dom/canvas/GeneratePlaceholderCanvasData.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef mozilla_dom_GeneratePlaceholderCanvasData_h
   8 #define mozilla_dom_GeneratePlaceholderCanvasData_h
   9
  10 #include "mozilla/StaticPrefs_privacy.h"
  11 #include "nsCOMPtr.h"
  12 #include "nsIRandomGenerator.h"
  13 #include "nsServiceManagerUtils.h"
  14
  15 #define RANDOM_BYTES_TO_SAMPLE 32
  16
  17 namespace mozilla {
  18 namespace dom {
  19
  20 /**
  21  * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
  22  * to generate random data for placeholder canvas by sampling
  23  * RANDOM_BYTES_TO_SAMPLE bytes and then returning it. If this fails or if
  24  * privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
  25  * nullptr.
  26  *
  27  * @return uint8_t*  output buffer
  28  */
  29 inline uint8_t* TryToGenerateRandomDataForPlaceholderCanvasData() {
  30   if (!StaticPrefs::privacy_resistFingerprinting_randomDataOnCanvasExtract()) {
  31     return nullptr;
  32   }
  33   nsresult rv;
  34   nsCOMPtr<nsIRandomGenerator> rg =
  35       do_GetService("@mozilla.org/security/random-generator;1", &rv);
  36   if (NS_FAILED(rv)) {
  37     return nullptr;
  38   }
  39   uint8_t* randomData;
  40   rv = rg->GenerateRandomBytes(RANDOM_BYTES_TO_SAMPLE, &randomData);
  41   if (NS_FAILED(rv)) {
  42     return nullptr;
  43   }
  44   return randomData;
  45 }
  46
  47 /**
  48  * If randomData not nullptr, repeats those bytes many times to fill buffer. If
  49  * randomData is nullptr, returns all-white pixel data.
  50  *
  51  * @param[in]   randomData  Buffer of RANDOM_BYTES_TO_SAMPLE bytes of random
  52  *                          data, or nullptr
  53  * @param[in]   size        Size of output buffer
  54  * @param[out]  buffer      Output buffer
  55  */
  56 inline void FillPlaceholderCanvas(uint8_t* randomData, uint32_t size,
  57                                   uint8_t* buffer) {
  58   if (!randomData) {
  59     memset(buffer, 0xFF, size);
  60     return;
  61   }
  62   auto remaining_to_fill = size;
  63   auto index = 0;
  64   while (remaining_to_fill > 0) {
  65     auto bytes_to_write = (remaining_to_fill > RANDOM_BYTES_TO_SAMPLE)
  66                               ? RANDOM_BYTES_TO_SAMPLE
  67                               : remaining_to_fill;
  68     memcpy(buffer + (index * RANDOM_BYTES_TO_SAMPLE), randomData,
  69            bytes_to_write);
  70     remaining_to_fill -= bytes_to_write;
  71     index++;
  72   }
  73   free(randomData);
  74 }
  75
  76 /**
  77  * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
  78  * to generate random canvas data by sampling RANDOM_BYTES_TO_SAMPLE bytes and
  79  * then repeating those bytes many times to fill the buffer. If this fails or
  80  * if privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
  81  * all-white, opaque pixel data.
  82  *
  83  * It is recommended that callers call this function instead of individually
  84  * calling TryToGenerateRandomDataForPlaceholderCanvasData and
  85  * FillPlaceholderCanvas unless there are requirements, like NoGC, that prevent
  86  * them from calling the RNG service inside this function.
  87  *
  88  * @param[in]   size    Size of output buffer
  89  * @param[out]  buffer  Output buffer
  90  */
  91 inline void GeneratePlaceholderCanvasData(uint32_t size, uint8_t* buffer) {
  92   uint8_t* randomData = TryToGenerateRandomDataForPlaceholderCanvasData();
  93   FillPlaceholderCanvas(randomData, size, buffer);
  94 }
  95
  96 }  // namespace dom
  97 }  // namespace mozilla
  98
  99 #endif  // mozilla_dom_GeneratePlaceholderCanvasData_h