qt: fix use-after-free pattern

qstring.toUtf8() returns a QByteArray object and .constData() returns
a pointer inside that object. It is not safe to store this pointer as
it will become invalid after the statement. Store a const reference
instead. (Due to scoping differences, some are copy-assigned though.)

In the UAT dialog, strlen(bytes.constData()) has also been replaced by
bytes.size() as an optimization.

Caught by ASAN.

Change-Id: Ie09f999a32d0ef1abaa1e658b9403b74bedffc37
Reviewed-on: https://code.wireshark.org/review/5528
Petri-Dish: Gerald Combs <gerald@wireshark.org>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Gerald Combs <gerald@wireshark.org>

1 files changed, 3 insertions, 4 deletions

diff --git a/ui/qt/display_filter_combo.cpp b/ui/qt/display_filter_combo.cpp
index 576a9a9b2e..edcad297fa 100644
--- a/ui/qt/display_filter_combo.cpp
+++ b/ui/qt/display_filter_combo.cpp
@@ -90,12 +90,11 @@ extern "C" void dfilter_recent_combo_write_all(FILE *rf) {
 
 void DisplayFilterCombo::writeRecent(FILE *rf) {
     int i;
-    const char *filter_str;
 
     for (i = 0; i < count(); i++) {
-        filter_str = itemText(i).toUtf8().constData();
-        if(filter_str && strlen(filter_str) > 0) {
-            fprintf(rf, RECENT_KEY_DISPLAY_FILTER ": %s\n", filter_str);
+        const QByteArray& filter = itemText(i).toUtf8();
+        if (!filter.isEmpty()) {
+            fprintf(rf, RECENT_KEY_DISPLAY_FILTER ": %s\n", filter.constData());
         }
     }
 }