Prevent excessive seeks in xbm detection

When auto-detecting an image type, the xbm handler would read
the entire file just to figure out that it's not an xbm. This patch
limits the read to maximum 4k and also breaks if line length >= 299.

Task-number: QT-4021
Reviewed-by: Robert Griebl

1 files changed, 17 insertions, 8 deletions

diff --git a/src/gui/image/qxbmhandler.cpp b/src/gui/image/qxbmhandler.cpp
index 0dd4e99..f9c2e0c 100644
--- a/src/gui/image/qxbmhandler.cpp
+++ b/src/gui/image/qxbmhandler.cpp
@@ -66,27 +66,36 @@ static inline int hex2byte(register char *p)
 static bool read_xbm_header(QIODevice *device, int& w, int& h)
 {
     const int buflen = 300;
+    const int maxlen = 4096;
     char buf[buflen + 1];
     QRegExp r1(QLatin1String("^#define[ \t]+[a-zA-Z0-9._]+[ \t]+"));
     QRegExp r2(QLatin1String("[0-9]+"));
 
     qint64 readBytes = 0;
+    qint64 totalReadBytes = 0;
 
-    // "#define .._width <num>"
-    readBytes = device->readLine(buf, buflen);
-    if (readBytes <= 0)
-	return false;
-    buf[readBytes - 1] = '\0';
+    buf[0] = '\0';
 
     // skip initial comment, if any
-    while (buf[0] != '#' && (readBytes = device->readLine( buf, buflen )) > 0) {}
+    while (buf[0] != '#') {
+        readBytes = device->readLine(buf, buflen);
+
+        // if readBytes >= buflen, it's very probably not a C file
+        if (readBytes <= 0 || readBytes >= buflen -1)
+            return false;
+
+        // limit xbm headers to the first 4k in the file to prevent
+        // excessive reads on non-xbm files
+        totalReadBytes += readBytes;
+        if (totalReadBytes >= maxlen)
+            return false;
+    }
 
-    if (readBytes <= 0)
-	return false;
     buf[readBytes - 1] = '\0';
     QString sbuf;
     sbuf = QString::fromLatin1(buf);
 
+    // "#define .._width <num>"
     if (r1.indexIn(sbuf) == 0 &&
          r2.indexIn(sbuf, r1.matchedLength()) == r1.matchedLength())
         w = QByteArray(&buf[r1.matchedLength()]).trimmed().toInt();