qstrncpy: don't call strncpy_s with invalid parameters

According to https://msdn.microsoft.com/en-us/library/5dae5d43.aspx,
strncpy_s' second argument must not be 0:

> If strDest or strSource is NULL, *or numberOfElements is 0*, the
> invalid parameter handler is invoked.

Move the existing check for len > 0 up to protect the strncpy_s
call, too.

Change-Id: I70d339ea60d4b76f3038b2e4e4756f6590a9bd31
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 15 insertions, 6 deletions

diff --git a/src/corelib/tools/qbytearray.cpp b/src/corelib/tools/qbytearray.cpp
index 00d15fd518..8bae505d76 100644
--- a/src/corelib/tools/qbytearray.cpp
+++ b/src/corelib/tools/qbytearray.cpp
@@ -208,13 +208,14 @@ char *qstrncpy(char *dst, const char *src, uint len)
 {
     if (!src || !dst)
         return 0;
+    if (len > 0) {
 #if defined(_MSC_VER) && _MSC_VER >= 1400
-    strncpy_s(dst, len, src, len-1);
+        strncpy_s(dst, len, src, len - 1);
 #else
-    strncpy(dst, src, len);
+        strncpy(dst, src, len);
 #endif
-    if (len > 0)
         dst[len-1] = '\0';
+    }
     return dst;
 }
 
diff --git a/tests/auto/corelib/tools/qbytearray/tst_qbytearray.cpp b/tests/auto/corelib/tools/qbytearray/tst_qbytearray.cpp
index f942eab800..6d1c7481a9 100644
--- a/tests/auto/corelib/tools/qbytearray/tst_qbytearray.cpp
+++ b/tests/auto/corelib/tools/qbytearray/tst_qbytearray.cpp
@@ -794,9 +794,17 @@ void tst_QByteArray::qstrncpy()
 {
     QByteArray src(1024, 'a'), dst(1024, 'b');
 
-    // singularities
-    QCOMPARE(::qstrncpy(0, 0,0), (char*)0);
-    QCOMPARE(::qstrncpy(dst.data(), 0, 0), (char*)0);
+    // dst == nullptr
+    QCOMPARE(::qstrncpy(0, src.data(),  0), (char*)0);
+    QCOMPARE(::qstrncpy(0, src.data(), 10), (char*)0);
+
+    // src == nullptr
+    QCOMPARE(::qstrncpy(dst.data(), 0,  0), (char*)0);
+    QCOMPARE(::qstrncpy(dst.data(), 0, 10), (char*)0);
+
+    // valid pointers, but len == 0
+    QCOMPARE(::qstrncpy(dst.data(), src.data(), 0), dst.data());
+    QCOMPARE(*dst.data(), 'b'); // must not have written to dst
 
     // normal copy
     QCOMPARE(::qstrncpy(dst.data(), src.data(), src.size()), dst.data());