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 // Run-length encode and decode R_ARM_RELATIVE relocations. 6 // 7 // R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the 8 // .rel.dyn section) in libchrome<version>.so, and the ELF standard 9 // representation of them is wasteful. 10 // 11 // A relocation is 8 bytes (16 bytes on 64 bit plaforms), split into offset 12 // and info fields. Offsets strictly increase, and each is commonly a 13 // few bytes different from its predecessor. There are long runs where 14 // the difference does not change. The info field is always 0x17. Example, 15 // from 'readelf -x4 libchrome.<version>.so': 16 // 17 // offset info offset info 18 // 808fef01 17000000 848fef01 17000000 ................ 19 // 888fef01 17000000 8c8fef01 17000000 ................ 20 // 908fef01 17000000 948fef01 17000000 ................ 21 // 22 // Run length encoding packs this data more efficiently, by representing it 23 // as a delta and a count of entries each differing from its predecessor 24 // by this delta. The above can be represented as a start address followed 25 // by an encoded count of 6 and offset difference of 4: 26 // 27 // start count diff 28 // 808fef01 00000006 00000004 29 // 30 // Because R_ARM_RELATIVE relocation offsets strictly increase, the complete 31 // set of R_ARM_RELATIVE relocations in libchrome.<version>.so can be 32 // represented by a single start address followed by one or more difference 33 // and count encoded word pairs: 34 // 35 // start run1 count run1 diff run2 count run2 diff 36 // 808fef01 00000006 00000004 00000010 00000008 ... 37 // 38 // Decoding regenerates R_ARM_RELATIVE relocations beginning at address 39 // 'start' and for each encoded run, incrementing the address by 'difference' 40 // for 'count' iterations and emitting a new R_ARM_RELATIVE relocation. 41 // 42 // Once encoded, data is prefixed by a single word count of packed delta and 43 // count pairs. A final run-length encoded R_ARM_RELATIVE relocations vector 44 // might therefore look something like: 45 // 46 // pairs start run 1 run 2 ... run 15 47 // 0000000f 808fef01 00000006 00000004 00000010 00000008 ... 48 // Interpreted as: 49 // pairs=15 start=.. count=6,delta=4 count=16,delta=8 50 51 #ifndef TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ 52 #define TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ 53 54 #include <stdint.h> 55 #include <string.h> 56 #include <vector> 57 58 #include "elf.h" 59 60 namespace relocation_packer { 61 62 // A RelocationRunLengthCodec packs vectors of R_ARM_RELATIVE relocations 63 // into more compact forms, and unpacks them to reproduce the pre-packed data. 64 class RelocationRunLengthCodec { 65 public: 66 // Encode R_ARM_RELATIVE relocations into a more compact form. 67 // |relocations| is a vector of R_ARM_RELATIVE relocation structs. 68 // |packed| is the vector of packed words into which relocations are packed. 69 static void Encode(const std::vector<Elf32_Rel>& relocations, 70 std::vector<Elf32_Word>* packed); 71 72 // Decode R_ARM_RELATIVE relocations from their more compact form. 73 // |packed| is the vector of packed relocations. 74 // |relocations| is a vector of unpacked R_ARM_RELATIVE relocation structs. 75 static void Decode(const std::vector<Elf32_Word>& packed, 76 std::vector<Elf32_Rel>* relocations); 77 }; 78 79 } // namespace relocation_packer 80 81 #endif // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ 82
