delta_encoder.cc revision 5f1c94371a64b3196d4be9466099bb892df9b88e 
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
12namespace 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.
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.
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