From 2adf444b5e73f1f43ac358581c88917c0eace8e8 Mon Sep 17 00:00:00 2001
From: Thiago Macieira <thiago.macieira@intel.com>
Date: Tue, 28 Sep 2021 18:41:04 -0700
Subject: [PATCH] QPlugin: change the generic metadata scan to forwards

The reason it scanned backwards was valid, but unfortunately that
doesn't work if the metadata magic appears in the debug info that comes
after the metadata.

Task-number: QTBUG-67461
Change-Id: I2bbf422288924c198645fffd16a9276767ca2589
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
---
 src/corelib/plugin/qlibrary.cpp | 38 ++++++++-------------------------
 1 file changed, 9 insertions(+), 29 deletions(-)

diff --git a/src/corelib/plugin/qlibrary.cpp b/src/corelib/plugin/qlibrary.cpp
index 634dc5a00cf..ee81b32bfb8 100644
--- a/src/corelib/plugin/qlibrary.cpp
+++ b/src/corelib/plugin/qlibrary.cpp
@@ -41,6 +41,7 @@
 #include "qlibrary_p.h"
 
 #include <q20algorithm.h>
+#include <qbytearraymatcher.h>
 #include <qdebug.h>
 #include <qendian.h>
 #include <qfile.h>
@@ -186,38 +187,17 @@ static qsizetype qt_find_pattern(const char *s, qsizetype s_len,
                              const char *pattern, ulong p_len)
 {
     /*
-      we search from the end of the file because on the supported
-      systems, the read-only data/text segments are placed at the end
-      of the file.  HOWEVER, when building with debugging enabled, all
-      the debug symbols are placed AFTER the data/text segments.
+      We used to search from the end of the file so we'd skip the code and find
+      the read-only data that usually follows. Unfortunately, in debug builds,
+      the debug sections come after and are usually much bigger than everything
+      else, making this process slower than necessary with debug plugins.
 
-      what does this mean?  when building in release mode, the search
-      is fast because the data we are looking for is at the end of the
-      file... when building in debug mode, the search is slower
-      because we have to skip over all the debugging symbols first
+      More importantly, the pattern string may exist in the debug information due
+      to it being used in the plugin in the first place.
     */
 
-    if (!s || !pattern || qsizetype(p_len) > s_len)
-        return -1;
-
-    size_t i, hs = 0, hp = 0, delta = s_len - p_len;
-
-    for (i = 0; i < p_len; ++i) {
-        hs += s[delta + i];
-        hp += pattern[i];
-    }
-    i = delta;
-    for (;;) {
-        if (hs == hp && qstrncmp(s + i, pattern, p_len) == 0)
-            return i;   // can't overflow, by construction
-        if (i == 0)
-            break;
-        --i;
-        hs -= s[i + p_len];
-        hs += s[i];
-    }
-
-    return -1;
+    static const QByteArrayMatcher matcher(QByteArray(pattern, p_len));
+    return matcher.indexIn(s, s_len);
 }
 
 /*