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 Elf_Data* data); 105 106 // Templated unpacker, helper for UnpackRelocations(). Rel type is one of 107 // ELF::Rel or ELF::Rela. 108 template <typename Rel> 109 bool UnpackTypedRelocations(const std::vector<uint8_t>& packed, 110 Elf_Data* data); 111 112 // Write ELF file changes. 113 void Flush(); 114 115 // File descriptor opened on the shared object. 116 int fd_; 117 118 // If set, pad rather than shrink .rel.dyn or .rela.dyn. Primarily for 119 // debugging, allows packing to be checked without affecting load addresses. 120 bool is_padding_relocations_; 121 122 // Libelf handle, assigned by Load(). 123 Elf* elf_; 124 125 // Sections that we manipulate, assigned by Load(). 126 Elf_Scn* relocations_section_; 127 Elf_Scn* dynamic_section_; 128 Elf_Scn* android_relocations_section_; 129 130 // Relocation type found, assigned by Load(). 131 enum { NONE = 0, REL, RELA } relocations_type_; 132 }; 133 134 } // namespace relocation_packer 135 136 #endif // TOOLS_RELOCATION_PACKER_SRC_ELF_FILE_H_ 137
