nixos/userborn: init

nixos/modules/module-list.nix

@@ -1347,6 +1347,7 @@
   ./services/system/systembus-notify.nix
   ./services/system/systemd-lock-handler.nix
   ./services/system/uptimed.nix
+  ./services/system/userborn.nix
   ./services/system/zram-generator.nix
   ./services/torrent/deluge.nix
   ./services/torrent/flexget.nix

nixos/modules/services/system/userborn.nix

@@ -0,0 +1,183 @@
+{
+  utils,
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+let
+
+  cfg = config.services.userborn;
+  userCfg = config.users;
+
+  userbornConfig = {
+    groups = lib.mapAttrsToList (username: opts: {
+      inherit (opts) name gid members;
+    }) config.users.groups;
+
+    users = lib.mapAttrsToList (username: opts: {
+      inherit (opts)
+        name
+        uid
+        group
+        description
+        home
+        password
+        hashedPassword
+        hashedPasswordFile
+        initialPassword
+        initialHashedPassword
+        ;
+      isNormal = opts.isNormalUser;
+      shell = utils.toShellPath opts.shell;
+    }) config.users.users;
+  };
+
+  userbornConfigJson = pkgs.writeText "userborn.json" (builtins.toJSON userbornConfig);
+
+  immutableEtc = config.system.etc.overlay.enable && !config.system.etc.overlay.mutable;
+  # The filenames created by userborn.
+  passwordFiles = [
+    "group"
+    "passwd"
+    "shadow"
+  ];
+
+in
+{
+
+  options.services.userborn = {
+
+    enable = lib.mkEnableOption "userborn";
+
+    package = lib.mkPackageOption pkgs "userborn" { };
+
+    passwordFilesLocation = lib.mkOption {
+      type = lib.types.str;
+      default = if immutableEtc then "/var/lib/nixos" else "/etc";
+      defaultText = lib.literalExpression ''if immutableEtc then "/var/lib/nixos" else "/etc"'';
+      description = ''
+        The location of the original password files.
+
+        If this is not `/etc`, the files are symlinked from this location to `/etc`.
+
+        The primary motivation for this is an immutable `/etc`, where we cannot
+        write the files directly to `/etc`.
+
+        However this an also serve other use cases, e.g. when `/etc` is on a `tmpfs`.
+      '';
+    };
+
+  };
+
+  config = lib.mkIf cfg.enable {
+
+    assertions = [
+      {
+        assertion = !(config.systemd.sysusers.enable && cfg.enable);
+        message = "You cannot use systemd-sysusers and Userborn at the same time";
+      }
+      {
+        assertion = config.system.activationScripts.users == "";
+        message = "system.activationScripts.users has to be empty to use userborn";
+      }
+      {
+        assertion = immutableEtc -> (cfg.passwordFilesLocation != "/etc");
+        message = "When `system.etc.overlay.mutable = false`, `services.userborn.passwordFilesLocation` cannot be set to `/etc`";
+      }
+    ];
+
+    system.activationScripts.users = lib.mkForce "";
+    system.activationScripts.hashes = lib.mkForce "";
+
+    systemd = {
+
+      # Create home directories, do not create /var/empty even if that's a user's
+      # home.
+      tmpfiles.settings.home-directories = lib.mapAttrs' (
+        username: opts:
+        lib.nameValuePair opts.home {
+          d = {
+            mode = opts.homeMode;
+            user = username;
+            inherit (opts) group;
+          };
+        }
+      ) (lib.filterAttrs (_username: opts: opts.home != "/var/empty") userCfg.users);
+
+      services.userborn = {
+        wantedBy = [ "sysinit.target" ];
+        requiredBy = [ "sysinit-reactivation.target" ];
+        after = [
+          "systemd-remount-fs.service"
+          "systemd-tmpfiles-setup-dev-early.service"
+        ];
+        before = [
+          "systemd-tmpfiles-setup-dev.service"
+          "sysinit.target"
+          "shutdown.target"
+          "sysinit-reactivation.target"
+        ];
+        conflicts = [ "shutdown.target" ];
+        restartTriggers = [
+          userbornConfigJson
+          cfg.passwordFilesLocation
+        ];
+        # This way we don't have to re-declare all the dependencies to other
+        # services again.
+        aliases = [ "systemd-sysusers.service" ];
+
+        unitConfig = {
+          Description = "Manage Users and Groups";
+          DefaultDependencies = false;
+        };
+
+        serviceConfig = {
+          Type = "oneshot";
+          RemainAfterExit = true;
+          TimeoutSec = "90s";
+
+          ExecStart = "${lib.getExe cfg.package} ${userbornConfigJson} ${cfg.passwordFilesLocation}";
+
+          ExecStartPre = lib.mkMerge [
+            (lib.mkIf (!config.system.etc.overlay.mutable) [
+              "${pkgs.coreutils}/bin/mkdir -p ${cfg.passwordFilesLocation}"
+            ])
+
+            # Make the source files writable before executing userborn.
+            (lib.mkIf (!userCfg.mutableUsers) (
+              lib.map (file: "-${pkgs.util-linux}/bin/umount ${cfg.passwordFilesLocation}/${file}") passwordFiles
+            ))
+          ];
+
+          # Make the source files read-only after userborn has finished.
+          ExecStartPost = lib.mkIf (!userCfg.mutableUsers) (
+            lib.map (
+              file:
+              "${pkgs.util-linux}/bin/mount --bind -o ro ${cfg.passwordFilesLocation}/${file} ${cfg.passwordFilesLocation}/${file}"
+            ) passwordFiles
+          );
+        };
+      };
+    };
+
+    # Statically create the symlinks to passwordFilesLocation when they're not
+    # inside /etc because we will not be able to do it at runtime in case of an
+    # immutable /etc!
+    environment.etc = lib.mkIf (cfg.passwordFilesLocation != "/etc") (
+      lib.listToAttrs (
+        lib.map (
+          file:
+          lib.nameValuePair file {
+            source = "${cfg.passwordFilesLocation}/${file}";
+            mode = "direct-symlink";
+          }
+        ) passwordFiles
+      )
+    );
+  };
+
+  meta.maintainers = with lib.maintainers; [ nikstur ];
+
+}

nixos/modules/system/etc/etc-activation.nix

@@ -19,8 +19,8 @@
           message = "`system.etc.overlay.enable` requires `boot.initrd.systemd.enable`";
         }
         {
-          assertion = (!config.system.etc.overlay.mutable) -> config.systemd.sysusers.enable;
-          message = "`system.etc.overlay.mutable = false` requires `systemd.sysusers.enable`";
+          assertion = (!config.system.etc.overlay.mutable) -> (config.systemd.sysusers.enable || config.services.userborn.enable);
+          message = "`!system.etc.overlay.mutable` requires `systemd.sysusers.enable` or `services.userborn.enable`";
         }
         {
           assertion = lib.versionAtLeast config.boot.kernelPackages.kernel.version "6.6";