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diff --git a/image/decoders/EXIF.cpp b/image/decoders/EXIF.cpp
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+++ b/image/decoders/EXIF.cpp
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+#include "EXIF.h"
+
+#include "mozilla/EndianUtils.h"
+#include "mozilla/StaticPrefs_image.h"
+
+namespace mozilla::image {
+
+// Section references in this file refer to the EXIF v2.3 standard, also known
+// as CIPA DC-008-Translation-2010.
+
+// See Section 4.6.4, Table 4.
+// Typesafe enums are intentionally not used here since we're comparing to raw
+// integers produced by parsing.
+enum class EXIFTag : uint16_t {
+  Orientation = 0x112,
+  XResolution = 0x11a,
+  YResolution = 0x11b,
+  PixelXDimension = 0xa002,
+  PixelYDimension = 0xa003,
+  ResolutionUnit = 0x128,
+  IFDPointer = 0x8769,
+};
+
+// See Section 4.6.2.
+enum EXIFType {
+  ByteType = 1,
+  ASCIIType = 2,
+  ShortType = 3,
+  LongType = 4,
+  RationalType = 5,
+  UndefinedType = 7,
+  SignedLongType = 9,
+  SignedRational = 10,
+};
+
+static const char* EXIFHeader = "Exif\0\0";
+static const uint32_t EXIFHeaderLength = 6;
+static const uint32_t TIFFHeaderStart = EXIFHeaderLength;
+
+struct ParsedEXIFData {
+  Orientation orientation;
+  Maybe<float> resolutionX;
+  Maybe<float> resolutionY;
+  Maybe<uint32_t> pixelXDimension;
+  Maybe<uint32_t> pixelYDimension;
+  Maybe<ResolutionUnit> resolutionUnit;
+};
+
+static float ToDppx(float aResolution, ResolutionUnit aUnit) {
+  constexpr float kPointsPerInch = 72.0f;
+  constexpr float kPointsPerCm = 1.0f / 2.54f;
+  switch (aUnit) {
+    case ResolutionUnit::Dpi:
+      return aResolution / kPointsPerInch;
+    case ResolutionUnit::Dpcm:
+      return aResolution / kPointsPerCm;
+  }
+  MOZ_CRASH("Unknown resolution unit?");
+}
+
+static Resolution ResolutionFromParsedData(const ParsedEXIFData& aData,
+                                           const gfx::IntSize& aRealImageSize) {
+  if (!aData.resolutionUnit || !aData.resolutionX || !aData.resolutionY) {
+    return {};
+  }
+
+  Resolution resolution{ToDppx(*aData.resolutionX, *aData.resolutionUnit),
+                        ToDppx(*aData.resolutionY, *aData.resolutionUnit)};
+
+  if (StaticPrefs::image_exif_density_correction_sanity_check_enabled()) {
+    if (!aData.pixelXDimension || !aData.pixelYDimension) {
+      return {};
+    }
+
+    const gfx::IntSize exifSize(*aData.pixelXDimension, *aData.pixelYDimension);
+
+    gfx::IntSize scaledSize = aRealImageSize;
+    resolution.ApplyTo(scaledSize.width, scaledSize.height);
+
+    if (exifSize != scaledSize) {
+      return {};
+    }
+  }
+
+  return resolution;
+}
+
+/////////////////////////////////////////////////////////////
+// Parse EXIF data, typically found in a JPEG's APP1 segment.
