Fix crash when text contains too many directional chars

In case a text to be layouted contains more than 128 directional characters
it causes the application to crash

The function initScriptAnalysisAndIsolatePairs() collects information of
RTL/LTR chaaracters into vector "isolatePairs". The size of the vector is
capped to 128. Later the function generateDirectionalRuns() iterates
the text again and tries to access items from the previously capped vector
above the upper bound.

Task-number: QTBUG-77819
Change-Id: Ibb7bf12c12b1db22f43ff46236518da3fdeed26a
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 24 insertions, 8 deletions

diff --git a/src/gui/text/qtextengine.cpp b/src/gui/text/qtextengine.cpp
index 2da13289bf..a7834587b1 100644
--- a/src/gui/text/qtextengine.cpp
+++ b/src/gui/text/qtextengine.cpp
@@ -399,6 +399,7 @@ struct QBidiAlgorithm {
                         analysis[i].bidiDirection = (level & 1) ? QChar::DirR : QChar::DirL;
                     runHasContent = true;
                     lastRunWithContent = -1;
+                    ++isolatePairPosition;
                 }
                 int runBeforeIsolate = runs.size();
                 ushort newLevel = isRtl ? ((stack.top().level + 1) | 1) : ((stack.top().level + 2) & ~1);
@@ -440,21 +441,19 @@ struct QBidiAlgorithm {
                 doEmbed(true, true, false);
                 break;
             case QChar::DirLRI:
-                Q_ASSERT(isolatePairs.at(isolatePairPosition).start == i);
                 doEmbed(false, false, true);
-                ++isolatePairPosition;
                 break;
             case QChar::DirRLI:
-                Q_ASSERT(isolatePairs.at(isolatePairPosition).start == i);
                 doEmbed(true, false, true);
-                ++isolatePairPosition;
                 break;
             case QChar::DirFSI: {
-                const auto &pair = isolatePairs.at(isolatePairPosition);
-                Q_ASSERT(pair.start == i);
-                bool isRtl = QStringView(text + pair.start + 1, pair.end - pair.start - 1).isRightToLeft();
+                bool isRtl = false;
+                if (isolatePairPosition < isolatePairs.size()) {
+                    const auto &pair = isolatePairs.at(isolatePairPosition);
+                    Q_ASSERT(pair.start == i);
+                    isRtl = QStringView(text + pair.start + 1, pair.end - pair.start - 1).isRightToLeft();
+                }
                 doEmbed(isRtl, false, true);
-                ++isolatePairPosition;
                 break;
             }
 
diff --git a/tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp b/tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp
index 9c477589f9..f0a32c2ed4 100644
--- a/tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp
+++ b/tests/auto/gui/text/qtextlayout/tst_qtextlayout.cpp
@@ -138,6 +138,7 @@ private slots:
     void noModificationOfInputString();
     void superscriptCrash_qtbug53911();
     void showLineAndParagraphSeparatorsCrash();
+    void tooManyDirectionalCharctersCrash_qtbug77819();
 
 private:
     QFont testFont;
@@ -2309,5 +2310,21 @@ void tst_QTextLayout::nbspWithFormat()
     QCOMPARE(layout.lineAt(1).textLength(), s2.length() + 1 + s3.length());
 }
 
+void tst_QTextLayout::tooManyDirectionalCharctersCrash_qtbug77819()
+{
+    QString data;
+    data += QString::fromUtf8("\xe2\x81\xa8"); // U+2068 FSI character
+    data += QString::fromUtf8("\xe2\x81\xa7"); // U+2067 RLI character
+
+    // duplicating the text
+    for (int i = 0; i < 10; i++)
+        data += data;
+
+    // Nothing to test. It must not crash in beginLayout().
+    QTextLayout tl(data);
+    tl.beginLayout();
+    tl.endLayout();
+}
+
 QTEST_MAIN(tst_QTextLayout)
 #include "tst_qtextlayout.moc"