1 files changed, 97 insertions, 0 deletions
diff --git a/dom/canvas/GeneratePlaceholderCanvasData.h b/dom/canvas/GeneratePlaceholderCanvasData.h
new file mode 100644
index 0000000000..87650e1827
--- /dev/null
+++ b/dom/canvas/GeneratePlaceholderCanvasData.h
@@ -0,0 +1,97 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_GeneratePlaceholderCanvasData_h
+#define mozilla_dom_GeneratePlaceholderCanvasData_h
+
+#include "mozilla/StaticPrefs_privacy.h"
+#include "nsCOMPtr.h"
+#include "nsIRandomGenerator.h"
+#include "nsServiceManagerUtils.h"
+
+#define RANDOM_BYTES_TO_SAMPLE 32
+
+namespace mozilla::dom {
+
+/**
+ * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
+ * to generate random data for placeholder canvas by sampling
+ * RANDOM_BYTES_TO_SAMPLE bytes and then returning it. If this fails or if
+ * privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
+ * nullptr.
+ *
+ * @return uint8_t*  output buffer
+ */
+inline uint8_t* TryToGenerateRandomDataForPlaceholderCanvasData() {
+  if (!StaticPrefs::privacy_resistFingerprinting_randomDataOnCanvasExtract()) {
+    return nullptr;
+  }
+  nsresult rv;
+  nsCOMPtr<nsIRandomGenerator> rg =
+      do_GetService("@mozilla.org/security/random-generator;1", &rv);
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+  uint8_t* randomData;
+  rv = rg->GenerateRandomBytes(RANDOM_BYTES_TO_SAMPLE, &randomData);
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+  return randomData;
+}
+
+/**
+ * If randomData not nullptr, repeats those bytes many times to fill buffer. If
+ * randomData is nullptr, returns all-white pixel data.
+ *
+ * @param[in]   randomData  Buffer of RANDOM_BYTES_TO_SAMPLE bytes of random
+ *                          data, or nullptr
+ * @param[in]   size        Size of output buffer
+ * @param[out]  buffer      Output buffer
+ */
+inline void FillPlaceholderCanvas(uint8_t* randomData, uint32_t size,
+                                  uint8_t* buffer) {
+  if (!randomData) {
+    memset(buffer, 0xFF, size);
+    return;
+  }
+  auto remaining_to_fill = size;
+  auto index = 0;
+  while (remaining_to_fill > 0) {
+    auto bytes_to_write = (remaining_to_fill > RANDOM_BYTES_TO_SAMPLE)
+                              ? RANDOM_BYTES_TO_SAMPLE
+                              : remaining_to_fill;
+    memcpy(buffer + (index * RANDOM_BYTES_TO_SAMPLE), randomData,
+           bytes_to_write);
+    remaining_to_fill -= bytes_to_write;
+    index++;
+  }
+  free(randomData);
+}
+
+/**
+ * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
+ * to generate random canvas data by sampling RANDOM_BYTES_TO_SAMPLE bytes and
+ * then repeating those bytes many times to fill the buffer. If this fails or
+ * if privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
+ * all-white, opaque pixel data.
+ *
+ * It is recommended that callers call this function instead of individually
+ * calling TryToGenerateRandomDataForPlaceholderCanvasData and
+ * FillPlaceholderCanvas unless there are requirements, like NoGC, that prevent
+ * them from calling the RNG service inside this function.
+ *
+ * @param[in]   size    Size of output buffer
+ * @param[out]  buffer  Output buffer
+ */
+inline void GeneratePlaceholderCanvasData(uint32_t size, uint8_t* buffer) {
+  uint8_t* randomData = TryToGenerateRandomDataForPlaceholderCanvasData();
+  FillPlaceholderCanvas(randomData, size, buffer);
+}
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_GeneratePlaceholderCanvasData_h