tools/relocation_packer/src/delta_encoder.cc

   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.
  17 void 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.
  42 void 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