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