commit 6b18f7ba0c983cb34395a3f77e6b9bb864511d3e Author: Igor Oliveira igor.oliveira@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 2c56afd2e02a..4d30c7762c14 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; @@ -2642,7 +2643,8 @@ 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; @@ -2650,6 +2652,12 @@ 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; @@ -2667,7 +2675,8 @@ 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) { @@ -2696,7 +2705,7 @@ 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 { @@ -2718,7 +2727,7 @@ 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"); @@ -2731,7 +2740,7 @@ 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 64348d308568..64437b3a0d23 100644 --- a/layout/generic/test/mochitest.ini +++ b/layout/generic/test/mochitest.ini @@ -144,3 +144,4 @@ 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>