Libreboot documentation: GRUB menu

- 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's 'memdisk' (which is itself stored in CBFS). -

Getting started

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

Which ROM image should I use?

- You can work directly with one of the ROM's already included in the libreboot ROM archives. For the purpose of - this tutorial it is assumed that your ROM 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 - ../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's and then flash it. -

Extract grubtest.cfg from the ROM image

- 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. -

- -

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 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 kernel releases, - these are not updated at all and you have to update them manually. - -

- -

- For the GRUB payload's 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 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 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. -

- -

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 -

Test it!

- - Now you have a modified ROM. Refer back to ../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. - -

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 ../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. - -

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 perists 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. -

- 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. -

How to change your default GRUB menu

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