Instrument document.scrollingElement.client[Width|Height] for

identifiability study.

This API can be used to compute the size of a client's scrollbar, which
is information which could be used to build a fingerprint to track users
across different origins.

This CL refactors the method used for instrumenting
HTMLElement.offset[Width|Height] introduced in
http://crrev.com/c/2436650 to support both use cases.

Bug: 973801
Change-Id: I518841aad9093b8df9368f2e6977ebab20536a37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472528Reviewed-by: Asanka Herath <asanka@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Dylan Cutler <dylancutler@google.com>
Cr-Commit-Position: refs/heads/master@{#818323}

third_party/blink/public/common/privacy_budget/identifiable_surface.h

@@ -229,8 +229,8 @@ class IdentifiableSurface {
   };
 
   enum class ScrollbarSurface : uint64_t {
-    kBodyOffsetWidth = 0,
-    kBodyOffsetHeight = 1,
+    kScrollingElementWidth = 0,
+    kScrollingElementHeight = 1,
     kElemScrollbarWidth = 2,
     kElemScrollbarHeight = 3,
   };

third_party/blink/renderer/core/dom/element.cc

@@ -33,6 +33,8 @@
 #include <utility>
 
 #include "cc/input/snap_selection_strategy.h"
+#include "third_party/blink/public/common/privacy_budget/identifiability_metric_builder.h"
+#include "third_party/blink/public/common/privacy_budget/identifiability_study_settings.h"
 #include "third_party/blink/public/mojom/scroll/scroll_into_view_params.mojom-blink.h"
 #include "third_party/blink/renderer/bindings/core/v8/dictionary.h"
 #include "third_party/blink/renderer/bindings/core/v8/script_promise_resolver.h"
@@ -1273,6 +1275,70 @@ int Element::clientTop() {
   return 0;
 }
 
+void Element::RecordScrollbarSizeForStudy(int measurement,
+                                          bool is_width,
+                                          bool is_offset) {
+  if (!IdentifiabilityStudySettings::Get()->IsTypeAllowed(
+          IdentifiableSurface::Type::kScrollbarSize))
+    return;
+
+  // Check for presence of a scrollbar.
+  PaintLayerScrollableArea* area;
+  if (this == GetDocument().ScrollingElementNoLayout()) {
+    auto* view = GetDocument().View();
+    if (!view)
+      return;
+    area = view->LayoutViewport();
+  } else {
+    auto* layout = GetLayoutBox();
+    if (!layout)
+      return;
+    area = layout->GetScrollableArea();
+  }
+  if (!area || area->HasOverlayOverflowControls())
+    return;
+
+  Scrollbar* scrollbar =
+      is_width ? area->VerticalScrollbar() : area->HorizontalScrollbar();
+  // We intentionally exclude platform overlay scrollbars since their size
+  // cannot be detected in JavaScript using the methods below.
+  if (!scrollbar)
+    return;
+
+  IdentifiableSurface::ScrollbarSurface surface;
+  int scrollbar_size;
+
+  // There are two common ways to detect the size of a scrollbar in a DOM
+  // window. They are:
+  // 1. Compute the difference of the window.inner[Width|Height] and the
+  //    corresponding document.scrollingElement.offset[Width|Height].
+  // 2. Any HTML element that insets the layout to fit a scrollbar, so it is
+  //    measurable by a JavaScript program on a site.
+  if (this == GetDocument().scrollingElement()) {
+    LocalDOMWindow* dom_window = GetDocument().domWindow();
+    scrollbar_size =
+        (is_width ? dom_window->innerWidth() : dom_window->innerHeight()) -
+        measurement;
+    surface =
+        is_width
+            ? IdentifiableSurface::ScrollbarSurface::kScrollingElementWidth
+            : IdentifiableSurface::ScrollbarSurface::kScrollingElementHeight;
+  } else if (is_offset) {
+    scrollbar_size = measurement - (is_width ? clientWidth() : clientHeight());
+    surface = is_width
+                  ? IdentifiableSurface::ScrollbarSurface::kElemScrollbarWidth
+                  : IdentifiableSurface::ScrollbarSurface::kElemScrollbarHeight;
+  } else {
+    return;
+  }
+
+  blink::IdentifiabilityMetricBuilder(GetDocument().UkmSourceID())
+      .Set(blink::IdentifiableSurface::FromTypeAndToken(
+               blink::IdentifiableSurface::Type::kScrollbarSize, surface),
+           scrollbar_size)
+      .Record(GetDocument().UkmRecorder());
+}
+
 int Element::clientWidth() {
   // When in strict mode, clientWidth for the document element should return the
   // width of the containing frame.
@@ -1293,29 +1359,40 @@ int Element::clientWidth() {
         // OverflowClipRect() may return infinite along a particular axis if
         // |layout_view| is not a scroll-container.
         DCHECK(layout_view->IsScrollContainer());
-        return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-                   layout_view->OverflowClipRect(PhysicalOffset()).Width(),
-                   layout_view->StyleRef())
-            .Round();
+        int result =
+            AdjustForAbsoluteZoom::AdjustLayoutUnit(
+                layout_view->OverflowClipRect(PhysicalOffset()).Width(),
+                layout_view->StyleRef())
+                .Round();
+        RecordScrollbarSizeForStudy(result, /* is_width= */ true,
+                                    /* is_offset= */ false);
+        return result;
       }
-      return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-                 LayoutUnit(layout_view->GetLayoutSize().Width()),
-                 layout_view->StyleRef())
-          .Round();
+      int result = AdjustForAbsoluteZoom::AdjustLayoutUnit(
+                       LayoutUnit(layout_view->GetLayoutSize().Width()),
+                       layout_view->StyleRef())
+                       .Round();
+      RecordScrollbarSizeForStudy(result, /* is_width= */ true,
+                                  /* is_offset= */ false);
+      return result;
     }
   }
 
