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 57 namespace relocation_packer { 58 59 // An ElfFile reads shared objects, and shuttles R_ARM_RELATIVE relocations 60 // between .rel.dyn and .android.rel.dyn sections. 61 class ElfFile { 62 public: ElfFile(int fd)63 explicit ElfFile(int fd) { memset(this, 0, sizeof(*this)); fd_ = fd; } ~ElfFile()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. SetPadding(bool flag)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