| // Copyright 2014 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. |
| |
| // Run-length encode and decode R_ARM_RELATIVE relocations. |
| // |
| // R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the |
| // .rel.dyn section) in libchrome<version>.so, and the ELF standard |
| // representation of them is wasteful. |
| // |
| // A relocation is 8 bytes (16 bytes on 64 bit plaforms), split into offset |
| // and info fields. Offsets strictly increase, and each is commonly a |
| // few bytes different from its predecessor. There are long runs where |
| // the difference does not change. The info field is always 0x17. Example, |
| // from 'readelf -x4 libchrome.<version>.so': |
| // |
| // offset info offset info |
| // 808fef01 17000000 848fef01 17000000 ................ |
| // 888fef01 17000000 8c8fef01 17000000 ................ |
| // 908fef01 17000000 948fef01 17000000 ................ |
| // |
| // Run length encoding packs this data more efficiently, by representing it |
| // as a delta and a count of entries each differing from its predecessor |
| // by this delta. The above can be represented as a start address followed |
| // by an encoded count of 6 and offset difference of 4: |
| // |
| // start count diff |
| // 808fef01 00000006 00000004 |
| // |
| // Because R_ARM_RELATIVE relocation offsets strictly increase, the complete |
| // set of R_ARM_RELATIVE relocations in libchrome.<version>.so can be |
| // represented by a single start address followed by one or more difference |
| // and count encoded word pairs: |
| // |
| // start run1 count run1 diff run2 count run2 diff |
| // 808fef01 00000006 00000004 00000010 00000008 ... |
| // |
| // Decoding regenerates R_ARM_RELATIVE relocations beginning at address |
| // 'start' and for each encoded run, incrementing the address by 'difference' |
| // for 'count' iterations and emitting a new R_ARM_RELATIVE relocation. |
| // |
| // Once encoded, data is prefixed by a single word count of packed delta and |
| // count pairs. A final run-length encoded R_ARM_RELATIVE relocations vector |
| // might therefore look something like: |
| // |
| // pairs start run 1 run 2 ... run 15 |
| // 0000000f 808fef01 00000006 00000004 00000010 00000008 ... |
| // Interpreted as: |
| // pairs=15 start=.. count=6,delta=4 count=16,delta=8 |
| |
| #ifndef TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ |
| #define TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ |
| |
| #include <stdint.h> |
| #include <string.h> |
| #include <vector> |
| |
| #include "elf.h" |
| |
| namespace relocation_packer { |
| |
| // A RelocationRunLengthCodec packs vectors of R_ARM_RELATIVE relocations |
| // into more compact forms, and unpacks them to reproduce the pre-packed data. |
| class RelocationRunLengthCodec { |
| public: |
| // Encode R_ARM_RELATIVE relocations into a more compact form. |
| // |relocations| is a vector of R_ARM_RELATIVE relocation structs. |
| // |packed| is the vector of packed words into which relocations are packed. |
| static void Encode(const std::vector<Elf32_Rel>& relocations, |
| std::vector<Elf32_Word>* packed); |
| |
| // Decode R_ARM_RELATIVE relocations from their more compact form. |
| // |packed| is the vector of packed relocations. |
| // |relocations| is a vector of unpacked R_ARM_RELATIVE relocation structs. |
| static void Decode(const std::vector<Elf32_Word>& packed, |
| std::vector<Elf32_Rel>* relocations); |
| }; |
| |
| } // namespace relocation_packer |
| |
| #endif // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ |