1 files changed, 95 insertions, 0 deletions
diff --git a/internal/namespace/namespace.go b/internal/namespace/namespace.go
index ab3797d..0f2618b 100644
--- a/internal/namespace/namespace.go
+++ b/internal/namespace/namespace.go
@@ -194,3 +194,98 @@ func Bootstrap() (err error) {
 
 	return nil
 }
+
+// BootstrapJoin joins the namespaces of the target PID and replaces the current process.
+func BootstrapJoin(targetPid int) (err error) {
+	if IsIsolated() {
+		return nil
+	}
+
+	var fdsToClose []int
+	defer func() {
+		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 {
+				return fmt.Errorf("argument %d contains null byte at position %d", i, j)
+			}
+		}
+	}
+
+	self, err := os.Executable()
+	if err != nil {
+		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-")
+	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)
+	}
+	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:]...)
+
+	for i, arg := range args {
+		for j := 0; j < len(arg); j++ {
+			if arg[j] == 0 {
+				return fmt.Errorf("launcher argument %d contains null byte at position %d", i, j)
+			}
+		}
+	}
+
+	// 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",
+	)
+
+	err = syscall.Exec(fmt.Sprintf("/proc/self/fd/%d", execFd), args, env)
+	if err != nil {
+		return fmt.Errorf("launcher exec failed: %w", err)
+	}
+
+	return nil
+}