[DeviceLight] Ensure to round lux value to avoid fingerprinting risk.

third_party/WebKit/Source/modules/device_light/DeviceLightDispatcher.cpp

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/device_light/DeviceLightDispatcher.h"
 
+#include <math.h>
+
 #include "modules/device_light/DeviceLightController.h"
 #include "public/platform/Platform.h"
 
+namespace {
+double EnsureRoundedLuxValue(double lux) {
+  // Make sure to round the lux value to nearest integer, to
+  // avoid too precise values and hence reduce fingerprinting risk.
+  // The special case when the lux value is infinity (no data can be
+  // provided) is simply returned as is.
+  return isinf(lux) ? lux : std::round(lux);

[palmer, 2017/03/23 19:00:19]

Apologies in advance for the drive-by. :)

A rounded double is still a high-precision value, though. I'd really like to get
it lower if possible, like down to 4 bits of precision. (Or whatever small range
still suits the intended applications.)

[timvolodine, 2017/04/20 01:34:30]

Acknowledged.

+}
+}  // namespace
+
 namespace blink {
 
 DeviceLightDispatcher& DeviceLightDispatcher::instance() {
   DEFINE_STATIC_LOCAL(DeviceLightDispatcher, deviceLightDispatcher,
                       (new DeviceLightDispatcher));
   return deviceLightDispatcher;
 }
 
 DeviceLightDispatcher::DeviceLightDispatcher() : m_lastDeviceLightData(-1) {}
 
 DeviceLightDispatcher::~DeviceLightDispatcher() {}
 
 DEFINE_TRACE(DeviceLightDispatcher) {
   PlatformEventDispatcher::trace(visitor);
 }
 
 void DeviceLightDispatcher::startListening() {
   Platform::current()->startListening(WebPlatformEventTypeDeviceLight, this);
 }
 
 void DeviceLightDispatcher::stopListening() {
   Platform::current()->stopListening(WebPlatformEventTypeDeviceLight);
   m_lastDeviceLightData = -1;
 }
 
 void DeviceLightDispatcher::didChangeDeviceLight(double value) {
-  m_lastDeviceLightData = value;
-  notifyControllers();
+  double newValue = EnsureRoundedLuxValue(value);
+  if (m_lastDeviceLightData != newValue) {
+    m_lastDeviceLightData = newValue;
+    notifyControllers();
+  }
 }
 
 double DeviceLightDispatcher::latestDeviceLightData() const {
   return m_lastDeviceLightData;
 }
 
 }  // namespace blink