From 3e3fdbe831f24365780383b3c45a3d53f23ba435 Mon Sep 17 00:00:00 2001
From: Robert Loehning <robert.loehning@qt.io>
Date: Tue, 11 Aug 2020 14:22:58 +0200
Subject: [PATCH] QXmlStreamReader: Don't resize readBuffer to a size it
 already has

Resizing it to 0 will cause it to allocate memory. This will then
cause append() to copy the data from the other string instead of
using copy on write.

Task-number: oss-fuzz-24347
Pick-to: 5.12 5.15
Change-Id: I581bd109f9b973e1c70b7b41b1f610a2ad5725b8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/serialization/qxmlstream.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/src/corelib/serialization/qxmlstream.cpp b/src/corelib/serialization/qxmlstream.cpp
index 1296908110..ba6e3059df 100644
--- a/src/corelib/serialization/qxmlstream.cpp
+++ b/src/corelib/serialization/qxmlstream.cpp
@@ -1484,7 +1484,8 @@ uint QXmlStreamReaderPrivate::getChar_helper()
     const int BUFFER_SIZE = 8192;
     characterOffset += readBufferPos;
     readBufferPos = 0;
-    readBuffer.resize(0);
+    if (readBuffer.size())
+        readBuffer.resize(0);
     if (decoder.isValid())
         nbytesread = 0;
     if (device) {