+/////////////////////////////////////////////////////////////
+EXIFData EXIFParser::ParseEXIF(const uint8_t* aData, const uint32_t aLength,
+                               const gfx::IntSize& aRealImageSize) {
+  if (!Initialize(aData, aLength)) {
+    return EXIFData();
+  }
+
+  if (!ParseEXIFHeader()) {
+    return EXIFData();
+  }
+
+  uint32_t offsetIFD;
+  if (!ParseTIFFHeader(offsetIFD)) {
+    return EXIFData();
+  }
+
+  JumpTo(offsetIFD);
+
+  ParsedEXIFData data;
+  ParseIFD(data);
+
+  return EXIFData{data.orientation,
+                  ResolutionFromParsedData(data, aRealImageSize)};
+}
+
+/////////////////////////////////////////////////////////
+// Parse the EXIF header. (Section 4.7.2, Figure 30)
+/////////////////////////////////////////////////////////
+bool EXIFParser::ParseEXIFHeader() {
+  return MatchString(EXIFHeader, EXIFHeaderLength);
+}
+
+/////////////////////////////////////////////////////////
+// Parse the TIFF header. (Section 4.5.2, Table 1)
+/////////////////////////////////////////////////////////
+bool EXIFParser::ParseTIFFHeader(uint32_t& aIFD0OffsetOut) {
+  // Determine byte order.
+  if (MatchString("MM\0*", 4)) {
+    mByteOrder = ByteOrder::BigEndian;
+  } else if (MatchString("II*\0", 4)) {
+    mByteOrder = ByteOrder::LittleEndian;
+  } else {
+    return false;
+  }
+
+  // Determine offset of the 0th IFD. (It shouldn't be greater than 64k, which
+  // is the maximum size of the entry APP1 segment.)
+  uint32_t ifd0Offset;
+  if (!ReadUInt32(ifd0Offset) || ifd0Offset > 64 * 1024) {
+    return false;
+  }
+
+  // The IFD offset is relative to the beginning of the TIFF header, which
+  // begins after the EXIF header, so we need to increase the offset
+  // appropriately.
+  aIFD0OffsetOut = ifd0Offset + TIFFHeaderStart;
+  return true;
+}
+
+// An arbitrary limit on the amount of pointers that we'll chase, to prevent bad
+// inputs getting us stuck.
+constexpr uint32_t kMaxEXIFDepth = 16;
+
+/////////////////////////////////////////////////////////
+// Parse the entries in IFD0. (Section 4.6.2)
+/////////////////////////////////////////////////////////
+void EXIFParser::ParseIFD(ParsedEXIFData& aData, uint32_t aDepth) {
+  if (NS_WARN_IF(aDepth > kMaxEXIFDepth)) {
+    return;
+  }
+
+  uint16_t entryCount;
+  if (!ReadUInt16(entryCount)) {
+    return;
+  }
+
+  for (uint16_t entry = 0; entry < entryCount; ++entry) {
+    // Read the fields of the 12-byte entry.
+    uint16_t tag;
+    if (!ReadUInt16(tag)) {
+      return;
+    }
+
+    uint16_t type;
+    if (!ReadUInt16(type)) {
+      return;
+    }
+
+    uint32_t count;
+    if (!ReadUInt32(count)) {
+      return;
+    }
+
+    switch (EXIFTag(tag)) {
+      case EXIFTag::Orientation:
+        // We should have an orientation value here; go ahead and parse it.
+        if (!ParseOrientation(type, count, aData.orientation)) {
+          return;
+        }
+        break;
+      case EXIFTag::ResolutionUnit:
+        if (!ParseResolutionUnit(type, count, aData.resolutionUnit)) {
+          return;
+        }
+        break;
+      case EXIFTag::XResolution:
+        if (!ParseResolution(type, count, aData.resolutionX)) {
+          return;
+        }
+        break;
+      case EXIFTag::YResolution:
+        if (!ParseResolution(type, count, aData.resolutionY)) {
+          return;
+        }
+        break;
+      case EXIFTag::PixelXDimension:
+        if (!ParseDimension(type, count, aData.pixelXDimension)) {
+          return;
+        }
+        break;
+      case EXIFTag::PixelYDimension:
+        if (!ParseDimension(type, count, aData.pixelYDimension)) {
+          return;
+        }
+        break;
+      case EXIFTag::IFDPointer: {
+        uint32_t offset;
+        if (!ReadUInt32(offset)) {
+          return;
+        }
+
+        ScopedJump jump(*this, offset + TIFFHeaderStart);
+        ParseIFD(aData, aDepth + 1);
+        break;
+      }
+
+      default:
+        Advance(4);
+        break;
+    }
+  }
+}
+
+bool EXIFParser::ReadRational(float& aOut) {
+  // Values larger than 4 bytes (like rationals) are specified as an offset into
+  // the TIFF header.
