tools/relocation_packer/src/elf_file.h

   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 relative relocations from the .rel.dyn
   8 // or .rela.dyn sections and pack into .android.rel.dyn or .android.rela.dyn,
   9 // and unpack to return the file to its pre-packed state.
  10 //
  11 // Files to be packed or unpacked must include an existing .android.rel.dyn
  12 // or android.rela.dyn section.  A standard libchrome.<version>.so will not
  13 // contain this section, so the following can be used to add one:
  14 //
  15 //   echo -n 'NULL' >/tmp/small
  16 //   if file libchrome.<version>.so | grep -q 'ELF 32'; then
  17 //     arm-linux-androideabi-objcopy
  18 //         --add-section .android.rel.dyn=/tmp/small
  19 //         libchrome.<version>.so libchrome.<version>.so.packed
  20 //   else
  21 //     aarch64-linux-android-objcopy
  22 //         --add-section .android.rela.dyn=/tmp/small
  23 //         libchrome.<version>.so libchrome.<version>.so.packed
  24 //   fi
  25 //   rm /tmp/small
  26 //
  27 // To use, open the file and pass the file descriptor to the constructor,
  28 // then pack or unpack as desired.  Packing or unpacking will flush the file
  29 // descriptor on success.  Example:
  30 //
  31 //   int fd = open(..., O_RDWR);
  32 //   ElfFile elf_file(fd);
  33 //   bool status;
  34 //   if (is_packing)
  35 //     status = elf_file.PackRelocations();
  36 //   else
  37 //     status = elf_file.UnpackRelocations();
  38 //   close(fd);
  39 //
  40 // SetPadding() causes PackRelocations() to pad .rel.dyn or .rela.dyn with
  41 // NONE-type entries rather than cutting a hole out of the shared object
  42 // file.  This keeps all load addresses and offsets constant, and enables
  43 // easier debugging and testing.
  44 //
  45 // A packed shared object file has all of its relative relocations
  46 // removed from .rel.dyn or .rela.dyn, and replaced as packed data in
  47 // .android.rel.dyn or .android.rela.dyn respectively.  The resulting file
  48 // is shorter than its non-packed original.
  49 //
  50 // Unpacking a packed file restores the file to its non-packed state, by
  51 // expanding the packed data in .android.rel.dyn or .android.rela.dyn,
  52 // combining the relative relocations with the data already in .rel.dyn
  53 // or .rela.dyn, and then writing back the now expanded section.
  54
  55 #ifndef TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_
  56 #define TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_
  57
  58 #include <string.h>
  59 #include <vector>
  60
  61 #include "elf.h"
  62 #include "libelf.h"
  63 #include "packer.h"
  64
  65 namespace relocation_packer {
  66
  67 // An ElfFile reads shared objects, and shuttles relative relocations
  68 // between .rel.dyn or .rela.dyn and .android.rel.dyn or .android.rela.dyn
  69 // sections.
  70 class ElfFile {
  71  public:
  72   explicit ElfFile(int fd)
  73       : fd_(fd), is_padding_relocations_(false), elf_(NULL),
  74         relocations_section_(NULL), dynamic_section_(NULL),
  75         android_relocations_section_(NULL), relocations_type_(NONE) {}
  76   ~ElfFile() {}
  77
  78   // Set padding mode.  When padding, PackRelocations() will not shrink
  79   // the .rel.dyn or .rela.dyn section, but instead replace relative with
  80   // NONE-type entries.
  81   // |flag| is true to pad .rel.dyn or .rela.dyn, false to shrink it.
  82   inline void SetPadding(bool flag) { is_padding_relocations_ = flag; }
  83
  84   // Transfer relative relocations from .rel.dyn or .rela.dyn to a packed
  85   // representation in .android.rel.dyn or .android.rela.dyn.  Returns true
  86   // on success.
  87   bool PackRelocations();
  88
  89   // Transfer relative relocations from a packed representation in
  90   // .android.rel.dyn or .android.rela.dyn to .rel.dyn or .rela.dyn.  Returns
  91   // true on success.
  92   bool UnpackRelocations();
  93
  94  private:
  95   // Load a new ElfFile from a filedescriptor.  If flushing, the file must
  96   // be open for read/write.  Returns true on successful ELF file load.
  97   // |fd| is an open file descriptor for the shared object.
  98   bool Load();
  99
 100   // Templated packer, helper for PackRelocations().  Rel type is one of
 101   // ELF::Rel or ELF::Rela.
 102   template <typename Rel>
 103   bool PackTypedRelocations(const std::vector<Rel>& relocations);
 104
 105   // Templated unpacker, helper for UnpackRelocations().  Rel type is one of
 106   // ELF::Rel or ELF::Rela.
 107   template <typename Rel>
 108   bool UnpackTypedRelocations(const std::vector<uint8_t>& packed);
 109
 110   // Write ELF file changes.
 111   void Flush();
 112
 113   // File descriptor opened on the shared object.
 114   int fd_;
 115
 116   // If set, pad rather than shrink .rel.dyn or .rela.dyn.  Primarily for
 117   // debugging, allows packing to be checked without affecting load addresses.
 118   bool is_padding_relocations_;
 119
 120   // Libelf handle, assigned by Load().
 121   Elf* elf_;
 122
 123   // Sections that we manipulate, assigned by Load().
 124   Elf_Scn* relocations_section_;
 125   Elf_Scn* dynamic_section_;
 126   Elf_Scn* android_relocations_section_;
 127
 128   // Relocation type found, assigned by Load().
 129   enum { NONE = 0, REL, RELA } relocations_type_;
 130 };
 131
 132 }  // namespace relocation_packer
 133
 134 #endif  // TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_