Modifying grub.cfg in CBFS

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Before you follow this guide, it is advisable that you have the ability to flash externally, just in case something goes wrong.

This guide assumes that you use the GNU GRUB bootloader as your default payload. In this configuration, GNU GRUB is flashed alongside coreboot and runs on bare metal as a native coreboot payload and does not use BIOS or UEFI services (but it can load and execute SeaBIOS, in addition to any other coreboot payload, by chainloading it).

In most circumstances, this guide will not benefit you. Libreboot’s default GRUB configuration file contains scripting logic within it that intelligently searches for GRUB partitions installed onto a partition on your SSD, HDD or USB drive installed on your computer. If such a file is found, libreboot’s default GRUB configuration is configured to switch automatically to that configuration. While not perfect, the logic does work with most configurations.

Therefore, you should only follow this guide if the automation (described above) does not work. It goes without saying that modifying the default GRUB configuration is risky, because a misconfiguration could create what’s called a soft brick where your machine is effectively useless and, in that scenario, may or may not require external flashing equipment for restoring the machine to a known state.

Compile flashrom and cbfstool

Libreboot does not currently distribute utilities pre-compiled. It only provides ROM images pre-compiled, where feasible. Therefore, you have to build the utilities from source.

As for the ROM, there are mainly three methods for obtaining a libreboot ROM image:

  1. Dump the contents of the the main boot flash on your system, which already has libreboot installed (with GNU GRUB as the default payload). Extract the GRUB configuration from that ROM image.
  2. Extract it from a libreboot ROM image supplied by the libreboot project, on the libreboot website or mirrors of the libreboot website.
  3. Build the ROM yourself, using the libreboot build system. Instructions for how to do this are covered in the following article: How to build libreboot from source

In either case, you will use the cbfstool supplied in the Libreboot build system. This can be found under coreboot/*/util/cbfstool/ as source code, where * can be any coreboot source code directory for a given mainboard. The directory named default should suffice.

Install the build dependencies. For Ubuntu 20.04 and similar, you can run the following command in the libreboot build system, from the root directory of the libreboot Git repository.

./build dependencies ubuntu2004

Then, download coreboot:

./download coreboot

Finally, compile the cbutils module:

./build module cbutils

Among other things, this will produce a cbfstool executable under any of the subdirectories in coreboot/ under `util/cbfstool/cbfstool

For example: coreboot/default/util/cbfstool/cbfstool

The cbfstool utility is what you shall use. It is used to manipulate CBFS (coreboot file system) which is a file system contained within the coreboot ROM image; as a coreboot distribution, libreboot inherits this technology.

You will also want to build flashrom which libreboot recommends for reading from and/or writing to the boot flash. In the libreboot build system, you can build it by running this command:

./build module flashrom

An executable will be available at flashrom/flashrom after you have done this.

Dump the boot flash

If you wish to modify your existing libreboot ROM, which was installed on your computer, you can use flashrom to acquire it.

Simply run the following, after using libreboot’s build system to compile flashrom:

sudo ./flashrom/flashrom -p internal -r dump.bin

If flashrom complains about multiple flash chip definitions, do what it says to rectify your command and run it again.

You may want to use the following, instead of -p internal: -p internal:laptop=force_I_want_a_brick,boardmismatch=force

Do not let the word brick fools you. This merely disables the safety checks in flashrom, which is sometimes necessary depending on what ROM was already flashed, versus the new ROM image.

The internal option assumes that internal read/write is possible; this is when you read from and/or write to the boot flash from an operating systems (usually GNU+Linux) that is running on the target system.

In other cases, you may need to connect an SPI programmer externally (with the machine powered down) and read the contents of the boot flash.

Learn how to externally reprogram these chips

Extract grub.cfg

Libreboot images that use the GNU GRUB bootloader will have two configuration files in CBFS:

We recommend that you modify grubtest.cfg first, and boot. Select the boot menu option for loading grubtest.cfg and verify that your new config works correctly. If it doesn’t, keep modifying grubtest.cfg until it does work. When that it done, copy the changes over to `grub.cfg

You can use the following commands to modify the contents of CBFS, where GRUB’s configuration file is concerned (dump.bin is the ROM that you dumped, or it could refer to the libreboot ROM image that you compiled or otherwise acquired).

Show the contents of CBFS, in your ROM:

cbfstool dump.bin print

Extract grub.cfg (substitude with grubtest.cfg as desired):

cbfstool dump.bin extract -n grub.cfg -f grub.cfg

You will now have a file named grub.cfg.

Make your desired modifications. You should then delete the old grub.cfg from your ROM image.

Insert new grub.cfg

Remove the old grub.cfg (substitute with grubtest.cfg as desired):

cbfstool dump.bin remove -n grub.cfg

Add your modified grub.cfg (substitute with grubtest.cfg as desired):

cbfstool dump.bin add -f grub.cfg -n grub.cfg -t raw

Flash the modified ROM image

Your modified dump.bin or other modified libreboot ROM can then be re-flashed using:

sudo ./flashrom -p internal -w dump.bin

If a -c option is required, use it and specify a flash chip name. This is only useful when flashrom complains about multiple flash chips being detected.

If flashrom complains about wrong chip/board, make sure that your ROM is for the correct system. If you’re sure, you can disable the safety checks by running this instead:

sudo ./flashrom -p internal:laptop=force_I_want_a_brick,boardmismatch=force -w dump.bin

If you need to use external flashing equipment, see the link above to the Raspberry Pi page.

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