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modGRUBShell.efi.md

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Introduction

With a modified GRUB shell it's possible to change the value of the variable previously found.

Fire up modGRUBShell.efi:

  • from a UEFI shell and navigate the FS (with cd and ls basic UNIX navigation commands) and find the EFI partition where it's located modGRUBShell.efi.
  • from OpenCore start the boot entry modGRUBShell.efi (by adding it to config.plist under Misc/Tools context)

You'll end up in a custom shell

Step 1: read the current value of the offset

There are different ways to interact with the desired offset, by using different commands. I'll list them below:

setup_var(2/_3) command

The syntax of the command is as it follows: setup_var <OFFSET>

Run it but in case you get an error like error: offset 0xXYZ is out of range use the following commands:

  • setup_var2 0xXYZ (if this doesn't work go with the next)
  • setup_var_3 0xYXZ

The difference between each of these commands is the VarStore name where the offset are looked in:

  • setup_var looks inside Setup
  • setup_var2 looks inside Custom (usually shitty InsydeH2O BIOS may have it)
  • setup_var3 looks inside Setup but discards variables that are too small (currently the threshold is 0x10 bytes), which allows skipping dummy varstores

setup_var_cv command

Recent firmwares often stores BIOS settings into multiple varstores, and sometimes most of them are not in default "Setup" name. setup_var_cv allows accessing varying size variables in varstore with the given name.

The usage is as it follows:

setup_var_cv nameOfVarStore offsetInVarStore [optional variable size] [optional value to write]

e.g. CFG Lock offset is 0x527 under Setup VarStoreName, with size of 1 and 0x0 as the value to write in the offset.

The resulting command will be setup_var_cv CpuSetup 0x43 0x01 0x00

Which command should I use first?

I'd recommend starting from setup_var(2/_3) and in case you have issues use setup_var_cv.

Conclusions

Once changed the offset value turn off your PC and turn it on again. Soft-reboot are not valid, therefore you need a cold reboot.

Credits