A modified grub allowing tweaking hidden BIOS settings.
based on grub with setup_var patch (invalid link now) and setup_var2 patch with setup_var_3 patch as a wordaround to duplicate Setup vairable.
As said in a guide about changing hidden "CFG Lock" BIOS setting, by using a modified GRUB shell, we can change any hidden UEFI BIOS settings. But here comes some errors on my Dell XPS 8930, so I have the shell patched and added a new command setup_var_3 for this situation.
On my PC, when I try to read/write values using the method above. It obtained a Setup variable but producing an error saying the offset is out of range. With extra investigation, I found there are two Setup variables in BIOS and the error comes from the very small (only 9 bytes in my situation) Setup variable.
With setup_var (without parameters), it said:
var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
...
var name: Setup, var size: 12, var guid: ec87d643-eba4-4bb5 - ...
So there are two Setup variables, and if using setup_var 0xC2 (the offset could be any value larger than first Setup variable size):
var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
...
successfully obtained "Setup" variable from VSS (got 9 (0x9) bytes).
error: offset is out of range
Here comes the error, but if using a smaller offset (smaller than first variable's size):
var name: Setup, var size: 12, var guid: 80e1202e-2697-4264 - ...
...
successfully obtained "Setup" variable from VSS (got 9 (0x9) bytes).
offset 0x01 is: 0x15
var name: Setup, var size: 12, var guid: ec87d643-eba4-4bb5 - ...
...
successfully obtained "Setup" variable from VSS (got 4304 (0x10d0) bytes).
offset 0x01 is: 0x00
So without the error, we can get correct value from the "real", second "Setup" variable. This is what the patch comes for.
This patch on two patches above adds the third setup_var command: setup_var_3. It supresses the error when the Setup variable is too small and doesn't look like a real Setup variable to avoid error terminating the program resulting in cannot access real Setup variable with large offset.
USE WITH CAUTION AND ENSURE YOU HAVE EXAMINED YOU ARE ACCESSING RIGHT SETUP VARIABLE OR YOU WILL RISK BRICKING YOUR COMPUTER!!!
As this is based on a relatively old grub version (2.00+20130519), I used Ubuntu 16.04 LTS with gcc 4.8 to compile it. (I haven't tested gcc 4.9 but gcc 5 won't work).
Build:
cd grub-2.00+20130519
./autogen.sh
./configure --with-platform=efi --prefix=<temporary install prefix>
make
make install
Generating modified GRUB shell:
cd <temporary install prefix>
./bin/mkstandalone -O x86_64-efi -o modGRUBShell.efi