   GetDocument().UpdateStyleAndLayoutForNode(this,
                                             DocumentUpdateReason::kJavaScript);
 
-  if (LayoutBox* layout_object = GetLayoutBox())
-    return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-               LayoutUnit(
-                   layout_object
-                       ->PixelSnappedClientWidthWithTableSpecialBehavior()),
-               layout_object->StyleRef())
-        .Round();
-  return 0;
+  int result = 0;
+  if (LayoutBox* layout_object = GetLayoutBox()) {
+    result =
+        AdjustForAbsoluteZoom::AdjustLayoutUnit(
+            LayoutUnit(layout_object
+                           ->PixelSnappedClientWidthWithTableSpecialBehavior()),
+            layout_object->StyleRef())
+            .Round();
+    RecordScrollbarSizeForStudy(result, /* is_width= */ true,
+                                /* is_offset= */ false);
+  }
+  return result;
 }
 
 int Element::clientHeight() {
@@ -1339,29 +1416,40 @@ int Element::clientHeight() {
         // OverflowClipRect() may return infinite along a particular axis if
         // |layout_view| is not a scroll-container.
         DCHECK(layout_view->IsScrollContainer());
-        return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-                   layout_view->OverflowClipRect(PhysicalOffset()).Height(),
-                   layout_view->StyleRef())
-            .Round();
+        int result =
+            AdjustForAbsoluteZoom::AdjustLayoutUnit(
+                layout_view->OverflowClipRect(PhysicalOffset()).Height(),
+                layout_view->StyleRef())
+                .Round();
+        RecordScrollbarSizeForStudy(result, /* is_width= */ false,
+                                    /* is_offset= */ false);
+        return result;
       }
-      return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-                 LayoutUnit(layout_view->GetLayoutSize().Height()),
-                 layout_view->StyleRef())
-          .Round();
+      int result = AdjustForAbsoluteZoom::AdjustLayoutUnit(
+                       LayoutUnit(layout_view->GetLayoutSize().Height()),
+                       layout_view->StyleRef())
+                       .Round();
+      RecordScrollbarSizeForStudy(result, /* is_width= */ false,
+                                  /* is_offset= */ false);
+      return result;
     }
   }
 
   GetDocument().UpdateStyleAndLayoutForNode(this,
                                             DocumentUpdateReason::kJavaScript);
 
-  if (LayoutBox* layout_object = GetLayoutBox())
-    return AdjustForAbsoluteZoom::AdjustLayoutUnit(
-               LayoutUnit(
-                   layout_object
-                       ->PixelSnappedClientHeightWithTableSpecialBehavior()),
-               layout_object->StyleRef())
-        .Round();
-  return 0;
+  int result = 0;
+  if (LayoutBox* layout_object = GetLayoutBox()) {
+    result = AdjustForAbsoluteZoom::AdjustLayoutUnit(
+                 LayoutUnit(
+                     layout_object
+                         ->PixelSnappedClientHeightWithTableSpecialBehavior()),
+                 layout_object->StyleRef())
+                 .Round();
+    RecordScrollbarSizeForStudy(result, /* is_width= */ false,
+                                /* is_offset= */ false);
+  }
+  return result;
 }
 
 LayoutBox* Element::GetLayoutBoxForScrolling() const {

third_party/blink/renderer/core/dom/element.h

@@ -934,6 +934,10 @@ class CORE_EXPORT Element : public ContainerNode, public Animatable {
   const ElementData* GetElementData() const { return element_data_.Get(); }
   UniqueElementData& EnsureUniqueElementData();
 
+  void RecordScrollbarSizeForStudy(int measurement,
+                                   bool is_width,
+                                   bool is_offset);
+
   void AddPropertyToPresentationAttributeStyle(MutableCSSPropertyValueSet*,
                                                CSSPropertyID,
                                                CSSValueID identifier);

third_party/blink/renderer/core/html/html_element.cc

@@ -26,8 +26,6 @@
 #include "third_party/blink/renderer/core/html/html_element.h"
 
