[tor-commits] [tor-browser] 44/81: Bug 23104: Add a default line height compensation

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pierov pushed a commit to branch tor-browser-102.4.0esr-12.0-1
in repository tor-browser.

commit 5604c67b658624cf8a1da020351654ce89cfaf96
Author: Igor Oliveira <igor.oliveira at posteo.net>
AuthorDate: 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 7bc03d10bd2a..befa9c8b1def 100644
--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -34,6 +34,7 @@
 #include "nsTableCellFrame.h"
 #include "nsTableFrame.h"
 #include "StickyScrollContainer.h"
+#include "nsContentUtils.h"
 
 using namespace mozilla;
 using namespace mozilla::css;
@@ -2715,7 +2716,8 @@ 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;
@@ -2723,6 +2725,12 @@ 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;
@@ -2740,7 +2748,8 @@ static nscoord GetNormalLineHeight(nsFontMetrics* aFontMetrics) {
   return normalLineHeight;
 }
 
-static inline nscoord ComputeLineHeight(const ComputedStyle* aComputedStyle,
+static inline nscoord ComputeLineHeight(nsIContent* aContent,
+                                        const ComputedStyle* aComputedStyle,
                                         nsPresContext* aPresContext,
                                         nscoord aBlockBSize,
                                         float aFontSizeInflation) {
@@ -2769,7 +2778,7 @@ static inline nscoord ComputeLineHeight(const ComputedStyle* aComputedStyle,
 
   RefPtr<nsFontMetrics> fm = nsLayoutUtils::GetFontMetricsForComputedStyle(
       aComputedStyle, aPresContext, aFontSizeInflation);
-  return GetNormalLineHeight(fm);
+  return GetNormalLineHeight(aContent, fm);
 }
 
 nscoord ReflowInput::GetLineHeight() const {
@@ -2806,7 +2815,7 @@ 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");
@@ -2819,7 +2828,7 @@ 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 448f8bab86ef..54926eb0c155 100644
--- a/layout/generic/test/mochitest.ini
+++ b/layout/generic/test/mochitest.ini
@@ -150,3 +150,4 @@ support-files =
     file_reframe_for_lazy_load_image.html
 [test_bug1655135.html]
 [test_bug1756831.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>

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