diff --git a/hardware_interface.h b/hardware_interface.h
index 58157d0..f55d700 100644
--- a/hardware_interface.h
+++ b/hardware_interface.h
@@ -19,18 +19,18 @@
 class HardwareInterface {
  public:
   // Returns the currently booted kernel partition. "/dev/sda2", for example.
-  virtual const std::string BootKernelDevice() = 0;
+  virtual std::string BootKernelDevice() const = 0;
 
   // Returns the currently booted rootfs partition. "/dev/sda3", for example.
-  virtual const std::string BootDevice() = 0;
+  virtual std::string BootDevice() const = 0;
 
   // Returns a list of all kernel partitions available (whether bootable or not)
-  virtual std::vector<std::string> GetKernelDevices() = 0;
+  virtual std::vector<std::string> GetKernelDevices() const = 0;
 
   // Is the specified kernel partition currently bootable, based on GPT flags?
   // Returns success.
   virtual bool IsKernelBootable(const std::string& kernel_device,
-                                bool* bootable) = 0;
+                                bool* bootable) const = 0;
 
   // Mark the specified kernel partition unbootable in GPT flags. We mark
   // the other kernel as bootable inside postinst, not inside the UE.
@@ -38,23 +38,23 @@
   virtual bool MarkKernelUnbootable(const std::string& kernel_device) = 0;
 
   // Returns true if this is an official Chrome OS build, false otherwise.
-  virtual bool IsOfficialBuild() = 0;
+  virtual bool IsOfficialBuild() const = 0;
 
   // Returns true if the boot mode is normal or if it's unable to
   // determine the boot mode. Returns false if the boot mode is
   // developer.
-  virtual bool IsNormalBootMode() = 0;
+  virtual bool IsNormalBootMode() const = 0;
 
   // Returns the HWID or an empty string on error.
-  virtual std::string GetHardwareClass() = 0;
+  virtual std::string GetHardwareClass() const = 0;
 
   // Returns the firmware version or an empty string if the system is
   // not running chrome os firmware.
-  virtual std::string GetFirmwareVersion() = 0;
+  virtual std::string GetFirmwareVersion() const = 0;
 
   // Returns the ec version or an empty string if the system is not
   // running a custom chrome os ec.
-  virtual std::string GetECVersion() = 0;
+  virtual std::string GetECVersion() const = 0;
 
   virtual ~HardwareInterface() {}
 };