cbwww/site/docs/gnulinux/grub_cbfs.md

Releases or or after Canoeboot 20231026 contain grub.cfg inside GRUB memdisk, inaccessible directly from CBFS, but the memdisk is inside grub.elf which gets put inside CBFS.

An override is possible, on new Canoeboot revisions. If grub.cfg is present in CBFS, Canoeboot'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. This contrasts older behaviour in Libreboot 20230625.

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.

In Libreboot 20230625 and older, GRUB memdisk always loaded the config from CBFS, which you would have to replace; this meant it was possible to brick your GRUB installation if you accidentally flashed without grub.cfg. In Canoeboot 20231026 and higher, such error is impossible; this behaviour is also present in Libreboot 20231021, upon which Canoeboot 20231026 is based.

This new behaviour is therefore much safer, under user error. However, it's still possible to flash a bad grub.cfg by misconfiguring it, which is why you should add grubtest.cfg first; when present, it will be available in the default menu, for testing, but the GRUB memdisk config will always be used first if no grub.cfg (as opposed to grubtest.cfg) exists in CBFS.

Insert new grub.cfg

You can find the default config under config/grub/config/grub.cfg in the Canoeboot build system,

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 Canoeboot 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.