Can I set an arbitrary nvram variable using rEFInd?

Question

Is there any way to customise the rEFInd bootloader to set a custom nvram variable, either in refind.conf or any other way? Specifically, I'd like to set a value for the boot-args variable.

It seems like an obvious/standard thing that someone might want to do, and I know that the OpenCore bootloader has a whole NVRAM configuration section to support it, but there doesn't seem to be anything in the docs about it for rEFInd.

Answer 1

rEFInd will not let you set an arbitrary NVRAM variable directly but you can use EDK2 to compile a small EFI driver. This is described in the WikiLeaks document EFI Basics: NVRAM Variables

EFI drivers placed in rEFInd driver directory are loaded and run automatically - see The rEFInd Boot Manager: Using EFI Drivers. Alternatively you could write a EFI program rather than driver and then use a manual stanza in rEFInd to call a shell script to call this and then the bootloader.

Another (probably easier) method would be to modify rEFInd. You can see in the source how it sets the csr-active-config NVRAM variable to turn SIP on and off - apple.h defines the GUID:

// The constants related to Apple's System Integrity Protection (SIP)....
#define CSR_GUID { 0x7c436110, 0xab2a, 0x4bbb, { 0xa8, 0x80, 0xfe, 0x41, 0x99, 0x5c, 0x9f, 0x82 } };

apple.c calls the EfivarSetRaw() function in lib.c

Status = EfivarSetRaw(&CsrGuid, L"csr-active-config", (CHAR8 *) &TargetCsr, 4, TRUE);
if (Status == EFI_SUCCESS)
    RecordgCsrStatus(TargetCsr, TRUE);
else
    SPrint(gCsrStatus, 255, L" Error setting System Integrity Protection code.");

Other NVRAM updates could be made in the same way - there are various other GUID listed in the WikiLeaks link in the first paragraph.

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If you want to pass a boot argument without updating NVRAM you can create a manual boot entry in rEFInd and specify the options in refind.conf. Additional options can be added using add_options - see the Creating Manual Boot Stanzas page of the documentation.

For example you can define submenu options like this and pick between them using F2 at boot.

menuentry "Catalina" {
    # Get loader GUID from macOS subvolume: diskutil info disk1s5|grep 'Volume UUID'
    icon \EFI\refind\themes\colourful\myicons\os_mac_silver.png
    volume "Preboot"
    loader \3B4B18C9-C57D-4F98-9168-C8D8B9F06EAD\System\Library\CoreServices\boot.efi
    submenuentry "Verbose" {
        add_options "-v"
    }
    submenuentry "Single User" {
        add_options "-v -s"
    }
    submenuentry "Safe Mode" {
        add_options "-v -x"
    }
    submenuentry "Recovery" {
        volume "Recovery"
        loader \3B4B18C9-C57D-4F98-9168-C8D8B9F06EAD\boot.efi
    }
}