refactor: improve resource management and cleanup patterns

- Simplify namespace bootstrapping by introducing `prepareLauncher` helper
- Implement a cleanup stack in `StartTunnel` to ensure orderly resource release on error
- Streamline temporary file and mount lifecycles in `ConfigureResolvConf` and `BlockHostServices`
- Ensure `Tunnel.Close()` also closes the underlying TUN device
- Reduce redundant manual cleanup calls using defer-based error handling

1 files changed, 42 insertions, 79 deletions

diff --git a/internal/namespace/namespace.go b/internal/namespace/namespace.go
index 0f2618b..6f56a84 100644
--- a/internal/namespace/namespace.go
+++ b/internal/namespace/namespace.go
@@ -84,7 +84,6 @@ func Bootstrap() (err error) {
 	}()
 
 	// 0. Validate current arguments for null bytes before proceeding.
-	// If any argument contains a null byte, syscall.Exec will fail with 'invalid argument'.
 	for i, arg := range os.Args {
 		for j := 0; j < len(arg); j++ {
 			if arg[j] == 0 {
@@ -98,57 +97,22 @@ func Bootstrap() (err error) {
 		return fmt.Errorf("failed to get executable path: %w", err)
 	}
 
-	// 1. Create a secure temporary file for the launcher binary.
-	// os.CreateTemp ensures a unique, unpredictable filename and restrictive permissions.
-	tmpFile, err := os.CreateTemp("", "wg-wrap-launcher-")
-	if err != nil {
-		return fmt.Errorf("failed to create temp launcher file: %w", err)
-	}
-	launcherPath := tmpFile.Name()
-
-	// 2. Write the embedded launcher binary to the temp file.
-	if _, err := tmpFile.Write(launcherBytes); err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to write launcher binary: %w", err)
-	}
-
-	// Ensure the binary is executable (0700)
-	if err := tmpFile.Chmod(0700); err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to set launcher permissions: %w", err)
-	}
-
-	// 2b. Open a read-only fd of the launcher to exec
-	execFd, err := syscall.Open(launcherPath, syscall.O_RDONLY, 0)
+	execFd, launcherPath, err := prepareLauncher()
 	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to open launcher for exec: %w", err)
+		return err
 	}
 	fdsToClose = append(fdsToClose, execFd)
+	_ = os.Remove(launcherPath) // Unlink early; fd remains valid
 
-	// Close the write file descriptor (to avoid ETXTBSY)
-	_ = tmpFile.Close()
-
-	// Unlink the file from disk (makes it invisible and ensures it is deleted on exit)
-	_ = os.Remove(launcherPath)
-
-	// Clear close-on-exec so it remains open across syscall.Exec
+	// Clear close-on-exec
 	if flags, err := unix.FcntlInt(uintptr(execFd), unix.F_GETFD, 0); err == nil {
 		_, _ = unix.FcntlInt(uintptr(execFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
 	}
 
 	// 3. Prepare arguments for the launcher.
-	// The launcher expects: launcher <command_to_run> [args...]
 	args := []string{self}
 	args = append(args, os.Args[1:]...)
 
-	// 4. Replace the current process with the launcher.
-	// We must check for null bytes in the arguments here because syscall.Exec
-	// (which calls execve) will return 'invalid argument' (EINVAL) if any
-	// string in the argv array contains a null byte.
 	for i, arg := range args {
 		for j := 0; j < len(arg); j++ {
 			if arg[j] == 0 {
@@ -164,7 +128,7 @@ func Bootstrap() (err error) {
 	}
 	fdsToClose = append(fdsToClose, hostNetFd)
 
-	// Clear close-on-exec so it remains open across syscall.Exec
+	// Clear close-on-exec
 	if flags, err := unix.FcntlInt(uintptr(hostNetFd), unix.F_GETFD, 0); err == nil {
 		_, _ = unix.FcntlInt(uintptr(hostNetFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
 	}
@@ -203,12 +167,13 @@ func BootstrapJoin(targetPid int) (err error) {
 
 	var fdsToClose []int
 	defer func() {
-		for _, fd := range fdsToClose {
-			_ = syscall.Close(fd)
+		if err != nil {
+			for _, fd := range fdsToClose {
+				_ = syscall.Close(fd)
+			}
 		}
 	}()
 
