# How to replace the default GRUB configuration file on a libreboot system

Libreboot 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.

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# Table of Contents

- Getting started
- Option A: don't flash a new ROM
- Option B: flash a new ROM

- Get the tools ready

- Locate the tools in the binary release
- Build the tools from source

- Which ROM image should I use?

- Use one of the provided ROM images
- Re-use the currently flashed ROM image

- Extract grubtest from the ROM image
- Re-insert the modified grubtest.cfg into the ROM image
- Test it!
- Final steps
- Troubleshooting

## Getting started

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

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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 machine (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 machine (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.

## Option A: don't flash a new ROM

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, #which_rom and #extract_grubtest 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..

This is all well and good, but what should you actually put in your GRUB configuration file? Read grub_config.html for more information.

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## Option B: flash a new ROM

Alternatively to editing the GRUB configuration file on your disk, you can also change the configuration that is stored in the CBFS on the flash chip.

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## Get the tools ready

You will need to use the cbfstool, rmodtool and flashrom tools. You can either use the pre-compiled binaries provided by the binary release of libreboot, or build the tools yourself from source from the libreboot source code.

### Locate the tools in the binary release

If you are working with the binary release libreboot_util, you will find the cbfstool and rmodtool binaries under ./cbfstool/{architecture}/ , where {architecture} refers to your hardware name for which your current running kernel is compiled for. You can find it by running:
$ uname -m

The flashrom binary is located under ./flashrom/{architecture}/

Whenever one of the tools is used in a command in this tutorial, it will be called as, e.g., ./cbfstool
Please adapt the paths accordingly (e.g.: cbfstool/i686/cbfstool).

### Build the tools from source

If you are working with libreboot_src, then you can run make command in libreboot_src/coreboot/util/cbfstool to build the cbfstool and rmodtool executables. See ../git/index.html#build_flashrom for instructions on how to build flashrom.

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## Which ROM image should I use?

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.

### Use one of the provided ROM images

Libreboot ROM images are distributed in separate tar archives, named {computer_model}.tar.xz. If you build from source without generated any release archives, the ROM images will be in ./bin/{computer_model}/. Make a working copy of the ROM you want to use:
$ cp ./bin/{computer_model}/{computer_model}_{keyboard_layout}_{mode}.rom libreboot.rom
{keyboard_layout} defines the keyboard layout that will be available on the GRUB console. This might be important if you protect GRUB with a password later on and want to insert non-ASCII characters. {mode} can be vesafb or txtmode and starts GRUB with a background image or in text mode.

### Re-use the currently flashed ROM image

If you want to re-use the ROM that you currently have flashed (and running) and then run:
$ sudo ./flashrom -p internal -r libreboot.rom
Notice that this is using "-r" (read) instead of "-w" (write). This will create a dump (copy) of your current firmware and name it libreboot.rom. You need to take ownership of the file. For example:
$ sudo chown yourusername:yourusername libreboot.rom
# chown yourusername:yourusername libreboot.rom

If you currently have flashed a ROM image from an older version, it is recommended to update first: basically, modify one of the latest ROM images and then flash it.

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## Extract grubtest.cfg from the ROM image

Display contents of ROM:
$ ./cbfstool libreboot.rom print

The libreboot.rom file contains your grub.cfg and grubtest.cfg files. grub.cfg will load first, but it has a menu entry for switching to the copy (grubtest.cfg). Thus, you should extract, modify and re-insert the grubtest.cfg first. This reduces your chance of making a mistake that could make your machine unbootable (or very hard to boot).

Extract grubtest.cfg from the ROM image:
$ ./cbfstool libreboot.rom extract -n grubtest.cfg -f grubtest.cfg

Make a copy of the original file, before you modify it:
$ cp grubtest.cfg grubtest_vanilla.cfg

Now you have a grubtest.cfg in the cbfstool directory.

This is all well and good, but what should you actually put in your GRUB configuration file? Read grub_config.html for more information.

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

So you've created your configuration. Now, how do you use it?

Delete the grubtest.cfg that remained inside the ROM:
$ ./cbfstool libreboot.rom remove -n grubtest.cfg

Display ROM contents and now you see grubtest.cfg no longer exists there:
$ ./cbfstool libreboot.rom print

Add the modified version that you just made:
$ ./cbfstool libreboot.rom add -n grubtest.cfg -f grubtest.cfg -t raw

Now display ROM contents again and see that it exists again:
$ ./cbfstool libreboot.rom print

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## Test it!

Now you have a modified ROM. Refer back to ../install/index.html#flashrom for information on how to flash it. 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.

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## Final steps

Create a copy of grubtest.cfg, called grub.cfg, which is the same except for one difference: change the menuentry 'Switch to grub.cfg' to 'Switch to grubtest.cfg' and inside it, change 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)
$ 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

Display ROM contents and now you see grub.cfg no longer exists there:
$ ./cbfstool libreboot.rom print

Add the modified version that you just made:
$ ./cbfstool libreboot.rom add -n grub.cfg -f grub.cfg -t raw

Now display ROM contents again and see that it exists again:
$ ./cbfstool libreboot.rom print

Now you have a modified ROM. Refer back to ../install/index.html#flashrom for information on how to flash it. Once you have done that, shut down and then boot up with your new configuration.

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## Troubleshooting

A user reported that segmentation faults occur with cbfstool when using this procedure depending on the size of the grub.cfg being re-insterted. In his case, a minimum size of 857 bytes was required. This could (at the time of this release) be a bug in cbfstool that should be investigated with the coreboot community. If cbfstool segfaults, then keep this in mind. 'strace' (or gdb? clang?) could be used for debugging. This was in libreboot 5th release (based on coreboot from late 2013), and I'm not sure if the issue persists in the current releases. I have not been able to reproduce it. strace (from that user) is here: cbfstool_libreboot5_strace. The issue has been reported by a few users, so it does not happen all the time: this bug (if it still exists) could (should) be reproduced.

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Copyright © 2014, 2015 Francis Rowe <info@gluglug.org.uk>
Copyright © 2015 Julian Mehne <juli@nmehne.de>
This document is released under the Creative Commons Attribution-ShareAlike 4.0 International Public License and all future versions. A copy of the license can be found at ../cc-by-sa-4.txt.

This document is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See ../cc-by-sa-4.txt for more information.