nixos/shadow: use su from sudo-rs when enabled (#495216)

nixos/modules/programs/shadow.nix

@@ -22,6 +22,13 @@ in
       '';
     };
 
+    security.shadow.su.package = lib.mkPackageOption pkgs [ "shadow" "su" ] {
+      extraDescription = ''
+        This can be overridden by other modules (e.g. sudo-rs) to provide
+        an alternative `su` implementation.
+      '';
+    };
+
     security.loginDefs = {
       package = lib.mkPackageOption pkgs "shadow" { };
 
@@ -262,7 +269,7 @@ in
           };
         in
         {
-          su = mkSetuidRoot "${cfg.package.su}/bin/su";
+          su = mkSetuidRoot "${config.security.shadow.su.package}/bin/su";
           sg = mkSetuidRoot "${cfg.package.out}/bin/sg";
           newgrp = mkSetuidRoot "${cfg.package.out}/bin/newgrp";
           newuidmap = mkSetuidRoot "${cfg.package.out}/bin/newuidmap";

nixos/modules/security/sudo-rs.nix

@@ -215,6 +215,8 @@ in
     ];
     security.sudo.enable = lib.mkDefault false;
 
+    security.shadow.su.package = lib.mkDefault cfg.package;
+
     security.sudo-rs.extraRules =
       let
         defaultRule =

nixos/tests/shadow.nix

@@ -171,5 +171,10 @@ in
         shadow.wait_for_file("/tmp/leo")
         assert "leo" in shadow.succeed("cat /tmp/leo")
         shadow.send_chars("logout\n")
+
+    with subtest("su wrapper should point to shadow by default"):
+        output = shadow.succeed("grep -aoP '/nix/store/[a-z0-9]{32}-[^\\x00]+' /run/wrappers/bin/su | head -1").strip()
+        assert "shadow" in output, \
+            f"su should come from shadow, but points to: {output}"
   '';
 }

nixos/tests/sudo-rs.nix

@@ -162,5 +162,10 @@ in
 
       with subtest("non-wheel users should be unable to run sudo thanks to execWheelOnly"):
           strict.fail('faketty -- su - noadmin -c "sudo --help"')
+
+      with subtest("su should come from sudo-rs"):
+          output = machine.succeed("grep -aoP '/nix/store/[a-z0-9]{32}-[^\\x00]+' /run/wrappers/bin/su | head -1").strip()
+          assert "sudo-rs" in output, \
+              f"su should come from sudo-rs, but points to: {output}"
     '';
 }