-	// Validate current arguments for null bytes before proceeding.
 	for i, arg := range os.Args {
 		for j := 0; j < len(arg); j++ {
 			if arg[j] == 0 {
@@ -222,48 +187,17 @@ func BootstrapJoin(targetPid int) (err error) {
 		return fmt.Errorf("failed to get executable path: %w", err)
 	}
 
-	// 1. Create a secure temporary file for the launcher binary.
-	tmpFile, err := os.CreateTemp("", "wg-wrap-launcher-")
+	execFd, launcherPath, err := prepareLauncher()
 	if err != nil {
-		return fmt.Errorf("failed to create temp launcher file: %w", err)
-	}
-	launcherPath := tmpFile.Name()
-
-	// 2. Write the embedded launcher binary to the temp file.
-	if _, err := tmpFile.Write(launcherBytes); err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to write launcher binary: %w", err)
-	}
-
-	// Ensure the binary is executable (0700)
-	if err := tmpFile.Chmod(0700); err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to set launcher permissions: %w", err)
-	}
-
-	// 2b. Open a read-only fd of the launcher to exec
-	execFd, err := syscall.Open(launcherPath, syscall.O_RDONLY, 0)
-	if err != nil {
-		_ = tmpFile.Close()
-		_ = os.Remove(launcherPath)
-		return fmt.Errorf("failed to open launcher for exec: %w", err)
+		return err
 	}
 	fdsToClose = append(fdsToClose, execFd)
-
-	// Close the write file descriptor (to avoid ETXTBSY)
-	_ = tmpFile.Close()
-
-	// Unlink the file from disk (makes it invisible and ensures it is deleted on exit)
 	_ = os.Remove(launcherPath)
 
-	// Clear close-on-exec so it remains open across syscall.Exec
 	if flags, err := unix.FcntlInt(uintptr(execFd), unix.F_GETFD, 0); err == nil {
 		_, _ = unix.FcntlInt(uintptr(execFd), unix.F_SETFD, flags&^unix.FD_CLOEXEC)
 	}
 
-	// 3. Prepare arguments for the launcher.
 	args := []string{self}
 	args = append(args, os.Args[1:]...)
 
@@ -275,8 +209,6 @@ func BootstrapJoin(targetPid int) (err error) {
 		}
 	}
 
-	// Set environment variables to tell the C launcher to join,
-	// and to tell the second wg-wrap instance that we are in a joined session.
 	env := append(os.Environ(),
 		fmt.Sprintf("WG_WRAP_JOIN_PID=%d", targetPid),
 		"WG_WRAP_JOINED=1",
@@ -289,3 +221,34 @@ func BootstrapJoin(targetPid int) (err error) {
 
 	return nil
 }
+
+func prepareLauncher() (int, string, error) {
+	tmpFile, err := os.CreateTemp("", "wg-wrap-launcher-")
+	if err != nil {
+		return 0, "", fmt.Errorf("failed to create temp launcher file: %w", err)
+	}
+	launcherPath := tmpFile.Name()
+
+	defer func() {
+		if err != nil {
+			_ = tmpFile.Close()
+			_ = os.Remove(launcherPath)
+		}
+	}()
+
+	if _, err = tmpFile.Write(launcherBytes); err != nil {
+		return 0, "", fmt.Errorf("failed to write launcher binary: %w", err)
+	}
+
+	if err = tmpFile.Chmod(0700); err != nil {
+		return 0, "", fmt.Errorf("failed to set launcher permissions: %w", err)
+	}
+
+	execFd, err := syscall.Open(launcherPath, syscall.O_RDONLY, 0)
+	if err != nil {
+		return 0, "", fmt.Errorf("failed to open launcher for exec: %w", err)
+	}
+
+	_ = tmpFile.Close()
+	return execFd, launcherPath, nil
+}