1// Copyright 2014 The Chromium Authors. All rights reserved. 2// Use of this source code is governed by a BSD-style license that can be 3// found in the LICENSE file. 4 5// Delta encode and decode relative relocations with addends. 6// 7// Relative relocations are the bulk of dynamic relocations (the 8// .rel.dyn or .rela.dyn sections) in libchrome<version>.so, and the ELF 9// standard representation of them is wasteful. .rel.dyn contains 10// relocations without addends, .rela.dyn relocations with addends. 11// 12// A relocation with an addend is 12 bytes on 32 bit platforms and 24 bytes 13// on 64 bit plaforms. It is split into offset, info, and addend fields. 14// Offsets strictly increase, and each is commonly a few bytes different 15// from its predecessor. Addends are less well behaved. The info field is 16// constant. Example, from 'readelf -x4 libchrome.<version>.so' 64 bit: 17// 18// offset info 19// 80949303 00000000 03040000 00000000 ................ 20// addend offset 21// fc015b00 00000000 88949303 00000000 ..[............. 22// info addend 23// 03040000 00000000 24025b00 00000000 ........$.[..... 24// offset info 25// 90949303 00000000 03040000 00000000 ................ 26// addend offset 27// 3c025b00 00000000 98949303 00000000 <.[............. 28// info addend 29// 03040000 00000000 50025b00 00000000 ........P.[..... 30// 31// The offset strictly increases, but the addend is unpredictable, so run 32// length encoding will not work well with this data. We can however pack 33// with delta encoding. The upper four bytes of the eight byte offset and 34// addend are invariably zeroes. The difference between adjacent offsets 35// is almost always small, and between adjacent addends is often small. And 36// info is constant and can be eliminated. 37// 38// Delta encoding reduces the size of the data modestly, so that the first 39// three relocations above can be represented as: 40// 41// initial offset initial addend offset delta addend delta 42// 00000000 03939480 00000000 005b01fc 00000000 00000008 00000000 00000028 43// offset delta addend delta ... 44// 00000000 00000008 00000000 0000009f 45// 46// The addend delta can be negative as well as positive, but overall the 47// deltas have a much smaller range than the input data. When encoded as 48// signed LEB128 the total data reduction becomes useful. 49 50#ifndef TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_ 51#define TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_ 52 53#include <vector> 54 55#include "elf.h" 56#include "elf_traits.h" 57 58namespace relocation_packer { 59 60// A RelocationDeltaCodec packs vectors of relative relocations with 61// addends into more compact forms, and unpacks them to reproduce the 62// pre-packed data. 63class RelocationDeltaCodec { 64 public: 65 // Encode relative relocations with addends into a more compact form. 66 // |relocations| is a vector of relative relocation with addend structs. 67 // |packed| is the vector of packed words into which relocations are packed. 68 static void Encode(const std::vector<ELF::Rela>& relocations, 69 std::vector<ELF::Sxword>* packed); 70 71 // Decode relative relocations with addends from their more compact form. 72 // |packed| is the vector of packed relocations. 73 // |relocations| is a vector of unpacked relative relocations. 74 static void Decode(const std::vector<ELF::Sxword>& packed, 75 std::vector<ELF::Rela>* relocations); 76}; 77 78} // namespace relocation_packer 79 80#endif // TOOLS_RELOCATION_PACKER_SRC_DELTA_ENCODER_H_ 81