elf_file.h revision f8ee788a64d60abd8f2d742a5fdedde054ecd910 
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// ELF shared object file updates handler.
6//
7// Provides functions to remove R_ARM_RELATIVE relocations from the .rel.dyn
8// section and pack them in .android.rel.dyn, and unpack to return the file
9// to its pre-packed state.
10//
11// Files to be packed or unpacked must include an existing .android.rel.dyn
12// section.  A standard libchrome.<version>.so will not contain this section,
13// so the following can be used to add one:
14//
15//   echo -n 'NULL' >/tmp/small
16//   arm-linux-gnueabi-objcopy
17//       --add-section .android.rel.dyn=/tmp/small
18//       libchrome.<version>.so libchrome.<version>.so.packed
19//   rm /tmp/small
20//
21// To use, open the file and pass the file descriptor to the constructor,
22// then pack or unpack as desired.  Packing or unpacking will flush the file
23// descriptor on success.  Example:
24//
25//   int fd = open(..., O_RDWR);
26//   ElfFile elf_file(fd);
27//   bool status;
28//   if (is_packing)
29//     status = elf_file.PackRelocations();
30//   else
31//     status = elf_file.UnpackRelocations();
32//   close(fd);
33//
34// SetPadding() causes PackRelocations() to pad .rel.dyn with R_ARM_NONE
35// entries rather than cutting a hole out of the shared object file.  This
36// keeps all load addresses and offsets constant, and enables easier
37// debugging and testing.
38//
39// A packed shared object file has all of its R_ARM_RELATIVE relocations
40// removed from .rel.dyn, and replaced as packed data in .android.rel.dyn.
41// The resulting file is shorter than its non-packed original.
42//
43// Unpacking a packed file restores the file to its non-packed state, by
44// expanding the packed data in android.rel.dyn, combining the R_ARM_RELATIVE
45// relocations with the data already in .rel.dyn, and then writing back the
46// now expanded .rel.dyn section.
47
48#ifndef TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_
49#define TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_
50
51#include <string.h>
52
53#include "elf.h"
54#include "libelf.h"
55#include "packer.h"
56
57namespace relocation_packer {
58
59// An ElfFile reads shared objects, and shuttles R_ARM_RELATIVE relocations
60// between .rel.dyn and .android.rel.dyn sections.
61class ElfFile {
62 public:
63  explicit ElfFile(int fd) { memset(this, 0, sizeof(*this)); fd_ = fd; }
64  ~ElfFile() {}
65
66  // Set padding mode.  When padding, PackRelocations() will not shrink
67  // the .rel.dyn section, but instead replace R_ARM_RELATIVE with
68  // R_ARM_NONE entries.
69  // |flag| is true to pad .rel.dyn, false to shrink it.
70  inline void SetPadding(bool flag) { is_padding_rel_dyn_ = flag; }
71
72  // Transfer R_ARM_RELATIVE relocations from .rel.dyn to a packed
73  // representation in .android.rel.dyn.  Returns true on success.
74  bool PackRelocations();
75
76  // Transfer R_ARM_RELATIVE relocations from a packed representation in
77  // .android.rel.dyn to .rel.dyn.  Returns true on success.
78  bool UnpackRelocations();
79
80 private:
81  // Load a new ElfFile from a filedescriptor.  If flushing, the file must
82  // be open for read/write.  Returns true on successful ELF file load.
83  // |fd| is an open file descriptor for the shared object.
84  bool Load();
85
86  // Write ELF file changes.
87  void Flush();
88
89  // If set, pad rather than shrink .rel.dyn.  Primarily for debugging,
90  // allows packing to be checked without affecting load addresses.
91  bool is_padding_rel_dyn_;
92
93  // File descriptor opened on the shared object.
94  int fd_;
95
96  // Libelf handle, assigned by Load().
97  Elf* elf_;
98
99  // Sections that we manipulate, assigned by Load().
100  Elf_Scn* rel_dyn_section_;
101  Elf_Scn* dynamic_section_;
102  Elf_Scn* android_rel_dyn_section_;
103};
104
105}  // namespace relocation_packer
106
107#endif  // TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_
108