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::dom {
  18
  19 /**
  20  * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
  21  * to generate random data for placeholder canvas by sampling
  22  * RANDOM_BYTES_TO_SAMPLE bytes and then returning it. If this fails or if
  23  * privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
  24  * nullptr.
  25  *
  26  * @return uint8_t*  output buffer
  27  */
  28 inline uint8_t* TryToGenerateRandomDataForPlaceholderCanvasData() {
  29   if (!StaticPrefs::privacy_resistFingerprinting_randomDataOnCanvasExtract()) {
  30     return nullptr;
  31   }
  32   nsresult rv;
  33   nsCOMPtr<nsIRandomGenerator> rg =
  34       do_GetService("@mozilla.org/security/random-generator;1", &rv);
  35   if (NS_FAILED(rv)) {
  36     return nullptr;
  37   }
  38   uint8_t* randomData;
  39   rv = rg->GenerateRandomBytes(RANDOM_BYTES_TO_SAMPLE, &randomData);
  40   if (NS_FAILED(rv)) {
  41     return nullptr;
  42   }
  43   return randomData;
  44 }
  45
  46 /**
  47  * If randomData not nullptr, repeats those bytes many times to fill buffer. If
  48  * randomData is nullptr, returns all-white pixel data.
  49  *
  50  * @param[in]   randomData  Buffer of RANDOM_BYTES_TO_SAMPLE bytes of random
  51  *                          data, or nullptr
  52  * @param[in]   size        Size of output buffer
  53  * @param[out]  buffer      Output buffer
  54  */
  55 inline void FillPlaceholderCanvas(uint8_t* randomData, uint32_t size,
  56                                   uint8_t* buffer) {
  57   if (!randomData) {
  58     memset(buffer, 0xFF, size);
  59     return;
  60   }
  61   auto remaining_to_fill = size;
  62   auto index = 0;
  63   while (remaining_to_fill > 0) {
  64     auto bytes_to_write = (remaining_to_fill > RANDOM_BYTES_TO_SAMPLE)
  65                               ? RANDOM_BYTES_TO_SAMPLE
  66                               : remaining_to_fill;
  67     memcpy(buffer + (index * RANDOM_BYTES_TO_SAMPLE), randomData,
  68            bytes_to_write);
  69     remaining_to_fill -= bytes_to_write;
  70     index++;
  71   }
  72   free(randomData);
  73 }
  74
  75 /**
  76  * When privacy.resistFingerprinting.randomDataOnCanvasExtract is true, tries
  77  * to generate random canvas data by sampling RANDOM_BYTES_TO_SAMPLE bytes and
  78  * then repeating those bytes many times to fill the buffer. If this fails or
  79  * if privacy.resistFingerprinting.randomDataOnCanvasExtract is false, returns
  80  * all-white, opaque pixel data.
  81  *
  82  * It is recommended that callers call this function instead of individually
  83  * calling TryToGenerateRandomDataForPlaceholderCanvasData and
  84  * FillPlaceholderCanvas unless there are requirements, like NoGC, that prevent
  85  * them from calling the RNG service inside this function.
  86  *
  87  * @param[in]   size    Size of output buffer
  88  * @param[out]  buffer  Output buffer
  89  */
  90 inline void GeneratePlaceholderCanvasData(uint32_t size, uint8_t* buffer) {
  91   uint8_t* randomData = TryToGenerateRandomDataForPlaceholderCanvasData();
  92   FillPlaceholderCanvas(randomData, size, buffer);
  93 }
  94
  95 }  // namespace mozilla::dom
  96
  97 #endif  // mozilla_dom_GeneratePlaceholderCanvasData_h