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 it's 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 exists 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.

## Table of Contents

- Getting started
- Don't want to flash a new ROM image?
- Build 'cbfstool' from source
- Which ROM image should I use?
- Extract grubtest from the ROM image
- Example modifications for grubtest.cfg
  - Trisquel GNU/Linux-libre
  - Parabola GNU/Linux-libre
- 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.

Install the build dependencies.

Back to top of page.

## Don't want to flash a new ROM image?

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.

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 #build_cbfstool, #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).

Back to top of page.

## Build 'cbfstool' 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 executable.

Alternatively if you are working with libreboot_bin, you will find binaries under ./cbfstool/

Back to top of page.

## Which ROM image should I use?

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

If you want to re-use the ROM that you currently have flashed (and running) then see ../git/index.html#build_flashrom 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.

Back to top of page.

## 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. You should extract, modify and re-insert the copy first. grub.cfg will load first, but it has a menu entry for switching to the copy (grubtest.cfg). 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

Now you have a grubtest.cfg in cbfstool directory. Edit it however you wish.

Back to top of page.

## Example modifications for grubtest.cfg

These are some common examples of ways in which the grubtest.cfg file can be modified.

### Trisquel GNU/Linux-libre

As an example, on my test system in /boot/grub/grub.cfg (on the HDD/SSD) I see for the main menu entry:

- linux /boot/vmlinuz-3.15.1-gnu.nonpae root=UUID=3a008e14-4871-497b-95e5-fb180f277951 ro crashkernel=384M-2G:64M,2G-:128M quiet splash $vt_handoff
- initrd /boot/initrd.img-3.15.1-gnu.nonpae

ro, quiet, splash, crashkernel=384M-2G:64M,2G-:128M and $vt_handoff can be safely ignored.

I use this to get my partition layout:
$ lsblk

In my case, I have no /boot partition, instead /boot is on the same partition as / on sda1. Yours might be different. In GRUB terms, sda means ahci0. 1 means msdos1, or gpt1, depending on whether I am using MBR or GPT partitioning. Thus, /dev/sda1 is GRUB is (ahci0,msdos1) or (ahci0,gpt1). In my case, I use MBR partitioning so it's (ahci0,msdos1). 'msdos' is a GRUB name simply because this partitioning type is traditionally used by MS-DOS. It doesn't mean that you have a proprietary OS.

Trisquel doesn't keep the filenames of kernels consistent, instead it keeps old kernels and new kernel updates are provided with the version in the filename. This can make GRUB payload a bit tricky. Fortunately, there are symlinks /vmlinuz and /initrd.img so if your /boot and / are on the same partition, you can set GRUB to boot from that. These are also updated automatically when installing kernel updates from your distributions apt-get repositories. Note: when using jxself kernel releases, these are not updated at all and you have to update them manually.

For the GRUB payload grubtest.cfg (in the 'Load Operating System' menu entry), we therefore have (in this example):
set root='ahci0,msdos1'
linux /vmlinuz root=UUID=3a008e14-4871-497b-95e5-fb180f277951
initrd /initrd.img

Optionally, you can convert the UUID to its real device name, for example /dev/sda1 in this case. sdX naming isn't very reliable, though, which is why UUID is used for most distributions.

Alternatively, if your /boot is on a separate partition then you cannot rely on the /vmlinuz and /initrd.img symlinks. Instead, go into /boot and create your own symlinks (update them manually when you install a new kernel update).
$ sudo -s
# cd /boot/
# rm -rf vmlinuz initrd.img
# ln -s kernel ksym
# ln -s initrd isym
# exit

Replace the underlined kernel and initrd filenames above with the actual filenames, of course.

Then your grubtest.cfg menu entry (for payload) becomes like that, for example if / was on sda2 and /boot was on sda1:
set root='ahci0,msdos1'
linux /ksym root=/dev/sda2
initrd /isym

There are lots of possible variations so please try to adapt.

### Parabola GNU/Linux-libre

You can basically adapt the above. Note however that Parabola does not keep old kernels still installed, and the file names are always consistent, so you don't need to boot from symlinks, you can just use the real thing directly.

Back to top of page.

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

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

Back to top of page.

## 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.

Back to top of page.

## 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.

Back to top of page.

## 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.

Back to top of page.

Copyright © 2014 Francis Rowe <info@gluglug.org.uk>
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 ../license.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 ../license.txt for more information.