From e5b072eca165430efc4d7a179011a42aab4470a2 Mon Sep 17 00:00:00 2001
From: Ivan Trubach <mr.trubach@icloud.com>
Date: Fri, 3 Mar 2023 02:31:38 +0300
Subject: [PATCH] nixos/iso-image: add an option to disable BIOS boot

This change adds an option to disable legacy BIOS boot support for ISO
images. The implementation uses syslinux package that currently does not
support non-x86 platforms and thus cannot be cross-compiled, e.g. from
AArch64 system.
---
 .../installer/cd-dvd/installation-cd-base.nix |  3 +++
 nixos/modules/installer/cd-dvd/iso-image.nix  | 19 +++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/nixos/modules/installer/cd-dvd/installation-cd-base.nix b/nixos/modules/installer/cd-dvd/installation-cd-base.nix
index 3f92b779d60a..3c7c7e30a0bf 100644
--- a/nixos/modules/installer/cd-dvd/installation-cd-base.nix
+++ b/nixos/modules/installer/cd-dvd/installation-cd-base.nix
@@ -21,6 +21,9 @@ with lib;
   # ISO naming.
   isoImage.isoName = "${config.isoImage.isoBaseName}-${config.system.nixos.label}-${pkgs.stdenv.hostPlatform.system}.iso";
 
+  # BIOS booting
+  isoImage.makeBiosBootable = true;
+
   # EFI booting
   isoImage.makeEfiBootable = true;
 
diff --git a/nixos/modules/installer/cd-dvd/iso-image.nix b/nixos/modules/installer/cd-dvd/iso-image.nix
index 86c2cd1a8c73..0245a522f7e1 100644
--- a/nixos/modules/installer/cd-dvd/iso-image.nix
+++ b/nixos/modules/installer/cd-dvd/iso-image.nix
@@ -535,6 +535,13 @@ in
       '';
     };
 
+    isoImage.makeBiosBootable = mkOption {
+      default = false;
+      description = lib.mdDoc ''
+        Whether the ISO image should be a BIOS-bootable disk.
+      '';
+    };
+
     isoImage.makeEfiBootable = mkOption {
       default = false;
       description = lib.mdDoc ''
@@ -693,7 +700,7 @@ in
     boot.loader.grub.enable = false;
 
     environment.systemPackages =  [ grubPkgs.grub2 grubPkgs.grub2_efi ]
-      ++ optional canx86BiosBoot pkgs.syslinux
+      ++ optional (config.isoImage.makeBiosBootable && canx86BiosBoot) pkgs.syslinux
     ;
 
     # In stage 1 of the boot, mount the CD as the root FS by label so
@@ -744,7 +751,7 @@ in
         { source = pkgs.writeText "version" config.system.nixos.label;
           target = "/version.txt";
         }
-      ] ++ optionals canx86BiosBoot [
+      ] ++ optionals (config.isoImage.makeBiosBootable && canx86BiosBoot) [
         { source = config.isoImage.splashImage;
           target = "/isolinux/background.png";
         }
@@ -771,7 +778,7 @@ in
         { source = config.isoImage.efiSplashImage;
           target = "/EFI/boot/efi-background.png";
         }
-      ] ++ optionals (config.boot.loader.grub.memtest86.enable && canx86BiosBoot) [
+      ] ++ optionals (config.boot.loader.grub.memtest86.enable && config.isoImage.makeBiosBootable && canx86BiosBoot) [
         { source = "${pkgs.memtest86plus}/memtest.bin";
           target = "/boot/memtest.bin";
         }
@@ -786,10 +793,10 @@ in
     # Create the ISO image.
     system.build.isoImage = pkgs.callPackage ../../../lib/make-iso9660-image.nix ({
       inherit (config.isoImage) isoName compressImage volumeID contents;
-      bootable = canx86BiosBoot;
+      bootable = config.isoImage.makeBiosBootable && canx86BiosBoot;
       bootImage = "/isolinux/isolinux.bin";
-      syslinux = if canx86BiosBoot then pkgs.syslinux else null;
-    } // optionalAttrs (config.isoImage.makeUsbBootable && canx86BiosBoot) {
+      syslinux = if config.isoImage.makeBiosBootable && canx86BiosBoot then pkgs.syslinux else null;
+    } // optionalAttrs (config.isoImage.makeUsbBootable && config.isoImage.makeBiosBootable && canx86BiosBoot) {
       usbBootable = true;
       isohybridMbrImage = "${pkgs.syslinux}/share/syslinux/isohdpfx.bin";
     } // optionalAttrs config.isoImage.makeEfiBootable {