+  uint32_t valueOffset;
+  if (!ReadUInt32(valueOffset)) {
+    return false;
+  }
+  ScopedJump jumpToHeader(*this, valueOffset + TIFFHeaderStart);
+  uint32_t numerator;
+  if (!ReadUInt32(numerator)) {
+    return false;
+  }
+  uint32_t denominator;
+  if (!ReadUInt32(denominator)) {
+    return false;
+  }
+  if (denominator == 0) {
+    return false;
+  }
+  aOut = float(numerator) / float(denominator);
+  return true;
+}
+
+bool EXIFParser::ParseResolution(uint16_t aType, uint32_t aCount,
+                                 Maybe<float>& aOut) {
+  if (!StaticPrefs::image_exif_density_correction_enabled()) {
+    Advance(4);
+    return true;
+  }
+  if (aType != RationalType || aCount != 1) {
+    return false;
+  }
+  float value;
+  if (!ReadRational(value)) {
+    return false;
+  }
+  if (value == 0.0f) {
+    return false;
+  }
+  aOut = Some(value);
+  return true;
+}
+
+bool EXIFParser::ParseDimension(uint16_t aType, uint32_t aCount,
+                                Maybe<uint32_t>& aOut) {
+  if (!StaticPrefs::image_exif_density_correction_enabled()) {
+    Advance(4);
+    return true;
+  }
+
+  if (aCount != 1) {
+    return false;
+  }
+
+  switch (aType) {
+    case ShortType: {
+      uint16_t value;
+      if (!ReadUInt16(value)) {
+        return false;
+      }
+      aOut = Some(value);
+      Advance(2);
+      break;
+    }
+    case LongType: {
+      uint32_t value;
+      if (!ReadUInt32(value)) {
+        return false;
+      }
+      aOut = Some(value);
+      break;
+    }
+    default:
+      return false;
+  }
+  return true;
+}
+
+bool EXIFParser::ParseResolutionUnit(uint16_t aType, uint32_t aCount,
+                                     Maybe<ResolutionUnit>& aOut) {
+  if (!StaticPrefs::image_exif_density_correction_enabled()) {
+    Advance(4);
+    return true;
+  }
+  if (aType != ShortType || aCount != 1) {
+    return false;
+  }
+  uint16_t value;
+  if (!ReadUInt16(value)) {
+    return false;
+  }
+  switch (value) {
+    case 2:
+      aOut = Some(ResolutionUnit::Dpi);
+      break;
+    case 3:
+      aOut = Some(ResolutionUnit::Dpcm);
+      break;
+    default:
+      return false;
+  }
+
+  // This is a 32-bit field, but the unit value only occupies the first 16 bits.
+  // We need to advance another 16 bits to consume the entire field.
+  Advance(2);
+  return true;
+}
+
+bool EXIFParser::ParseOrientation(uint16_t aType, uint32_t aCount,
+                                  Orientation& aOut) {
+  // Sanity check the type and count.
+  if (aType != ShortType || aCount != 1) {
+    return false;
+  }
+
+  uint16_t value;
+  if (!ReadUInt16(value)) {
+    return false;
+  }
+
+  switch (value) {
+    case 1:
+      aOut = Orientation(Angle::D0, Flip::Unflipped);
+      break;
+    case 2:
+      aOut = Orientation(Angle::D0, Flip::Horizontal);
+      break;
+    case 3:
+      aOut = Orientation(Angle::D180, Flip::Unflipped);
+      break;
+    case 4:
+      aOut = Orientation(Angle::D180, Flip::Horizontal);
+      break;
+    case 5:
+      aOut = Orientation(Angle::D90, Flip::Horizontal);
+      break;
+    case 6:
+      aOut = Orientation(Angle::D90, Flip::Unflipped);
+      break;
+    case 7:
+      aOut = Orientation(Angle::D270, Flip::Horizontal);
+      break;
+    case 8:
+      aOut = Orientation(Angle::D270, Flip::Unflipped);
+      break;
+    default:
+      return false;
+  }
+
+  // This is a 32-bit field, but the orientation value only occupies the first
+  // 16 bits. We need to advance another 16 bits to consume the entire field.
