--- title: Modifying grub.cfg in CBFS x-toc-enable: true ... 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 GRUB bootloader as your default payload. In this configuration, 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 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](../build/) 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: ./update trees -f coreboot Finally, compile the `cbutils` payload (and you will then have the utils): ./build grub Among other things, this will produce a `cbfstool` executable under any of the subdirectories in `src/coreboot/` under `util/cbfstool/cbfstool`. For example: `src/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: ./update trees -b flashrom An executable will be available at `src/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 ./src/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 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](../install/spi.md) Extract grub.cfg ================ Releases *after* Libreboot 20230625 ----------------------------------- Releases after Libreboot 20230625 contain `grub.cfg` inside the GRUB memdisk, inaccessible directly from CBFS, but the memdisk is inside `grub.elf` which gets put inside CBFS. An override is possible, on these Libreboot revisions. If `grub.cfg` is present in CBFS, Libreboot's GRUB will use *that* and not the memdisk one; it will not auto-switch to `grubtest.cfg`, but the test config will be available in the menu to switch to, if present. You can find `grub.cfg` under lbmk (for this purpose, it's best to use the lbmk one, not the release one - unless you're using a release after 20230625). Find it at path: `config/grub/config/grub.cfg`. So, you can *add* `grubtest.cfg` as normal, test that, and then *add* `grub.cfg` once you're happy, and it will override the default. Libreboot 20230625 and below: ---------------------------- libreboot images that use the GRUB bootloader will have *two* configuration files in CBFS: * `grub.cfg` * `grubtest.cfg` 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 =================== NOTE: As stated above, releases after Libreboot 20230625 only default to the config in memdisk, and lack a CBFS config, so you can skip the *remove* step below and just directly add the new `grub.cfg` - unless you already added one before, in which case removal is required first. 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.