Added version 0.2 from V.

crcutil-1.0/code/protected_crc.h

@@ -0,0 +1,61 @@
+// Copyright 2010 Google Inc.  All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Protects CRC tables with its own CRC.
+// CRC tables get corrupted too, and if corruption is
+// not caught, data poisoning becomes a reality.
+
+#ifndef CRCUTIL_PROTECTED_CRC_H_
+#define CRCUTIL_PROTECTED_CRC_H_
+
+namespace crcutil {
+
+#pragma pack(push, 16)
+
+// Class CrcImplementation should not have virtual functions:
+// vptr is stored as the very first field, vptr value is defined
+// at runtime, so it is impossible to CRC(*this) once and
+// guarantee that this value will not change from run to run.
+//
+template<typename CrcImplementation> class ProtectedCrc
+    : public CrcImplementation {
+ public:
+  typedef typename CrcImplementation::Crc Crc;
+
+  // Returns check value that the caller should compare
+  // against pre-computed, trusted constant.
+  //
+  // Computing SelfCheckValue() after CRC initialization,
+  // storing it in memory, and periodically checking against
+  // stored value may not work: if CRC tables were initialized
+  // incorrectly and/or had been corrupted during initialization,
+  // CheckValue() will return garbage. Garbage in, garbage out.
+  // Consequitive checks will not detect a problem, the application
+  // will happily produce and save the data with corrupt CRC.
+  //
+  // The application should call SelfCheckValue() regularly:
+  // 1. First and foremost, on every CRC mismatch.
+  // 2. After CRC'ing the  data but before sending it out or writing it.
+  // 3. Worst case, every Nth CRC'ed byte or every Nth call to CRC.
+  //
+  Crc SelfCheckValue() const {
+    return CrcDefault(this, sizeof(*this), 0);
+  }
+} GCC_ALIGN_ATTRIBUTE(16);
+
+#pragma pack(pop)
+
+}  // namespace crcutil
+
+#endif  // CRCUTIL_PROTECTED_CRC_H_