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make grub_cbfs.md a bit clearer 

a lot of the instructions are really old

make it relevant to the way libreboot currently works

some legacy information has been removed, pertaining to
libreboot 20230625 which is oldstable; 20240612 is the
current stable release, so the 20230625 information no
longer needs to be written here.

Signed-off-by: Leah Rowe <info@minifree.org>
master
Leah Rowe 2024-08-26 22:36:21 +01:00
parent c6fde88de7
commit 30cae6536a
1 changed files with 62 additions and 132 deletions
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194
site/docs/linux/grub_cbfs.md
View File

@ -9,26 +9,13 @@ now, as of 27 January 2024, which is a fork of flashrom.
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).
Libreboot's own GRUB configuration automatically scans for one provided by
your distro, and this automation will usually work. Sometimes, you might wish
to override it with your own custom menuentry or additional logic in the GRUB
config. You can configure GRUB however you like, and this topic is vast so what
to actually *put in the config* will not be covered here.
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.
This guide will simply teach you how to modify the config, but not what to put.
Compile flashprog and cbfstool
=============================
@ -55,20 +42,28 @@ 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
Install the build dependencies. For Debian and similar, you can run
the following command in the libreboot build system, from the root directory
of the libreboot Git repository.
./mk dependencies ubuntu2004
./mk dependencies debian
Then, download coreboot:
Determine what coreboot tree you need for your board. For example, if building
for `x200_8mb`, check `config/coreboot/x200_8mb/target.cfg` and it might
say `tree="default"` - in this case, the coreboot tree is named `default`.
./mk -f coreboot
Then, download coreboot (we'll assume the `default` tree is correct):
./mk -f coreboot default
Finally, compile the `cbutils` payload (and you will then have the utils):
./mk -b grub
GRUB is multi-tree, but for GRUB utilities that's fine because we don't patch
those in any tree; we only patch the GRUB kernel to add various drivers and
extra crypto such as argon2.
Among other things, this will produce a `cbfstool` executable under any of the
subdirectories in `src/coreboot/` under `util/cbfstool/cbfstool`.
@ -78,6 +73,13 @@ 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 can compile cbfstool and ifdtool for the given coreboot tree, e.g.:
./mk -d coreboot default
This will create `elf/cbfstool/default/cbfbstool`
and `elf/ifdtool/default/ifdtool`.
You will also want to build `flashprog` 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:
@ -90,125 +92,53 @@ this.
Dump the boot flash
===================
If you wish to modify your *existing* libreboot ROM, which was installed on
your computer, you can use `flashprog` to acquire it.
[Learn how to externally reprogram these chips](../install/spi.md) and use
the `-r` option in flashprog; alternatively, for internal flash access,
look at the [main flashing guide](../install/).
Simply run the following, after using libreboot's build system to compile
flashprog:
sudo ./elf/flashprog/flashprog -p internal -r dump.bin
If flashprog 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 flashprog, 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
================
Libreboot 20231021 or newer
-----------------------------------
Releases or or after 20231021 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 image).
Find it at path (in current lbmk): `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 or older
----------------------------
NOTE: This information will probably be deleted after a certain time has passed.
Libreboot changed a lot, as of release 20231021 in reference to 20230625, so it
may take a while before people adjust; therefore, this information is provided
for reference, but you should consider it to be deprecated:
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.
Those guides show how to dump the flash contents, which you are advised to do.
Insert new grub.cfg
===================
NOTE: As stated above, releases on or after Libreboot 20231021 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.
If you already have a `grub.cfg` in cbfstool, you can extract and modify that
one, e.g.:
Remove the old `grub.cfg` (substitute with `grubtest.cfg` as desired):
cbfstool libreboot.rom extract -n grub.cfg -f grub.cfg
cbfstool dump.bin remove -n grub.cfg
Now remove it:
Add your modified `grub.cfg` (substitute with `grubtest.cfg` as desired):
cbfstool libreboot.rom remove -n grub.cfg
cbfstool dump.bin add -f grub.cfg -n grub.cfg -t raw
It's important that you re-add `grub.cfg` before flashing:
cbfstool libreboot.rom add -f grub.cfg -n grub.cfg
Repeat this for `grubtest.cfg` if you wish.
If you're using a default Libreboot image, there *is no `grub.cfg` in flash*.
This is because there is also a default one embedded inside the GRUB binary,
which is inside CBFS. So:
The coreboot image has its own filesystem, CBFS, and within CBFS is the GRUB
binary, and within the GRUB binary is another filesystem called memdisk, where
the default GRUB configuration is located.
You can insert a `grub.cfg` into CBFS and it will override the one in memdisk.
Check your board, e.g. `x200_8mb`, look at the file: `config/coreboot/x200_8mb/target.cfg`
and check for `grubtree` - if it's not sot, then it's `default`, otherwise check
what it's set to.
We'll assume it's `default`, so the
file `config/grub/default/config/payload` is your GRUB config; this will be the
same as what yo uhave in memdisk.
Modify *that* file, or the one you extracted if you already inserted a custom
one before, and you will re-insert it when you're done.
Flash the modified ROM image
============================
Your modified `dump.bin` or other modified libreboot ROM can then be re-flashed
using:
sudo ./flashprog -p internal -w dump.bin
If a `-c` option is required, use it and specify a flash chip name. This is
only useful when `flashprog` complains about multiple flash chips being
detected.
If flashprog 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 ./flashprog -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.
Check the [Libreboot flashing guide](../install/) which says how to flash the
new image.