Make POSIX transition rule parser more robust

The POSIX rule parser used by QTzTimeZonePrivate recklessly assumed
that, if splitting the rule on a dot produced more than one part, it
necessarily produced at least three. That's true for well-formed POSIX
rules, but we should catch the case of malformed rules.

Likewise, when calculating the dates of transitions, splitting the
date rule on dots might produce too few fragments; and the fragments
might not parse as valid numbers, or might be out of range for their
respective fields in a date. Check all these cases, too.

Added a test that crashed previously. Changed
QTimeZone::offsetFromUtc() so that its "return zero on invalid"
applies also to the case where the backend returns invalid, in
support of this.

Fixes: QTBUG-92808
Change-Id: Ica383a7a987465483341bdef8dcfd42edb6b43d6
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Robert Löhning <robert.loehning@qt.io>
(cherry picked from commit 964f91fd25a59654905c5a68d3cbccedab9ebb5a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qtimezoneprivate_tz.cpp

@@ -351,6 +351,11 @@ static QByteArray parseTzPosixRule(QDataStream &ds)
 
 static QDate calculateDowDate(int year, int month, int dayOfWeek, int week)
 {
+    if (dayOfWeek == 0) // Sunday; we represent it as 7, POSIX uses 0
+        dayOfWeek = 7;
+    else if (dayOfWeek & ~7 || month < 1 || month > 12 || week < 1 || week > 5)
+        return QDate();
+
     QDate date(year, month, 1);
     int startDow = date.dayOfWeek();
     if (startDow <= dayOfWeek)
@@ -365,28 +370,34 @@ static QDate calculateDowDate(int year, int month, int dayOfWeek, int week)
 
 static QDate calculatePosixDate(const QByteArray &dateRule, int year)
 {
+    bool ok;
     // Can start with M, J, or a digit
     if (dateRule.at(0) == 'M') {
         // nth week in month format "Mmonth.week.dow"
         QList<QByteArray> dateParts = dateRule.split('.');
-        int month = dateParts.at(0).mid(1).toInt();
+        if (dateParts.count() > 2) {
-        int week = dateParts.at(1).toInt();
+            int month = dateParts.at(0).mid(1).toInt(&ok);
-        int dow = dateParts.at(2).toInt();
+            int week = ok ? dateParts.at(1).toInt(&ok) : 0;
-        if (dow == 0) // Sunday; we represent it as 7
+            int dow = ok ? dateParts.at(2).toInt(&ok) : 0;
-            dow = 7;
+            if (ok)
                 return calculateDowDate(year, month, dow, week);
+        }
     } else if (dateRule.at(0) == 'J') {
         // Day of Year ignores Feb 29
-        int doy = dateRule.mid(1).toInt();
+        int doy = dateRule.mid(1).toInt(&ok);
+        if (ok && doy > 0 && doy < 366) {
             QDate date = QDate(year, 1, 1).addDays(doy - 1);
-        if (QDate::isLeapYear(date.year()))
+            if (QDate::isLeapYear(date.year()) && date.month() > 2)
                 date = date.addDays(-1);
             return date;
+        }
     } else {
         // Day of Year includes Feb 29
-        int doy = dateRule.toInt();
+        int doy = dateRule.toInt(&ok);
+        if (ok && doy > 0 && doy <= 366)
             return QDate(year, 1, 1).addDays(doy - 1);
     }
+    return QDate();
 }
 
 // returns the time in seconds, INT_MIN if we failed to parse
@@ -576,7 +587,8 @@ static QList<QTimeZonePrivate::Data> calculatePosixTransitions(const QByteArray
         result << data;
         return result;
     }
-
+    if (parts.count() < 3 || parts.at(1).isEmpty() || parts.at(2).isEmpty())
+        return result; // Malformed.
 
     // Get the std to dst transtion details
     QList<QByteArray> dstParts = parts.at(1).split('/');
@@ -596,6 +608,9 @@ static QList<QTimeZonePrivate::Data> calculatePosixTransitions(const QByteArray
     else
         stdTime = QTime(2, 0, 0);
 
+    if (dstDateRule.isEmpty() || stdDateRule.isEmpty() || !dstTime.isValid() || !stdTime.isValid())
+        return result; // Malformed.
+
     // Limit year to the range QDateTime can represent:
     const int minYear = int(QDateTime::YearRange::First);
     const int maxYear = int(QDateTime::YearRange::Last);

tests/auto/corelib/time/qtimezone/tst_qtimezone.cpp

@@ -67,6 +67,7 @@ private Q_SLOTS:
     void isValidId_data();
     void isValidId();
     void serialize();
+    void malformed();
     // Backend tests
     void utcTest();
     void icuTest();
@@ -961,6 +962,21 @@ void tst_QTimeZone::serialize()
         QSKIP("No serialization enabled");
 }
 
+void tst_QTimeZone::malformed()
+{
+    // Regression test for QTBUG-92808
+    // Strings that look enough like a POSIX zone specifier that the constructor
+    // accepts them, but the specifier is invalid.
+    // Must not crash or trigger assertions when calling offsetFromUtc()
+    const QDateTime now = QDateTime::currentDateTime();
+    QTimeZone barf("QUT4tCZ0 , /");
+    if (barf.isValid())
+        barf.offsetFromUtc(now);
+    barf = QTimeZone("QtC+09,,MA");
+    if (barf.isValid())
+        barf.offsetFromUtc(now);
+}
+
 void tst_QTimeZone::utcTest()
 {
 #ifdef QT_BUILD_INTERNAL