p1..
Maybe I am doing something wrong..
Have not used chromeos kernel config