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81 lines
3.5 KiB
C++
81 lines
3.5 KiB
C++
// Copyright 2014 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// Delta encode and decode relative relocations with addends.
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//
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// Relative relocations are the bulk of dynamic relocations (the
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// .rel.dyn or .rela.dyn sections) in libchrome.<version>.so, and the ELF
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// standard representation of them is wasteful. .rel.dyn contains
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// relocations without addends, .rela.dyn relocations with addends.
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//
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// A relocation with an addend is 12 bytes on 32 bit platforms and 24 bytes
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// on 64 bit plaforms. It is split into offset, info, and addend fields.
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// Offsets strictly increase, and each is commonly a few bytes different
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// from its predecessor. Addends are less well behaved. The info field is
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// constant. Example, from 'readelf -x4 libchrome.<version>.so' 64 bit:
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//
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// offset info
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// 80949303 00000000 03040000 00000000 ................
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// addend offset
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// fc015b00 00000000 88949303 00000000 ..[.............
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// info addend
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// 03040000 00000000 24025b00 00000000 ........$.[.....
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// offset info
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// 90949303 00000000 03040000 00000000 ................
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// addend offset
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// 3c025b00 00000000 98949303 00000000 <.[.............
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// info addend
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// 03040000 00000000 50025b00 00000000 ........P.[.....
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//
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// The offset strictly increases, but the addend is unpredictable, so run
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// length encoding will not work well with this data. We can however pack
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// with delta encoding. The upper four bytes of the eight byte offset and
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// addend are invariably zeroes. The difference between adjacent offsets
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// is almost always small, and between adjacent addends is often small. And
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// info is constant and can be eliminated.
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//
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// Delta encoding reduces the size of the data modestly, so that the first
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// three relocations above can be represented as:
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//
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// initial offset initial addend offset delta addend delta
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// 00000000 03939480 00000000 005b01fc 00000000 00000008 00000000 00000028
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// offset delta addend delta ...
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// 00000000 00000008 00000000 0000009f
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//
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// The addend delta can be negative as well as positive, but overall the
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// deltas have a much smaller range than the input data. When encoded as
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// signed LEB128 the total data reduction becomes useful.
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#ifndef TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_
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#define TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_
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#include <vector>
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#include "elf.h"
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#include "elf_traits.h"
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namespace relocation_packer {
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// A RelocationDeltaCodec packs vectors of relative relocations with
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// addends into more compact forms, and unpacks them to reproduce the
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// pre-packed data.
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class RelocationDeltaCodec {
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public:
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// Encode relative relocations with addends into a more compact form.
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// |relocations| is a vector of relative relocation with addend structs.
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// |packed| is the vector of packed words into which relocations are packed.
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static void Encode(const std::vector<ELF::Rela>& relocations,
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std::vector<ELF::Sxword>* packed);
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// Decode relative relocations with addends from their more compact form.
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// |packed| is the vector of packed relocations.
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// |relocations| is a vector of unpacked relative relocations.
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static void Decode(const std::vector<ELF::Sxword>& packed,
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std::vector<ELF::Rela>* relocations);
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};
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} // namespace relocation_packer
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#endif // TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_
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