--- /dev/null	2020-09-04 12:37:41.765504620 +0200
+++ new/src/hotspot/share/memory/metaspace/msAllocationGuard.hpp	2020-09-04 13:57:38.397271049 +0200
@@ -0,0 +1,119 @@
+/*
+ * Copyright (c) 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2020 SAP SE. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_MEMORY_METASPACE_MSALLOCATIONGUARD_HPP
+#define SHARE_MEMORY_METASPACE_MSALLOCATIONGUARD_HPP
+
+#include "memory/allocation.hpp"
+#include "memory/metaspace/msChunklevel.hpp"
+#include "utilities/debug.hpp"
+#include "utilities/globalDefinitions.hpp"
+
+// In Debug builds, Metadata in Metaspace can be optionally guarded - enclosed in canaries -
+// to detect memory overwriters.
+//
+// These canaries are periodically checked, e.g. when the Metaspace is purged in a context
+// of a GC.
+
+// The canaries precede any allocated block...
+//
+// +---------------+
+// |  'METAMETA'   |
+// +---------------+
+// |  block size   |
+// +---------------+
+// |  block...     |
+// .               .
+// .               .
+// .               .
+// |               |
+// +---------------+
+// . <padding>     .
+// +---------------+
+// |  'METAMETA'   |
+// +---------------+
+// |  block size   |
+// +---------------+
+// |  block...     |
+
+// ... and since the blocks are allocated via pointer bump and closely follow each other,
+// one block's prefix is its predecessor's suffix, so apart from the last block all
+// blocks have an overwriter canary on both ends.
+//
+
+// Note: this feature is only available in debug, and is activated using
+//  -XX:+MetaspaceGuardAllocations. When active, it disables deallocation handling - since
+//  freeblock handling in the freeblock lists would get too complex - so one may run leaks
+//  in deallocation-heavvy scenarios (e.g. lots of class redefinitions).
+//
+
+namespace metaspace {
+
+#ifdef ASSERT
+
+struct Prefix {
+  static const uintx EyeCatcher =
+      NOT_LP64(0x77698465) LP64_ONLY(0x7769846577698465ULL); // "META" resp "METAMETA"
+
+  const uintx _mark;
+  const size_t _word_size;   // raw word size including prefix
+  // MetaWord payload [0];   // varsized (but unfortunately not all our compilers understand that)
+
+  Prefix(size_t word_size)
+    : _mark(EyeCatcher),
+      _word_size(word_size)
+  {}
+
+  MetaWord* payload() const {
+    return (MetaWord*)(this + 1);
+  }
+
+  void check() const {
+    assert(_mark == EyeCatcher, "Corrupt block: missing eyecatcher (at " PTR_FORMAT ").", p2i(this));
+    assert(_word_size > 0 && _word_size < chunklevel::MAX_CHUNK_WORD_SIZE,
+           "Corrupt block: invalid size " SIZE_FORMAT " (at " PTR_FORMAT ").", _word_size, p2i(this));
+  }
+
+};
+
+// The prefix structure must be aligned to MetaWord size.
+STATIC_ASSERT((sizeof(Prefix) & WordAlignmentMask) == 0);
+
+inline size_t prefix_size() {
+  return sizeof(Prefix);
+}
+
+// Given a pointer to a memory area, establish the prefix at the start of that area and
+// return the starting pointer to the payload.
+inline MetaWord* establish_prefix(MetaWord* p_raw, size_t raw_word_size) {
+  const Prefix* pp = new(p_raw)Prefix(raw_word_size);
+  return pp->payload();
+}
+
+#endif
+
+} // namespace metaspace
+
+#endif // SHARE_MEMORY_METASPACE_MSALLOCATIONGUARD_HPP