Fix: Auto-detect unprivileged user and use XDG_RUNTIME_DIR for default root

.github/linters/urunc-dict.txt

@@ -329,6 +329,7 @@ unikontainer
 unikontainers
 unikraft
 urandom
+userns
 urfave
 urunc
 urunc's

cmd/urunc/main.go

@@ -118,6 +118,18 @@ func main() {
 			// stateCommand,
 		},
 		Before: func(_ context.Context, cmd *cli.Command) (context.Context, error) {
+			if !cmd.IsSet("root") {
+				xdgRuntimeDir := os.Getenv("XDG_RUNTIME_DIR")
+				if xdgRuntimeDir != "" && ShouldHonorXDGRuntimeDir() {
+					root := xdgRuntimeDir + "/urunc"
+					if err := prepareXDGRuntimeDir(root); err != nil {
+						return nil, err
+					}
+					if err := cmd.Set("root", root); err != nil {
+						return nil, err
+					}
+				}
+			}
 			if err := reviseRootDir(cmd); err != nil {
 				return nil, err
 			}

cmd/urunc/utils.go

@@ -22,6 +22,7 @@ import (
 	"os/exec"
 	"syscall"
 
+	"github.com/moby/sys/userns"
 	"github.com/sirupsen/logrus"
 	"github.com/urfave/cli/v3"
 	"github.com/urunc-dev/urunc/pkg/unikontainers"
@@ -130,3 +131,32 @@ func fatalWithCode(err error, ret int) {
 	}
 	os.Exit(ret)
 }
+
+// ShouldHonorXDGRuntimeDir reports whether the runtime should use XDG_RUNTIME_DIR
+// for the default root directory (e.g. /run/user/UID/urunc instead of /run/urunc).
+// It returns true for non-root processes and for root inside a user namespace
+// when not running as the "root" user (e.g. rootless Podman).
+func ShouldHonorXDGRuntimeDir() bool {
+	if os.Geteuid() != 0 {
+		return true
+	}
+	if !userns.RunningInUserNS() {
+		// euid == 0 in the initial ns (real root): use /run/urunc for backward compatibility.
+		return false
+	}
+	// euid == 0 inside a user namespace (rootless): honor XDG_RUNTIME_DIR unless USER=root.
+	u, ok := os.LookupEnv("USER")
+	return !ok || u != "root"
+}
+
+// prepareXDGRuntimeDir prepares the XDG_RUNTIME_DIR directory according to the XDG specification.
+// It creates the directory with proper permissions and sets the sticky bit to prevent auto-pruning.
+func prepareXDGRuntimeDir(root string) error {
+	if err := os.MkdirAll(root, 0o700); err != nil {
+		return fmt.Errorf("the path in $XDG_RUNTIME_DIR must be writable by the user: %w", err)
+	}
+	if err := os.Chmod(root, os.FileMode(0o700)|os.ModeSticky); err != nil {
+		return fmt.Errorf("you should check permission of the path in $XDG_RUNTIME_DIR: %w", err)
+	}
+	return nil
+}