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"Reboot to {OS}" scripts for rEFInd Next Boot selection

Reboot to {OS}

This a collection of notes and files used in my quest to create "Reboot to Windows" and "Reboot to Linux" scripts (and desktop shortcuts) for Linux and Windows respectively that automatically reboot my system and instruct rEFInd to auto-select the appropriate OS entry.

General Information

The key for achieving this is to modify the EFI Variable PreviousBoot with GUID 36d08fa7-cf0b-42f5-8f14-68df73ed3740, which rEFInd uses to store the last entry selected in the menu and, if using the + default entry, will be used to select the default OS. By doing this, we trick rEFInd into booting the OS we choose without having to be physically there to press the keyboard.

This variable seems to use the following format:

  • 4 bytes, 07 00 00 00 (although Windows ignores this)
  • The text string of the entry, in UTF-16 Little Endian (no BOM)
  • 4 bytes, 20 00 00 00 (effectively: a space and a NUL character)

The variable doesn't need to contain the full text of the entry, either: Any substring will match. I don't know what rEFInd does in case of multiple matches; I believe it stops after the first. It's up to you to put everything in there or just a substring.

Select Next Boot OS from Linux

Linux exposes all EFI variables via efivarfs in the directory /sys/firmware/efi/efivars/, with file names {NAME}-{GUID}. Specifically, the relevant variable is at /sys/firmware/efi/efivars/PreviousBoot-36d08fa7-cf0b-42f5-8f14-68df73ed3740. These files contain the value of the variable in NVRAM and can be modified (by root only). Most of them will have the immutable flag set, to prevent errors, so you must call chattr -i /path/to/efivar before attempting to modify them.

This is enough to edit the default rEFInd entry: Write to the efivar file with the format specified in the previous point and the name of the entry you want selected and you're done. See the linux.refind-next-boot.py file below for a ready to use script that will set the value to this variable to its first command line argument with the appropriate format.

If you place that script (renamed to refind-next-boot) in your $PATH and give it the appropriate file permissions, you can just run:

sudo refind-next-boot 'Microsoft'
systemctl reboot

Those two commands can be conviniently placed in a script or desktop launcher so that you can reboot to Windows directly. You might want to add yourself to the sudoers file so that you can run that command with no password, in wich case remember to adequately secure the script: Set root as its owner and group and set permissions to 0755 or more restrictive.

And this is it. That was the easy part.

Select Next Boot OS from Windows

Ok, this is where it gets tricky. Windows has no way of giving you access to the EFI variables other than using the Windows API, specifically via GetFirmwareEnvironmentVariable/SetFirmwareEnvironmentVariable. These functions bot receive the name of the variable, its GUID surrounded by curly braces, a buffer to read/write from/to, respectively, and the length of the buffer or the data.

To call those two functions, the running process needs elevated privileges and a modification to the user access token, which aparently is a thing in Windows. All of this is only available via the Windows API, of course, so you'll need to write some C/C++ code.

Below is a script program that works essentially like the python script but for Windows. It needs to be compiled, which I painfully did using Visual Studio, a experience I wouldn't want to repeat. It works the same: Just call it with the name of the entry you want to boot or a substring of that set. Afterwards, you are free to power off or shut down your system using whatever method and rEFInd will just select the correct entry.

Of course, Windows being Windows, creating a desktop shortcut that has an icon and is just double-click-and-forget is a bit more tricky than the Linux equivalent. First, you'll need to place the compiled program someplace and set it to run as administrator (right click, Propertied, Compatibility, check Run as administrator). After that, in that same folder, create a .bat file that calls our program and restarts:

refind-next-boot "linux"
shutdown -t 0 -r

Now create a shortcut to that .bat file, place it in your desktop, give it a proper icon and name and voilà, a "Reboot to Linux" button! It will bother you with a few console windows and a UAC dialog, yes, but it's better than nothing.

#!/usr/bin/env python3
import subprocess
import sys
EFIVAR_NAME = 'PreviousBoot'
EFIVAR_GUID = '36d08fa7-cf0b-42f5-8f14-68df73ed3740'
EFIVAR_PREFIX = '/sys/firmware/efi/efivars'
PREFIX = b'\x07\x00\x00\x00'
SUFFIX = b'\x20\x00\x00\x00'
if len(sys.argv) != 2:
print('error: must pass exactly one argument', file=sys.stderr)
sys.exit(1)
text = sys.argv[1]
filename = '{}/{}-{}'.format(EFIVAR_PREFIX, EFIVAR_NAME, EFIVAR_GUID)
retcode = subprocess.call(['chattr', '-i', filename])
if retcode != 0:
sys.exit(42 + retcode)
with open(filename, 'wb') as f:
content = PREFIX + bytes(text, 'utf-16-le') + SUFFIX
f.write(content)
#include <windows.h>
#include <strsafe.h>
#include <iostream>
const LPCTSTR STR_VARNAME = L"PreviousBoot";
const LPCTSTR STR_VARGUID = L"{36d08fa7-cf0b-42f5-8f14-68df73ed3740}";
void ErrorExit() {
DWORD error = GetLastError();
LPTSTR errorText = nullptr;
FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
nullptr, error, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPTSTR) &errorText, 0, nullptr);
std::wcerr << L"Error " << error << L": " << errorText << std::endl;
LocalFree(errorText);
ExitProcess(error);
}
int main(int argc, char** argv) {
if (argc != 2) {
std::wcerr << L"Error: Must have exactly one command line argument" << std::endl;
ExitProcess(1);
}
/* get the privileges necessary */
HANDLE hToken;
if (!OpenProcessToken(GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES | TOKEN_QUERY, &hToken)) {
ErrorExit();
}
TOKEN_PRIVILEGES tkp;
LookupPrivilegeValue(nullptr, SE_SYSTEM_ENVIRONMENT_NAME, &tkp.Privileges[0].Luid);
tkp.PrivilegeCount = 1;
tkp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
AdjustTokenPrivileges(hToken, FALSE, &tkp, 0, nullptr, 0);
if (GetLastError() != ERROR_SUCCESS) {
ErrorExit();
}
/* construct the efivar content */
char* sStr = argv[1];
DWORD nStrSize = strlen(sStr);
DWORD nVarSize = 4 + (2 * nStrSize);
BYTE* lpVarData = (BYTE*)LocalAlloc(LPTR, nVarSize);
lpVarData[nVarSize - 4] = 0x20;
for (DWORD i = 0; i < nStrSize; i++) {
lpVarData[(2 * i)] = sStr[i];
lpVarData[1 + (2 * i)] = 0x00;
}
/* write the efivar contents to the efivar */
DWORD dwSetResult = SetFirmwareEnvironmentVariable(STR_VARNAME, STR_VARGUID, lpVarData, nVarSize);
if (!dwSetResult) {
ErrorExit();
}
}
@jordanwilson230

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commented Jul 16, 2018

@Darkhogg How did you compile the code? The errors I am getting, for example:

error C2440: 'initializing': cannot convert from 'const wchar_t [13]' to 'LPCTSTR'

...and similar. I dont know C++ but it would be of great use if I was able to get it working.

@opcon

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commented Jul 31, 2018

@jordanwilson230 I don't know if you've fixed this already, but you need to change the CharacterSet in your project settings to Unicode, see https://stackoverflow.com/a/13977501

@Darkhogg

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commented Sep 27, 2018

Oh, I didn't notice I had comments here!

@jordanwilson230 I don't remember if I'm being honest, It was a trial and error process for the most part. As soon as I had a working binary, I copy-pasted the code here and never touched it again. Sorry. I might try to tweak this file so it's more easily compiled without having to use VS, it's something I've wanted to do for a while.

@skannerdarkly

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commented Dec 13, 2018

It worked for me! Thanks for the great codes. I was able to compile without much of an issue after downloading Visual Studio community (for free). Created a simple .bat file:
refind-next-boot.exe "vmlinux"
(sudo is not a windows command) and I was off and running.

@ashehata

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commented Dec 27, 2018

Thanks! I was able to get this to work by just using "printf "<HEX_BYTES>" > " and it works perfectly. Now my question is do you think it's possible to also set a temporary timeout of 0 for that next OS since we already know we want it?

@jamezrin

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commented Jan 5, 2019

Instead of using modifying the efivar it's possible to set use_nvram to false in the refind configuration and the previous boot will be stored in vars/PreviousBoot. I'm not sure if it's better to make refind use the efivars or not, but that's another alternative.

By the way, thanks for this guide, you did an awesome job.

@Darkhogg

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commented Mar 5, 2019

@skannerdarkly glad to hear that! I did the same plus a few indirections to avoid the UAC dialog, creating a service and a few other things.
@ashehata I don't think so, unless you edit the refind config when running this AND after starting the system, and I wish you the best of luck modifying the ESP from Windows... Or maybe contribute to rEFInd, I don't think it would be very hard to do, but their code is in SourceForge and a bit messy, so yeah...
@jamezrin OH, I didn't know that, I will investigate. Is it a new feature? I don't recall reading about that when I did this...

@cyperous

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commented Apr 2, 2019

I can't get this to work in Windows 10, I tried first compiling in linux with the following command.

x86_64-w64-mingw32-g++ -municode -static -o refind-next-boot.exe refind-next-boot.cpp but I had to change main to wmain

It compiled without issue but after getting no errors when running rewind-next-boot "ubuntu" I restarted only to find out that I was back in windows like the program did nothing.

I then tried installing VS on a different machine and changing the character set to Unicode and this required no changes to the code. copy the compiled exe to the machine I was using and still nothing (restarted back into windows).

I even went a step further and made a binary copy of the efivars from linux and modified the code in windows to actully write that same binary file to the efivars using this program and even verified the value using GetFirmwareEnvironmentVariable and same thing restarted into Windows again. Right now the only way to get back to Ubuntu is to have direct access to the machine :-( sad face.

If anyone has any ideas why this won't work I'm up for suggestions at this point I've spent to much time screwing around with this. I'm giving up.

@cyperous

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commented Apr 3, 2019

I found a different solutions that doesn't involve using nvram and setting environment and uses the refind configuration file.

I added a file called default.conf to the EFIs refind folder and included the file in refind.conf

then I wrote an app that modifies the default.conf file to set the default boot selection.

Here is the link to the GitHub project refind-next-boot

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