systemd-stage-1: Enable backdoor in nixos tests

nixos/doc/manual/release-notes/rl-2311.section.md

@@ -554,3 +554,5 @@ The module update takes care of the new config syntax and the data itself (user
 - `teleport` has been upgraded from major version 12 to major version 14. Please see upstream [upgrade instructions](https://goteleport.com/docs/management/operations/upgrading/) and release notes for versions [13](https://goteleport.com/docs/changelog/#1300-050823) and [14](https://goteleport.com/docs/changelog/#1400-092023). Note that Teleport does not officially support upgrades across more than one major version at a time. If you're running Teleport server components, it is recommended to first upgrade to an intermediate 13.x version by setting `services.teleport.package = pkgs.teleport_13`. Afterwards, this option can be removed to upgrade to the default version (14).
 
 - The Linux kernel module `msr` (see [`msr(4)`](https://man7.org/linux/man-pages/man4/msr.4.html)), which provides an interface to read and write the model-specific registers (MSRs) of an x86 CPU, can now be configured via `hardware.cpu.x86.msr`.
+
+- There is a new NixOS option when writing NixOS tests `testing.initrdBackdoor`, that enables `backdoor.service` in initrd. Requires `boot.initrd.systemd.enable` to be enabled. Boot will pause in stage 1 at `initrd.target`, and will listen for commands from the `Machine` python interface, just like stage 2 normally does. This enables commands to be sent to test and debug stage 1. Use `machine.switch_root()` to leave stage 1 and proceed to stage 2.

nixos/lib/test-driver/test_driver/machine.py

@@ -1278,3 +1278,19 @@ class Machine:
     def run_callbacks(self) -> None:
         for callback in self.callbacks:
             callback()
+
+    def switch_root(self) -> None:
+        """
+        Transition from stage 1 to stage 2. This requires the
+        machine to be configured with `testing.initrdBackdoor = true`
+        and `boot.initrd.systemd.enable = true`.
+        """
+        self.wait_for_unit("initrd.target")
+        self.execute(
+            "systemctl isolate --no-block initrd-switch-root.target 2>/dev/null >/dev/null",
+            check_return=False,
+            check_output=False,
+        )
+        self.wait_for_console_text(r"systemd\[1\]:.*Switching root\.")
+        self.connected = False
+        self.connect()

nixos/modules/testing/test-instrumentation.nix

@@ -6,10 +6,15 @@
 with lib;
 
 let
+  cfg = config.testing;
+
   qemu-common = import ../../lib/qemu-common.nix { inherit lib pkgs; };
 
   backdoorService = {
-    wantedBy = [ "multi-user.target" ];
+    wantedBy = [ "sysinit.target" ];
+    unitConfig.DefaultDependencies = false;
+    conflicts = [ "shutdown.target" "initrd-switch-root.target" ];
+    before = [ "shutdown.target" "initrd-switch-root.target" ];
     requires = [ "dev-hvc0.device" "dev-${qemu-common.qemuSerialDevice}.device" ];
     after = [ "dev-hvc0.device" "dev-${qemu-common.qemuSerialDevice}.device" ];
     script =
@@ -18,7 +23,9 @@ let
         export HOME=/root
         export DISPLAY=:0.0
 
+        if [[ -e /etc/profile ]]; then
             source /etc/profile
+        fi
 
         # Don't use a pager when executing backdoor
         # actions. Because we use a tty, commands like systemctl
@@ -49,9 +56,59 @@ in
 
 {
 
+  options.testing = {
+
+    initrdBackdoor = lib.mkEnableOption (lib.mdDoc ''
+      enable backdoor.service in initrd. Requires
+      boot.initrd.systemd.enable to be enabled. Boot will pause in
+      stage 1 at initrd.target, and will listen for commands from the
+      Machine python interface, just like stage 2 normally does. This
+      enables commands to be sent to test and debug stage 1. Use
+      machine.switch_root() to leave stage 1 and proceed to stage 2.
+    '');
+
+  };
+
   config = {
 
-    systemd.services.backdoor = backdoorService
+    assertions = [
+      {
+        assertion = cfg.initrdBackdoor -> config.boot.initrd.systemd.enable;
+        message = ''
+          testing.initrdBackdoor requires boot.initrd.systemd.enable to be enabled.
+        '';
+      }
+    ];
+
+    systemd.services.backdoor = backdoorService;
+
+    boot.initrd.systemd = lib.mkMerge [
+      {
+        contents."/etc/systemd/journald.conf".text = ''
+          [Journal]
+          ForwardToConsole=yes
+          MaxLevelConsole=debug
+        '';
+
+        extraConfig = config.systemd.extraConfig;
+      }
+
+      (lib.mkIf cfg.initrdBackdoor {
+        # Implemented in machine.switch_root(). Suppress the unit by
+        # making it a noop without removing it, which would break
+        # initrd-parse-etc.service
+        services.initrd-cleanup.serviceConfig.ExecStart = [
+          # Reset
+          ""
+          # noop
+          "/bin/true"
+        ];
+
+        services.backdoor = backdoorService;
+
+        contents."/usr/bin/env".source = "${pkgs.coreutils}/bin/env";
+      })
+    ];
 
     # Prevent agetty from being instantiated on the serial device, since it
     # interferes with the backdoor (writes to it will randomly fail
@@ -107,12 +164,6 @@ in
         MaxLevelConsole=debug
       '';
 
-    boot.initrd.systemd.contents."/etc/systemd/journald.conf".text = ''
-      [Journal]
-      ForwardToConsole=yes
-      MaxLevelConsole=debug
-    '';
-
     systemd.extraConfig = ''
       # Don't clobber the console with duplicate systemd messages.
       ShowStatus=no
@@ -126,8 +177,6 @@ in
       DefaultDeviceTimeoutSec=300
     '';
 
-    boot.initrd.systemd.extraConfig = config.systemd.extraConfig;
-
     boot.consoleLogLevel = 7;
 
     # Prevent tests from accessing the Internet.

nixos/tests/systemd-initrd-simple.nix

@@ -2,16 +2,19 @@ import ./make-test-python.nix ({ lib, pkgs, ... }: {
   name = "systemd-initrd-simple";
 
   nodes.machine = { pkgs, ... }: {
-    boot.initrd.systemd = {
-      enable = true;
-      emergencyAccess = true;
-    };
+    testing.initrdBackdoor = true;
+    boot.initrd.systemd.enable = true;
     virtualisation.fileSystems."/".autoResize = true;
   };
 
   testScript = ''
     import subprocess
 
+    with subtest("testing initrd backdoor"):
+        machine.wait_for_unit("initrd.target")
+        machine.succeed("systemctl status initrd-fs.target")
+        machine.switch_root()
+
     with subtest("handover to stage-2 systemd works"):
         machine.wait_for_unit("multi-user.target")
         machine.succeed("systemd-analyze | grep -q '(initrd)'")  # direct handover
@@ -37,6 +40,7 @@ import ./make-test-python.nix ({ lib, pkgs, ... }: {
         subprocess.check_call(["qemu-img", "resize", "vm-state-machine/machine.qcow2", "+1G"])
 
         machine.start()
+        machine.switch_root()
         newAvail = machine.succeed("df --output=avail / | sed 1d")
 
         assert int(oldAvail) < int(newAvail), "File system did not grow"