| // 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 relative relocations. |
| // |
| // Relative relocations are the bulk of dynamic relocations (the |
| // .rel.dyn or .rela.dyn sections) in libchrome.<version>.so, and the ELF |
| // standard representation of them is wasteful. .rel.dyn contains |
| // relocations without addends, .rela.dyn relocations with addends. |
| // |
| // A relocation with no addend is 8 bytes on 32 bit platforms and 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 constant. Example, from 'readelf -x4 libchrome.<version>.so' 32 bit: |
| // |
| // 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 |
| // 01ef8f80 00000006 00000004 |
| // |
| // Because relative relocation offsets strictly increase, the complete |
| // set of 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 |
| // 01ef8f80 00000006 00000004 00000010 00000008 ... |
| // |
| // Decoding regenerates relative relocations beginning at address |
| // 'start' and for each encoded run, incrementing the address by 'difference' |
| // for 'count' iterations and emitting a new relative relocation. |
| // |
| // Once encoded, data is prefixed by a single word count of packed delta and |
| // count pairs. A final run-length encoded relative relocations vector |
| // might therefore look something like: |
| // |
| // pairs start run 1 run 2 ... run 15 |
| // 0000000f 01ef8f80 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 <vector> |
| |
| #include "elf.h" |
| #include "elf_traits.h" |
| |
| namespace relocation_packer { |
| |
| // A RelocationRunLengthCodec packs vectors of relative relocations |
| // into more compact forms, and unpacks them to reproduce the pre-packed data. |
| class RelocationRunLengthCodec { |
| public: |
| // Encode relative relocations into a more compact form. |
| // |relocations| is a vector of relative relocation structs. |
| // |packed| is the vector of packed words into which relocations are packed. |
| static void Encode(const std::vector<ELF::Rel>& relocations, |
| std::vector<ELF::Xword>* packed); |
| |
| // Decode relative relocations from their more compact form. |
| // |packed| is the vector of packed relocations. |
| // |relocations| is a vector of unpacked relative relocation structs. |
| static void Decode(const std::vector<ELF::Xword>& packed, |
| std::vector<ELF::Rel>* relocations); |
| }; |
| |
| } // namespace relocation_packer |
| |
| #endif // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_ |