+  Advance(2);
+  return true;
+}
+
+bool EXIFParser::Initialize(const uint8_t* aData, const uint32_t aLength) {
+  if (aData == nullptr) {
+    return false;
+  }
+
+  // An APP1 segment larger than 64k violates the JPEG standard.
+  if (aLength > 64 * 1024) {
+    return false;
+  }
+
+  mStart = mCurrent = aData;
+  mLength = mRemainingLength = aLength;
+  mByteOrder = ByteOrder::Unknown;
+  return true;
+}
+
+void EXIFParser::Advance(const uint32_t aDistance) {
+  if (mRemainingLength >= aDistance) {
+    mCurrent += aDistance;
+    mRemainingLength -= aDistance;
+  } else {
+    mCurrent = mStart;
+    mRemainingLength = 0;
+  }
+}
+
+void EXIFParser::JumpTo(const uint32_t aOffset) {
+  if (mLength >= aOffset) {
+    mCurrent = mStart + aOffset;
+    mRemainingLength = mLength - aOffset;
+  } else {
+    mCurrent = mStart;
+    mRemainingLength = 0;
+  }
+}
+
+bool EXIFParser::MatchString(const char* aString, const uint32_t aLength) {
+  if (mRemainingLength < aLength) {
+    return false;
+  }
+
+  for (uint32_t i = 0; i < aLength; ++i) {
+    if (mCurrent[i] != aString[i]) {
+      return false;
+    }
+  }
+
+  Advance(aLength);
+  return true;
+}
+
+bool EXIFParser::MatchUInt16(const uint16_t aValue) {
+  if (mRemainingLength < 2) {
+    return false;
+  }
+
+  bool matched;
+  switch (mByteOrder) {
+    case ByteOrder::LittleEndian:
+      matched = LittleEndian::readUint16(mCurrent) == aValue;
+      break;
+    case ByteOrder::BigEndian:
+      matched = BigEndian::readUint16(mCurrent) == aValue;
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Should know the byte order by now");
+      matched = false;
+  }
+
+  if (matched) {
+    Advance(2);
+  }
+
+  return matched;
+}
+
+bool EXIFParser::ReadUInt16(uint16_t& aValue) {
+  if (mRemainingLength < 2) {
+    return false;
+  }
+
+  bool matched = true;
+  switch (mByteOrder) {
+    case ByteOrder::LittleEndian:
+      aValue = LittleEndian::readUint16(mCurrent);
+      break;
+    case ByteOrder::BigEndian:
+      aValue = BigEndian::readUint16(mCurrent);
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Should know the byte order by now");
+      matched = false;
+  }
+
+  if (matched) {
+    Advance(2);
+  }
+
+  return matched;
+}
+
+bool EXIFParser::ReadUInt32(uint32_t& aValue) {
+  if (mRemainingLength < 4) {
+    return false;
+  }
+
+  bool matched = true;
+  switch (mByteOrder) {
+    case ByteOrder::LittleEndian:
+      aValue = LittleEndian::readUint32(mCurrent);
+      break;
+    case ByteOrder::BigEndian:
+      aValue = BigEndian::readUint32(mCurrent);
+      break;
+    default:
+      MOZ_ASSERT_UNREACHABLE("Should know the byte order by now");
+      matched = false;
+  }
+
+  if (matched) {
+    Advance(4);
+  }
+
+  return matched;
+}
+
+}  // namespace mozilla::image