First implementation of subuid/subgid manipulation module

nixos/modules/config/users-groups.nix

@@ -100,6 +100,36 @@ let
         description = "The path to the user's shell.";
       };
 
+      subUidRanges = mkOption {
+        type = types.listOf types.optionSet;
+        default = [];
+        example = [
+          { startUid = 1000; count = 1; }
+          { startUid = 100001; count = 65534; }
+        ];
+        options = [ subordinateUidRange ];
+        description = ''
+          Subordinate user ids that user is allowed to use.
+          They are set into <filename>/etc/subuid</filename> and are used
+          by <literal>newuidmap</literal> for user namespaces.
+        '';
+      };
+
+      subGidRanges = mkOption {
+        type = types.listOf types.optionSet;
+        default = [];
+        example = [
+          { startGid = 100; count = 1; }
+          { startGid = 1001; count = 999; }
+        ];
+        options = [ subordinateGidRange ];
+        description = ''
+          Subordinate group ids that user is allowed to use.
+          They are set into <filename>/etc/subgid</filename> and are used
+          by <literal>newgidmap</literal> for user namespaces.
+        '';
+      };
+
       createHome = mkOption {
         type = types.bool;
         default = false;
@@ -211,6 +241,36 @@ let
 
   };
 
+  subordinateUidRange = {
+    startUid = mkOption {
+      type = types.int;
+      description = ''
+        Start of the range of subordinate user ids that user is
+        allowed to use.
+      '';
+    };
+    count = mkOption {
+      type = types.int;
+      default = 1;
+      description = ''Count of subordinate user ids'';
+    };
+  };
+
+  subordinateGidRange = {
+    startGid = mkOption {
+      type = types.int;
+      description = ''
+        Start of the range of subordinate group ids that user is
+        allowed to use.
+      '';
+    };
+    count = mkOption {
+      type = types.int;
+      default = 1;
+      description = ''Count of subordinate group ids'';
+    };
+  };
+
   getGroup = gname:
     let
       groups = mapAttrsToList (n: g: g) (
@@ -265,6 +325,20 @@ let
     ))
   );
 
+  mkSubuidEntry = user: concatStrings (
+    map (range: "${user.name}:${toString range.startUid}:${toString range.count}\n")
+        user.subUidRanges);
+
+  subuidFile = concatStrings (map mkSubuidEntry (
+    sortOn "uid" (attrValues cfg.extraUsers)));
+
+  mkSubgidEntry = user: concatStrings (
+    map (range: "${user.name}:${toString range.startGid}:${toString range.count}\n")
+        user.subGidRanges);
+
+  subgidFile = concatStrings (map mkSubgidEntry (
+    sortOn "uid" (attrValues cfg.extraUsers)));
+
   # If mutableUsers is true, this script adds all users/groups defined in
   # users.extra{Users,Groups} to /etc/{passwd,group} iff there isn't any
   # existing user/group with the same name in those files.
@@ -504,6 +578,15 @@ in {
     # for backwards compatibility
     system.activationScripts.groups = stringAfter [ "users" ] "";
 
+    environment.etc."subuid" = {
+      text = subuidFile;
+      mode = "0644";
+    };
+    environment.etc."subgid" = {
+      text = subgidFile;
+      mode = "0644";
+    };
+
     assertions = [
       { assertion = !cfg.enforceIdUniqueness || (uidsAreUnique && gidsAreUnique);
         message = "uids and gids must be unique!";