grub: add boot.loader.grub.efiInstallAsRemovable

Closes #16374

nixos/modules/installer/tools/nixos-generate-config.pl

@@ -527,8 +527,11 @@ EOF
   # Use the GRUB 2 boot loader.
   boot.loader.grub.enable = true;
   boot.loader.grub.version = 2;
+  # boot.loader.grub.efiSupport = true;
+  # boot.loader.grub.efiInstallAsRemovable = true;
+  # boot.loader.efi.efiSysMountPoint = "/boot/efi";
   # Define on which hard drive you want to install Grub.
-  # boot.loader.grub.device = "/dev/sda";
+  # boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only
 EOF
         }
 

nixos/modules/system/boot/loader/grub/grub.nix

@@ -55,7 +55,7 @@ let
       inherit (cfg)
         version extraConfig extraPerEntryConfig extraEntries
         extraEntriesBeforeNixOS extraPrepareConfig configurationLimit copyKernels
-        default fsIdentifier efiSupport gfxmodeEfi gfxmodeBios;
+        default fsIdentifier efiSupport efiInstallAsRemovable gfxmodeEfi gfxmodeBios;
       path = (makeBinPath ([
         pkgs.coreutils pkgs.gnused pkgs.gnugrep pkgs.findutils pkgs.diffutils pkgs.btrfs-progs
         pkgs.utillinux ] ++ (if cfg.efiSupport && (cfg.version == 2) then [pkgs.efibootmgr ] else [])
@@ -357,6 +357,44 @@ in
         '';
       };
 
+      efiInstallAsRemovable = mkOption {
+        default = false;
+        example = true;
+        type = types.bool;
+        description = ''
+          Whether to invoke <literal>grub-install</literal> with
+          <literal>--removable</literal>.
+
+          Unless turn this on, GRUB will install itself somewhere (exactly
+          where depends on other config variables) in
+          <literal>boot.loader.efi.efiSysMountPoint</literal>. If you've set
+          <literal>boot.loader.efi.canTouchEfiVariables</literal> *AND* you
+          are currently booted in UEFI mode, then GRUB will use
+          <literal>efibootmgr</literal> to modify the boot order in the NVRAM's
+          EFI variables of your computer to include this location. If you are
+          *not* booted in UEFI mode at the time grub is being installed, the
+          NVRAM will not be modified, and your system will not find GRUB at
+          boot time. GRUB will still succeed (althgouh you'll see a warning
+          printed "<literal>efibootmgr: EFI variables are not supported on
+          this system.</literal>").
+
+          If you do turn this feature on, then GRUB will install itself
+          in a specific special location within <literal>efiSysMountPoint</literal>
+          (namely <literal>EFI/boot/boot$arch.efi</literal>) which is the firmwares
+          are hardcoded to try first, regardless of NVRAM EFI variables.
+
+          To summarize, turn this on if:
+          <itemizedlist>
+            <listitem>You are installing NixOS and want it to boot in UEFI mode,
+            but you are currently booted in legacy mode</listitem>
+            <listitem>You want to make a drive that will boot regardless of
+            the NVRAM state of the computer (like a USB "removable" drive)</listitem>
+            <listitem>You simply dislike the idea of depending on some NVRAM
+            state to make your drive bootable</listitem>
+          </itemizedlist>
+        '';
+      };
+
       enableCryptodisk = mkOption {
         default = false;
         type = types.bool;
@@ -484,6 +522,14 @@ in
           assertion = !cfg.trustedBoot.enable || cfg.trustedBoot.systemHasTPM == "YES_TPM_is_activated";
           message = "Trusted GRUB can break the system! Confirm that the system has an activated TPM by setting 'systemHasTPM'.";
         }
+        {
+          assertion = cfg.efiInstallAsRemovable -> cfg.efiSupport;
+          message = "If you wish to to use boot.loader.grub.efiInstallAsRemovable, then turn on boot.loader.grub.efiSupport";
+        }
+        {
+          assertion = cfg.efiInstallAsRemovable -> !config.boot.loader.efi.canTouchEfiVariables;
+          message = "If you wish to to use boot.loader.grub.efiInstallAsRemovable, then turn off boot.loader.efi.canTouchEfiVariables";
+        }
       ] ++ flip concatMap cfg.mirroredBoots (args: [
         {
           assertion = args.devices != [ ];

nixos/modules/system/boot/loader/grub/install-grub.pl

@@ -60,6 +60,7 @@ my $grubTargetEfi = get("grubTargetEfi");
 my $bootPath = get("bootPath");
 my $storePath = get("storePath");
 my $canTouchEfiVariables = get("canTouchEfiVariables");
+my $efiInstallAsRemovable = get("efiInstallAsRemovable");
 my $efiSysMountPoint = get("efiSysMountPoint");
 my $gfxmodeEfi = get("gfxmodeEfi");
 my $gfxmodeBios = get("gfxmodeBios");
@@ -544,13 +545,15 @@ if (($requireNewInstall != 0) && ($efiTarget eq "no" || $efiTarget eq "both")) {
 # install EFI GRUB
 if (($requireNewInstall != 0) && ($efiTarget eq "only" || $efiTarget eq "both")) {
     print STDERR "installing the GRUB $grubVersion EFI boot loader into $efiSysMountPoint...\n";
+    my @command = ("$grubEfi/sbin/grub-install", "--recheck", "--target=$grubTargetEfi", "--boot-directory=$bootPath", "--efi-directory=$efiSysMountPoint");
     if ($canTouchEfiVariables eq "true") {
-        system("$grubEfi/sbin/grub-install", "--recheck", "--target=$grubTargetEfi", "--boot-directory=$bootPath", "--efi-directory=$efiSysMountPoint", "--bootloader-id=$bootloaderId") == 0
+        push @command, "--bootloader-id=$bootloaderId";
-                or die "$0: installation of GRUB EFI into $efiSysMountPoint failed\n";
     } else {
-        system("$grubEfi/sbin/grub-install", "--recheck", "--target=$grubTargetEfi", "--boot-directory=$bootPath", "--efi-directory=$efiSysMountPoint", "--no-nvram") == 0
+        push @command, "--no-nvram";
-                or die "$0: installation of GRUB EFI into $efiSysMountPoint failed\n";
+        push @command, "--removable" if $efiInstallAsRemovable eq "true";
     }
+
+    (system @command) == 0 or die "$0: installation of GRUB EFI into $efiSysMountPoint failed\n";
 }