Libreboot documentation: GRUB menu

Libreboot on X60/T60/macbook21 uses the GRUB2 payload, which means that the grub.cfg (which is where your GRUB menu comes from) is inside the ROM along with GRUB itself, executed by libreboot directly in the SPI 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 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. In this case, libreboot is configured such that the grub.cfg exists directly inside CBFS instead of inside the grub.elf payload's 'memdisk' (which is itself stored in CBFS).

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

Download libreboot_src.tar.gz or libreboot_bin.tar.gz from http://libreboot.org/
If you downloaded libreboot from git, refer to ../index.html#build_meta before continuing.

First, install the build dependencies.

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, then you can run ./builddeps-cbfstool command inside libreboot_bin/; a cbfstool and rmodtool executable will appear under libreboot_bin/

You can work directly with one of the ROM's already included in libreboot_bin.tar.gz. For the purpose of this tutorial it is assumed that your ROM is named 'libreboot_usqwerty.rom' so please make sure to adapt.

If you want to re-use the ROM that you currently have flashed (and running) then see ../index.html#build_flashrom and then run:
$ sudo ./flashrom -p internal -r libreboot_usqwerty.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_usqwerty.rom. You need to take ownership of the file. For example:
$ sudo chown yourusername:yourusername libreboot_usqwerty.rom
# chown yourusername:yourusername libreboot_usqwerty.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's and then flash it.

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

The libreboot_usqwerty.rom file contains your grub.cfg, along with a copy called grubtest.cfg. 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 grub.cfg from the ROM:
$ ./cbfstool libreboot_usqwerty.rom extract -n grubtest.cfg -f grubtest.cfg

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

## Example modification (Trisquel GNU/Linux, can also be adapted, or copied, for other apt-get distros)

Look at the 3 lines under the menu entry 'Load Operating System'; these are all you need to change.

As an example, on my test system in /boot/grub/grub.cfg 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

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 GRUB's name simply because this partitioning type is traditionally used by MS-DOS. It doesn't mean 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.org/linux-libre kernels, these are not updated at all and you have to update them manually.

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

Optionally, you can convert the UUID to it's 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 grub.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 ro quiet splash
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.

Now you have your modified grub.cfg. (right?)

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

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

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

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

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

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

Rename (just reduce confusion):
$ mv grubtest.cfg grub.cfg

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

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

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

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

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

If all went well, GRUB should now boot your system the way you intended.

Anecdotally, 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). Not sure if the issue perists in the 6th release (based on coreboot from June 1st, 2014 at the time of writing); I have never personally encountered the bug. strace (from that user) is here: cbfstool_libreboot5_strace. The issue has been reported by a few users, so does not happen all the time: this bug (if it still exists) could (should) be reproduced.

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.