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// Copyright 2018 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#ifndef COMPONENTS_ZUCCHINI_REFERENCE_BYTES_MIXER_H_
#define COMPONENTS_ZUCCHINI_REFERENCE_BYTES_MIXER_H_
#include <stdint.h>
#include <memory>
#include <vector>
#include "base/macros.h"
#include "components/zucchini/buffer_view.h"
#include "components/zucchini/image_utils.h"
namespace zucchini {
class Disassembler;
// References encoding may be quite complex in some architectures (e.g., ARM),
// requiring bit-level manipulation. In general, bits in a reference body fall
// under 2 categories:
// - Operation bits: Instruction op code, conditionals, or structural data.
// - Payload bits: Actual target data of the reference. These may be absolute,
// or be displacements relative to instruction pointer / program counter.
// During patch application,
// Old reference bytes = {old operation, old payload},
// is transformed to
// New reference bytes = {new operation, new payload}.
// New image bytes are written by three sources:
// (1) Direct copy from old image to new image for matched blocks.
// (2) Bytewise diff correction.
// (3) Dedicated reference target correction.
//
// For references whose operation and payload bits are stored in easily
// separable bytes (e.g., rel32 reference in X86), (2) can exclude payload bits.
// So during patch application, (1) naively copies everything, (2) fixes
// operation bytes only, and (3) fixes payload bytes only.
//
// For architectures with references whose operation and payload bits may mix
// within shared bytes (e.g., ARM rel32), a dilemma arises:
// - (2) cannot ignores shared bytes, since otherwise new operation bits not
// properly transfer.
// - Having (2) always overwrite these bytes would reduce the benefits of
// reference correction, since references are likely to change.
//
// Our solution applies a hybrid approach: For each matching old / new reference
// pair, define:
// Mixed reference bytes = {new operation, old payload},
//
// During patch generation, we compute bytewise correction from old reference
// bytes to the mixed reference bytes. So during patch application, (2) only
// corrects operation bit changes (and skips if they don't change), and (3)
// overwrites old payload bits to new payload bits.
// A base class for (stateful) mixed reference byte generation. This base class
// serves as a stub. Architectures whose references store operation bits and
// payload bits can share common bytes (e.g., ARM rel32) should override this.
class ReferenceBytesMixer {
public:
ReferenceBytesMixer();
virtual ~ReferenceBytesMixer();
// Returns a new ReferenceBytesMixer instance that's owned by the caller.
static std::unique_ptr<ReferenceBytesMixer> Create(
const Disassembler& src_dis,
const Disassembler& dst_dis);
// Returns the number of bytes that need to be mixed for references with given
// |type|. Returns 0 if no mixing is required.
virtual int NumBytes(uint8_t type) const;
// Computes mixed reference bytes by combining (a) "payload bits" from an
// "old" reference of |type| at |old_base[old_offset]| with (b) "operation
// bits" from a "new" reference of |type| at |new_base[new_offset]|. Returns
// the result as ConstBufferView, which is valid only until the next call to
// Mix().
virtual ConstBufferView Mix(uint8_t type,
ConstBufferView::const_iterator old_base,
offset_t old_offset,
ConstBufferView::const_iterator new_base,
offset_t new_offset);
private:
DISALLOW_COPY_AND_ASSIGN(ReferenceBytesMixer);
};
} // namespace zucchini
#endif // COMPONENTS_ZUCCHINI_REFERENCE_BYTES_MIXER_H_
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