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// Run-length encode and decode relative relocations.
6//
7// Relative relocations are the bulk of dynamic relocations (the
8// .rel.dyn or .rela.dyn sections) in libchrome<version>.so, and the ELF
9// standard representation of them is wasteful.  .rel.dyn contains
10// relocations without addends, .rela.dyn relocations with addends.
11//
12// A relocation with no addend is 8 bytes on 32 bit platforms and 16 bytes
13// on 64 bit plaforms, split into offset and info fields.  Offsets strictly
14// increase, and each is commonly a few bytes different from its predecessor.
15// There are long runs where the difference does not change.  The info field
16// is constant.  Example, from 'readelf -x4 libchrome.<version>.so' 32 bit:
17//
18//   offset   info     offset   info
19//   808fef01 17000000 848fef01 17000000 ................
20//   888fef01 17000000 8c8fef01 17000000 ................
21//   908fef01 17000000 948fef01 17000000 ................
22//
23// Run length encoding packs this data more efficiently, by representing it
24// as a delta and a count of entries each differing from its predecessor
25// by this delta.  The above can be represented as a start address followed
26// by an encoded count of 6 and offset difference of 4:
27//
28//   start    count    diff
29//   01ef8f80 00000006 00000004
30//
31// Because relative relocation offsets strictly increase, the complete
32// set of relative relocations in libchrome.<version>.so can be
33// represented by a single start address followed by one or more difference
34// and count encoded word pairs:
35//
36//   start    run1 count run1 diff  run2 count run2 diff
37//   01ef8f80 00000006   00000004   00000010   00000008 ...
38//
39// Decoding regenerates relative relocations beginning at address
40// 'start' and for each encoded run, incrementing the address by 'difference'
41// for 'count' iterations and emitting a new relative relocation.
42//
43// Once encoded, data is prefixed by a single word count of packed delta and
44// count pairs.  A final run-length encoded relative relocations vector
45// might therefore look something like:
46//
47//   pairs    start    run 1             run 2             ... run 15
48//   0000000f 01ef8f80 00000006 00000004 00000010 00000008 ...
49// Interpreted as:
50//   pairs=15 start=.. count=6,delta=4   count=16,delta=8
51
52#ifndef TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_
53#define TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_
54
55#include <vector>
56
57#include "elf.h"
58#include "elf_traits.h"
59
60namespace relocation_packer {
61
62// A RelocationRunLengthCodec packs vectors of relative relocations
63// into more compact forms, and unpacks them to reproduce the pre-packed data.
64class RelocationRunLengthCodec {
65 public:
66  // Encode relative relocations into a more compact form.
67  // |relocations| is a vector of relative relocation structs.
68  // |packed| is the vector of packed words into which relocations are packed.
69  static void Encode(const std::vector<ELF::Rel>& relocations,
70                     std::vector<ELF::Xword>* packed);
71
72  // Decode relative relocations from their more compact form.
73  // |packed| is the vector of packed relocations.
74  // |relocations| is a vector of unpacked relative relocation structs.
75  static void Decode(const std::vector<ELF::Xword>& packed,
76                     std::vector<ELF::Rel>* relocations);
77};
78
79}  // namespace relocation_packer
80
81#endif  // TOOLS_RELOCATION_PACKER_SRC_RUN_LENGTH_ENCODER_H_
82