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| 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_ |
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