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 #include "delta_encoder.h" 6 7 #include <vector> 8 9 #include "debug.h" 10 #include "elf_traits.h" 11 12 namespace relocation_packer { 13 14 // Encode relative relocations with addends into a delta encoded (packed) 15 // representation. Represented as simple r_offset and r_addend delta pairs, 16 // with an implicit neutral element at the start. Encode(const std::vector<ELF::Rela> & relocations,std::vector<ELF::Sxword> * packed)17void RelocationDeltaCodec::Encode(const std::vector<ELF::Rela>& relocations, 18 std::vector<ELF::Sxword>* packed) { 19 // One relocation is sufficient for delta encoding. 20 if (relocations.size() < 1) 21 return; 22 23 // Start with the element count, then append the delta pairs. 24 packed->push_back(relocations.size()); 25 26 ELF::Addr offset = 0; 27 ELF::Sxword addend = 0; 28 29 for (size_t i = 0; i < relocations.size(); ++i) { 30 const ELF::Rela* relocation = &relocations[i]; 31 CHECK(ELF_R_TYPE(relocation->r_info) == ELF::kRelativeRelocationCode); 32 33 packed->push_back(relocation->r_offset - offset); 34 offset = relocation->r_offset; 35 packed->push_back(relocation->r_addend - addend); 36 addend = relocation->r_addend; 37 } 38 } 39 40 // Decode relative relocations with addends from a delta encoded (packed) 41 // representation. Decode(const std::vector<ELF::Sxword> & packed,std::vector<ELF::Rela> * relocations)42void RelocationDeltaCodec::Decode(const std::vector<ELF::Sxword>& packed, 43 std::vector<ELF::Rela>* relocations) { 44 // We need at least one packed pair after the packed pair count to be 45 // able to unpack. 46 if (packed.size() < 3) 47 return; 48 49 // Ensure that the packed data offers enough pairs. There may be zero 50 // padding on it that we ignore. 51 CHECK(static_cast<size_t>(packed[0]) <= (packed.size() - 1) >> 1); 52 53 ELF::Addr offset = 0; 54 ELF::Sxword addend = 0; 55 56 // The first packed vector element is the pairs count. Start uncondensing 57 // pairs at the second, and finish at the end of the pairs data. 58 const size_t pairs_count = packed[0]; 59 for (size_t i = 1; i < 1 + (pairs_count << 1); i += 2) { 60 offset += packed[i]; 61 addend += packed[i + 1]; 62 63 // Generate a relocation for this offset and addend pair. 64 ELF::Rela relocation; 65 relocation.r_offset = offset; 66 relocation.r_info = ELF_R_INFO(0, ELF::kRelativeRelocationCode); 67 relocation.r_addend = addend; 68 relocations->push_back(relocation); 69 } 70 } 71 72 } // namespace relocation_packer 73