[tbb-commits] [tor-browser/tor-browser-88.0-10.5-1] Bug 23104: Add a default line height compensation

[sysrqb at torproject.org]

commit 79899765deded0eb0c730a20e0ed5a2f05c2d1f1
Author: Igor Oliveira <igor.oliveira at posteo.net>
Date:   Sun Dec 10 18:16:59 2017 -0200

    Bug 23104: Add a default line height compensation
    
    Many fonts have issues with their vertical metrics. they
    are used to influence the height of ascenders and depth
    of descenders. Gecko uses it to calculate the line height
    (font height + ascender + descender), however because of
    that idiosyncratic behavior across multiple operating
    systems, it can be used to identify the user's OS.
    
    The solution proposed in the patch uses a default factor
    to be multiplied with the font size, simulating the concept
    of ascender and descender. This way all operating
    systems will have the same line height only and only if the
    frame is outside the chrome.
---
 layout/generic/ReflowInput.cpp             | 19 +++++++++---
 layout/generic/test/mochitest.ini          |  1 +
 layout/generic/test/test_tor_bug23104.html | 50 ++++++++++++++++++++++++++++++
 3 files changed, 65 insertions(+), 5 deletions(-)

diff --git a/layout/generic/ReflowInput.cpp b/layout/generic/ReflowInput.cpp
index 4ab0a5d0ecb7..61d303e95438 100644
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -31,6 +31,7 @@
 #include "mozilla/SVGUtils.h"
 #include "mozilla/dom/HTMLInputElement.h"
 #include "nsGridContainerFrame.h"
+#include "nsContentUtils.h"
 
 using namespace mozilla;
 using namespace mozilla::css;
@@ -2651,7 +2652,8 @@ void ReflowInput::CalculateBlockSideMargins() {
 
 // For risk management, we use preference to control the behavior, and
 // eNoExternalLeading is the old behavior.
-static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
+static nscoord GetNormalLineHeight(nsIContent* aContent,
+                                   nsFontMetrics* aFontMetrics) {
   MOZ_ASSERT(nullptr != aFontMetrics, "no font metrics");
 
   nscoord normalLineHeight;
@@ -2659,6 +2661,12 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
   nscoord externalLeading = aFontMetrics->ExternalLeading();
   nscoord internalLeading = aFontMetrics->InternalLeading();
   nscoord emHeight = aFontMetrics->EmHeight();
+
+  if (nsContentUtils::ShouldResistFingerprinting() &&
+      !aContent->IsInChromeDocument()) {
+    return NSToCoordRound(emHeight * NORMAL_LINE_HEIGHT_FACTOR);
+  }
+
   switch (GetNormalLineHeightCalcControl()) {
     case eIncludeExternalLeading:
       normalLineHeight = emHeight + internalLeading + externalLeading;
@@ -2676,7 +2684,8 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
   return normalLineHeight;
 }
 
-static inline nscoord ComputeLineHeight(ComputedStyle* aComputedStyle,
+static inline nscoord ComputeLineHeight(nsIContent* aContent,
+                                        ComputedStyle* aComputedStyle,
                                         nsPresContext* aPresContext,
                                         nscoord aBlockBSize,
                                         float aFontSizeInflation) {
@@ -2705,7 +2714,7 @@ static inline nscoord ComputeLineHeight(ComputedStyle* aComputedStyle,
 
   RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetFontMetricsForComputedStyle(
       aComputedStyle, aPresContext, aFontSizeInflation);
-  return GetNormalLineHeight(fm);
+  return GetNormalLineHeight(aContent, fm);
 }
 
 nscoord ReflowInput::CalcLineHeight() const {
@@ -2727,7 +2736,7 @@ nscoord ReflowInput::CalcLineHeight(nsIContent* aContent,
                                     float aFontSizeInflation) {
   MOZ_ASSERT(aComputedStyle, "Must have a ComputedStyle");
 
-  nscoord lineHeight = ComputeLineHeight(aComputedStyle, aPresContext,
+  nscoord lineHeight = ComputeLineHeight(aContent, aComputedStyle, aPresContext,
                                          aBlockBSize, aFontSizeInflation);
 
   NS_ASSERTION(lineHeight >= 0, "ComputeLineHeight screwed up");
@@ -2740,7 +2749,7 @@ nscoord ReflowInput::CalcLineHeight(nsIContent* aContent,
     if (!lh.IsNormal()) {
       RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetFontMetricsForComputedStyle(
           aComputedStyle, aPresContext, aFontSizeInflation);
-      nscoord normal = GetNormalLineHeight(fm);
+      nscoord normal = GetNormalLineHeight(aContent, fm);
       if (lineHeight < normal) {
         lineHeight = normal;
       }
diff --git a/layout/generic/test/mochitest.ini b/layout/generic/test/mochitest.ini
index 072b11da7eb6..3c377e0781ed 100644
--- a/layout/generic/test/mochitest.ini
+++ b/layout/generic/test/mochitest.ini
@@ -145,3 +145,4 @@ skip-if = debug == true || tsan # the test is slow. tsan: bug 1612707
 support-files =
     file_reframe_for_lazy_load_image.html
 [test_bug1655135.html]
+[test_tor_bug23104.html]
diff --git a/layout/generic/test/test_tor_bug23104.html b/layout/generic/test/test_tor_bug23104.html
new file mode 100644
index 000000000000..8ff1d2190c45
--- /dev/null
+++ b/layout/generic/test/test_tor_bug23104.html
@@ -0,0 +1,50 @@
+<!DOCTYPE HTML>
+<meta charset="UTF-8">
+<html>
+<head>
+  <title>Test for Tor Bug #23104: CSS line-height reveals the platform Tor browser is running</title>
+  <script type="application/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
+  <script type="application/javascript" src="/tests/SimpleTest/SpawnTask.js"></script>
+  <style type="text/css">
+    span {
+      background-color: #000;
+      color: #fff;
+      font-size: 16.5px;
+    }
+  </style>
+</head>
+<body>
+<span id="test1">Test1</span>
+<span id="test2">كلمة</span>
+<span id="test3">ação</span>
+<script>
+
+let setPref = async function (key, value) {
+  await SpecialPowers.pushPrefEnv({"set": [[key, value]]});
+}
+
+function getStyle(el, styleprop) {
+  el = document.getElementById(el);
+  return document.defaultView.getComputedStyle(el, null).getPropertyValue(styleprop);
+}
+
+function validateElement(elementName, isFingerprintResistent) {
+  var fontSize = getStyle(elementName, 'font-size');
+  var lineHeight = getStyle(elementName, 'line-height');
+  var validationCb = isFingerprintResistent ? is : isnot;
+  validationCb(parseFloat(lineHeight), Math.round(parseFloat(fontSize)) * 1.2, 'Line Height validation');
+}
+
+add_task(async function() {
+  await setPref("layout.css.line-height.normal-as-resolved-value.enabled", false);
+  for (let resistFingerprintingValue of [true, false]) {
+    await setPref("privacy.resistFingerprinting", resistFingerprintingValue);
+    for (let elementId of ['test1', 'test2', 'test3']) {
+      validateElement(elementId, resistFingerprintingValue);
+    }
+  }
+});
+
+</script>
+</body>
+</html>