How to replace the default GRUB configuration file

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Libreboot on x86 uses the GRUB payload by default, which means that the GRUB configuration file (where your GRUB menu comes from) is stored directly alongside libreboot and its GRUB payload executable, inside the flash chip. In context, this means that installing distributions and managing them is handled slightly differently compared to traditional BIOS systems.

A libreboot (or coreboot) ROM image is not simply “flat”; there is an actual filesystem inside called CBFS (coreboot filesystem). A utility called ‘cbfstool’ allows you to change the contents of the ROM image. In this case, libreboot is configured such that the ‘grub.cfg’ and ‘grubtest.cfg’ files exist directly inside CBFS instead of inside the GRUB payload ‘memdisk’ (which is itself stored in CBFS).

You can either modify the GRUB configuration stored in the flash chip, or you can modify a GRUB configuration file on the main storage which the libreboot GRUB payload will automatically search for.

Here is an excellent writeup about CBFS (coreboot filesystem): http://lennartb.home.xs4all.nl/coreboot/col5.html.

This guide is *only* for the GRUB payload. If you use the depthcharge payload, ignore this section entirely.

Table of Contents

Introduction

Download the latest release from libreboot.org
If you downloaded from git, refer to ../git/#build_meta before continuing.

There are several advantages to modifying the GRUB configuration stored in CBFS, but this also means that you have to flash a new libreboot ROM image on your system (some users feel intimidated by this, to say the least). Doing so can be risky if not handled correctly, because it can result in a bricked system (recovery is easy if you have the equipment for it, but most people don’t). If you aren’t up to that then don’t worry; it is possible to use a custom GRUB menu without flashing a new image, by loading a GRUB configuration from a partition on the main storage instead.

1st option: don’t re-flash

By default, GRUB in libreboot is configured to scan all partitions on the main storage for /boot/grub/libreboot_grub.cfg or /grub/libreboot_grub.cfg(for systems where /boot is on a dedicated partition), and then use it automatically.

Simply create your custom GRUB configuration and save it to /boot/grub/libreboot_grub.cfg on the running system. The next time you boot, GRUB (in libreboot) will automatically switch to this configuration file. This means that you do not have to re-flash, recompile or otherwise modify libreboot at all!

Ideally, your distribution should automatically generate a libreboot_grub.cfg file that is written specifically under the assumption that it will be read and used on a libreboot system that uses GRUB as a payload. If your distribution does not do this, then you can try to add that feature yourself or politely ask someone involved with or otherwise knowledgeable about the distribution to do it for you. The libreboot_grub.cfg could either contain the full configuration, or it could chainload another GRUB ELF executable (built to be used as a coreboot payload) that is located in a partition on the main storage.

If you want to adapt a copy of the existing libreboot GRUB configuration and use that for the libreboot_grub.cfg file, then follow #tools, #rom and #extract_testconfig to get the grubtest.cfg. Rename grubtest.cfg to libreboot_grub.cfg and save it to /boot/grub/ on the running system where it is intended to be used. Modify the file at that location however you see fit, and then stop reading this guide (the rest of this page is irrelevant to you); in libreboot_grub.cfg on disk, if you are adapting it based on grub.cfg from CBFS then remove the check for libreboot_grub.cfg otherwise it will loop..

2nd option: re-flash

You can modify what is stored inside the flash chip quite easily. Read on to find out how.

Acquire the necessary utilities

Use cbfstool and flashrom. There are available in the libreboot_util release archive, or they can be compiled (see ../git/#build_flashrom). Flashrom is also available from the repositories: # pacman -S flashrom

Acquiring the correct ROM image

You can either work directly with one of the ROM images already included in the libreboot ROM archives, or re-use the ROM that you have currently flashed. For the purpose of this tutorial it is assumed that your ROM image file is named libreboot.rom, so please make sure to adapt.

ROM images are included pre-compiled in libreboot. You can also dump your current firmware, using flashrom: $ sudo flashrom -p internal -r libreboot.rom # flashrom -p internal -r libreboot.rom If you are told to specify the chip, add the option -c {your chip} to the command, for example:

# flashrom -c MX25L6405 -p internal -r libreboot.rom

Extract grubtest.cfg from the ROM image

You can check the contents of the ROM image, inside CBFS:

$ cd \.../libreboot\_util/cbfstool** $ ./cbfstool libreboot.rom

print**

The files grub.cfg and grubtest.cfg should be present. grub.cfg is loaded by default, with a menuentry for switching to grubtest.cfg. In this tutorial, you will first modify and test grubtest.cfg. This is to reduce the possibility of bricking your device, so DO NOT SKIP THIS!

Extract grubtest.cfg from the ROM image:

$ ./cbfstool libreboot.rom extract -n grubtest.cfg -f grubtest.cfg

Modify the grubtest.cfg accordingly.

Re-insert the modified grubtest.cfg into the ROM image

Once your grubtest.cfg is modified and saved, delete the unmodified config from the ROM image:

$ ./cbfstool libreboot.rom remove -n grubtest.cfg

Next, insert the modified version:
$ ./cbfstool libreboot.rom add -n grubtest.cfg -f grubtest.cfg -t raw

Testing

Now you have a modified ROM. Refer back to ../install/#flashrom for information on how to flash it. $ cd /libreboot_util # ./flash update libreboot.rom
Ocassionally, coreboot changes the name of a given board. If flashrom complains about a board mismatch, but you are sure that you chose the correct ROM image, then run this alternative command: # ./flash forceupdate libreboot.rom You should see
“Verifying flash... VERIFIED.”** written at the end of the flashrom output. Once you have done that, shut down and then boot up with your new test configuration.**

Choose (in GRUB) the menu entry that switches to grubtest.cfg. If it works, then your config is safe and you can continue below.

If it does not work like you want it to, if you are unsure or sceptical in any way, then re-do the steps above until you get it right! Do *not* proceed past this point unless you are 100% sure that your new configuration is safe (or desirable) to use.

Final steps

When you are satisfied booting from grubtest.cfg, you can create a copy of grubtest.cfg, called grub.cfg. This is the same except for one difference: the menuentry ‘Switch to grub.cfg’ will be changed to ‘Switch to grubtest.cfg’ and inside it, all instances of grub.cfg to grubtest.cfg. This is so that the main config still links (in the menu) to grubtest.cfg, so that you don’t have to manually switch to it, in case you ever want to follow this guide again in the future (modifying the already modified config). From /libreboot_util/cbfstool, do:
$ sed -e ‘s:(cbfsdisk)/grub.cfg:(cbfsdisk)/grubtest.cfg:g’ -e ‘s:Switch to grub.cfg:Switch to grubtest.cfg:g’ < grubtest.cfg > grub.cfg
Delete the grub.cfg that remained inside the ROM:

$ ./cbfstool libreboot.rom remove -n grub.cfg

Add the modified version that you just made:

$ ./cbfstool libreboot.rom add -n grub.cfg -f grub.cfg -t raw

Now you have a modified ROM. Again, refer back to ../install/#flashrom for information on how to flash it. It’s the same method as you used before. Shut down and then boot up with your new configuration.

Copyright © 2014, 2015 Leah Rowe info@minifree.org
Copyright © 2015 Jeroen Quint jezza@diplomail.ch
This page is available under the CC BY SA 4.0

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