 #include "base/stl_util.h"
-#include "third_party/blink/public/common/privacy_budget/identifiability_metric_builder.h"
-#include "third_party/blink/public/common/privacy_budget/identifiability_study_settings.h"
 #include "third_party/blink/renderer/bindings/core/v8/js_event_handler_for_content_attribute.h"
 #include "third_party/blink/renderer/bindings/core/v8/string_or_trusted_script.h"
 #include "third_party/blink/renderer/bindings/core/v8/string_treat_null_as_empty_string_or_trusted_script.h"
@@ -1497,67 +1495,6 @@ int HTMLElement::offsetTopForBinding() {
   return 0;
 }
 
-void HTMLElement::RecordScrollbarSizeForStudy(int offset_measurement,
-                                              bool is_width) {
-  if (!IdentifiabilityStudySettings::Get()->IsTypeAllowed(
-          IdentifiableSurface::Type::kScrollbarSize))
-    return;
-
-  // Check for presence of a scrollbar.
-  PaintLayerScrollableArea* area;
-  if (this == GetDocument().ScrollingElementNoLayout()) {
-    auto* view = GetDocument().View();
-    if (!view)
-      return;
-    area = view->LayoutViewport();
-  } else {
-    auto* layout = GetLayoutBox();
-    if (!layout)
-      return;
-    area = layout->GetScrollableArea();
-  }
-  if (!area || area->HasOverlayOverflowControls())
-    return;
-
-  Scrollbar* scrollbar =
-      is_width ? area->VerticalScrollbar() : area->HorizontalScrollbar();
-  // We intentionally exclude platform overlay scrollbars since their size
-  // cannot be detected in JavaScript using the methods below.
-  if (!scrollbar)
-    return;
-
-  IdentifiableSurface::ScrollbarSurface surface;
-  int scrollbar_size;
-
-  // There are two common ways to detect the size of a scrollbar in a DOM
-  // window. They are:
-  // 1. Compute the difference of the window.inner[Width|Height] and the
-  //    corresponding document.scrollingElement.offset[Width|Height].
-  // 2. Any HTML element that insets the layout to fit a scrollbar, so it is
-  //    measurable by a JavaScript program on a site.
-  if (this == GetDocument().scrollingElement()) {
-    LocalDOMWindow* dom_window = GetDocument().domWindow();
-    scrollbar_size =
-        (is_width ? dom_window->innerWidth() : dom_window->innerHeight()) -
-        offset_measurement;
-    surface = is_width
-                  ? IdentifiableSurface::ScrollbarSurface::kBodyOffsetWidth
-                  : IdentifiableSurface::ScrollbarSurface::kBodyOffsetHeight;
-  } else {
-    scrollbar_size =
-        offset_measurement - (is_width ? clientWidth() : clientHeight());
-    surface = is_width
-                  ? IdentifiableSurface::ScrollbarSurface::kElemScrollbarWidth
-                  : IdentifiableSurface::ScrollbarSurface::kElemScrollbarHeight;
-  }
-
-  blink::IdentifiabilityMetricBuilder(GetDocument().UkmSourceID())
-      .Set(blink::IdentifiableSurface::FromTypeAndToken(
-               blink::IdentifiableSurface::Type::kScrollbarSize, surface),
-           scrollbar_size)
-      .Record(GetDocument().UkmRecorder());
-}
-
 int HTMLElement::offsetWidthForBinding() {
   GetDocument().EnsurePaintLocationDataValidForNode(
       this, DocumentUpdateReason::kJavaScript);
@@ -1569,7 +1506,8 @@ int HTMLElement::offsetWidthForBinding() {
             LayoutUnit(layout_object->PixelSnappedOffsetWidth(offset_parent)),
             layout_object->StyleRef())
             .Round();
-    RecordScrollbarSizeForStudy(result, /* isWidth= */ true);
+    RecordScrollbarSizeForStudy(result, /* isWidth= */ true,
+                                /* is_offset= */ true);
   }
   return result;
 }
@@ -1586,7 +1524,8 @@ int HTMLElement::offsetHeightForBinding() {
             LayoutUnit(layout_object->PixelSnappedOffsetHeight(offset_parent)),
             layout_object->StyleRef())
             .Round();
-    RecordScrollbarSizeForStudy(result, /* isWidth= */ false);
+    RecordScrollbarSizeForStudy(result, /* is_width= */ false,
+                                /* is_offset= */ true);
   }
   return result;
 }

third_party/blink/renderer/core/html/html_element.h

@@ -212,8 +212,6 @@ class CORE_EXPORT HTMLElement : public Element {
 
   void HandleKeypressEvent(KeyboardEvent&);
 
-  void RecordScrollbarSizeForStudy(int, bool);
-
   static AttributeTriggers* TriggersForAttributeName(
       const QualifiedName& attr_name);