[Backport] Security bug 1161379

Manual backport of patch originally reviewed on
https://chromium-review.googlesource.com/c/chromium/src/+/2600386:
[privacy_budget] Remove unnecessary kCanvasReadback metrics.

The identifiability metrics recorded under kCanvasReadback surface type
used two conflicting sources as inputs: the CanvasRenderingContext
type, and the paint-op digest.

There are known collisions between resulting IdentifiableSurface values
from the two sources, which makes it impossible to losslessly separate
the two during analysis.

While the fact that a canvas readback happened is interesting, it
doesn't help determine the observed diversity of clients. Hence this
change removes one of those sources: the CanvasRenderingContext type.

Bug: 1161379
Change-Id: I770cb631c9c4afe4c36d1b129aaf61410db25d43
Commit-Queue: Asanka Herath <[email protected]>
Reviewed-by: Caleb Raitto <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Cr-Commit-Position: refs/heads/master@{#847480}
Reviewed-by: Jüri Valdmann <[email protected]>

Author: asankah

chromium/third_party/blink/renderer/core/offscreencanvas/offscreen_canvas.cc

@@ -222,13 +222,6 @@ ImageBitmap* OffscreenCanvas::transferToImageBitmap(
                                       "ImageBitmap construction failed");
   }
 
-  RecordIdentifiabilityMetric(
-      blink::IdentifiableSurface::FromTypeAndToken(
-          blink::IdentifiableSurface::Type::kCanvasReadback,
-          context_ ? context_->GetContextType()
-                   : CanvasRenderingContext::kContextTypeUnknown),
-      0);
-
   return image;
 }
 

chromium/third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc

@@ -38,8 +38,6 @@
 #include "third_party/blink/public/common/features.h"
 #include "third_party/blink/public/common/privacy_budget/identifiability_metric_builder.h"
 #include "third_party/blink/public/common/privacy_budget/identifiability_metrics.h"
-#include "third_party/blink/public/common/privacy_budget/identifiability_study_settings.h"
-#include "third_party/blink/public/common/privacy_budget/identifiable_surface.h"
 #include "third_party/blink/public/platform/platform.h"
 #include "third_party/blink/public/platform/task_type.h"
 #include "third_party/blink/renderer/bindings/modules/v8/rendering_context.h"
@@ -682,13 +680,6 @@ ImageData* CanvasRenderingContext2D::getImageData(
     int sw,
     int sh,
     ExceptionState& exception_state) {
-  const IdentifiableSurface surface = IdentifiableSurface::FromTypeAndToken(
-      IdentifiableSurface::Type::kCanvasReadback, GetContextType());
-  if (IdentifiabilityStudySettings::Get()->ShouldSample(surface)) {
-    blink::IdentifiabilityMetricBuilder(ukm_source_id_)
-        .Set(surface, 0)
-        .Record(ukm_recorder_);
-  }
   return BaseRenderingContext2D::getImageData(sx, sy, sw, sh, exception_state);
 }
 

chromium/third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc

@@ -4667,16 +4667,6 @@ void WebGLRenderingContextBase::readPixels(
     GLenum format,
     GLenum type,
     MaybeShared<DOMArrayBufferView> pixels) {
-  if (IdentifiabilityStudySettings::Get()->ShouldSample(
-          blink::IdentifiableSurface::Type::kCanvasReadback)) {
-    const auto& ukm_params = GetUkmParameters();
-    blink::IdentifiabilityMetricBuilder(ukm_params.source_id)
-        .Set(blink::IdentifiableSurface::FromTypeAndToken(
-                 blink::IdentifiableSurface::Type::kCanvasReadback,
-                 GetContextType()),
-             0)
-        .Record(ukm_params.ukm_recorder);
-  }
   ReadPixelsHelper(x, y, width, height, format, type, pixels.View(), 0);
 }