QString: Respect precision when reading data for %.*s format string

If we disregard the precision we may read a very large string that we
subsequently discard. Furthermore, people use this to read
non-null-terminated strings, which randomly crashes.

Change-Id: Ifa255dbe71c82d3d4fb46adfef7a9dc74bd40cee
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit e99e07cb5c939ca5bbb1dfdeb66c862d6cd4f2f2)
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/text/qstring.cpp

@@ -6772,13 +6772,17 @@ QString QString::vasprintf(const char *cformat, va_list ap)
                 if (length_mod == lm_l) {
                     const ushort *buff = va_arg(ap, const ushort*);
                     const ushort *ch = buff;
-                    while (*ch != 0)
+                    while (precision != 0 && *ch != 0) {
                         ++ch;
+                        --precision;
+                    }
                     subst.setUtf16(buff, ch - buff);
-                } else
+                } else if (precision == -1) {
                     subst = QString::fromUtf8(va_arg(ap, const char*));
-                if (precision != -1)
+                } else {
-                    subst.truncate(precision);
+                    const char *buff = va_arg(ap, const char*);
+                    subst = QString::fromUtf8(buff, qstrnlen(buff, precision));
+                }
                 ++c;
                 break;
             }

tests/auto/corelib/text/qstring/tst_qstring.cpp

@@ -584,6 +584,7 @@ private slots:
     void isValidUtf16_data();
     void isValidUtf16();
     void unicodeStrings();
+    void vasprintfWithPrecision();
 };
 
 template <class T> const T &verifyZeroTermination(const T &t) { return t; }
@@ -6937,6 +6938,35 @@ void tst_QString::isValidUtf16()
     QTEST(string.isValidUtf16(), "valid");
 }
 
+static QString doVasprintf(const char *msg, ...) {
+    va_list args;
+    va_start(args, msg);
+    const QString result = QString::vasprintf(msg, args);
+    va_end(args);
+    return result;
+}
+
+void tst_QString::vasprintfWithPrecision()
+{
+    {
+        const char *msg = "Endpoint %.*s with";
+        static const char arg0[3] = { 'a', 'b', 'c' };
+        static const char arg1[4] = { 'a', 'b', 'c', '\0' };
+        QCOMPARE(doVasprintf(msg, 3, arg0), QStringLiteral("Endpoint abc with"));
+        QCOMPARE(doVasprintf(msg, 9, arg1), QStringLiteral("Endpoint abc with"));
+        QCOMPARE(doVasprintf(msg, 0, nullptr), QStringLiteral("Endpoint  with"));
+    }
+
+    {
+        const char *msg = "Endpoint %.*ls with";
+        static const ushort arg0[3] = { 'a', 'b', 'c' };
+        static const ushort arg1[4] = { 'a', 'b', 'c', '\0' };
+        QCOMPARE(doVasprintf(msg, 3, arg0), QStringLiteral("Endpoint abc with"));
+        QCOMPARE(doVasprintf(msg, 9, arg1), QStringLiteral("Endpoint abc with"));
+        QCOMPARE(doVasprintf(msg, 0, nullptr), QStringLiteral("Endpoint  with"));
+    }
+}
+
 QTEST_APPLESS_MAIN(tst_QString)
 
 #include "tst_qstring.moc"