1 // Copyright (c) 2013 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 #include "crazy_linker_elf_loader.h" 6 7 #include <limits.h> // For PAGE_SIZE and PAGE_MASK 8 9 #include "crazy_linker_debug.h" 10 #include "linker_phdr.h" 11 12 #define PAGE_START(x) ((x) & PAGE_MASK) 13 #define PAGE_OFFSET(x) ((x) & ~PAGE_MASK) 14 #define PAGE_END(x) PAGE_START((x) + (PAGE_SIZE - 1)) 15 16 namespace crazy { 17 18 #define MAYBE_MAP_FLAG(x, from, to) (((x) & (from)) ? (to) : 0) 19 #define PFLAGS_TO_PROT(x) \ 20 (MAYBE_MAP_FLAG((x), PF_X, PROT_EXEC) | \ 21 MAYBE_MAP_FLAG((x), PF_R, PROT_READ) | \ 22 MAYBE_MAP_FLAG((x), PF_W, PROT_WRITE)) 23 ElfLoader()24 ElfLoader::ElfLoader() 25 : fd_(), 26 path_(NULL), 27 phdr_num_(0), 28 phdr_mmap_(NULL), 29 phdr_table_(NULL), 30 phdr_size_(0), 31 file_offset_(0), 32 wanted_load_address_(0), 33 load_start_(NULL), 34 load_size_(0), 35 load_bias_(0), 36 loaded_phdr_(NULL) {} 37 ~ElfLoader()38 ElfLoader::~ElfLoader() { 39 if (phdr_mmap_) { 40 // Deallocate the temporary program header copy. 41 munmap(phdr_mmap_, phdr_size_); 42 } 43 } 44 LoadAt(const char * lib_path,off_t file_offset,uintptr_t wanted_address,Error * error)45 bool ElfLoader::LoadAt(const char* lib_path, 46 off_t file_offset, 47 uintptr_t wanted_address, 48 Error* error) { 49 50 LOG("%s: lib_path='%s', file_offset=%p, load_address=%p\n", 51 __FUNCTION__, 52 lib_path, 53 file_offset, 54 wanted_address); 55 56 // Check that the load address is properly page-aligned. 57 if (wanted_address != PAGE_START(wanted_address)) { 58 error->Format("Load address is not page aligned (%08x)", wanted_address); 59 return false; 60 } 61 wanted_load_address_ = reinterpret_cast<void*>(wanted_address); 62 63 // Check that the file offset is also properly page-aligned. 64 // PAGE_START() can't be used here due to the compiler complaining about 65 // comparing signed (off_t) and unsigned (size_t) values. 66 if ((file_offset & static_cast<off_t>(PAGE_SIZE - 1)) != 0) { 67 error->Format("File offset is not page aligned (%08x)", file_offset); 68 return false; 69 } 70 file_offset_ = file_offset; 71 72 // Open the file. 73 if (!fd_.OpenReadOnly(lib_path)) { 74 error->Format("Can't open file: %s", strerror(errno)); 75 return false; 76 } 77 78 if (file_offset && fd_.SeekTo(file_offset) < 0) { 79 error->Format( 80 "Can't seek to file offset %08x: %s", file_offset, strerror(errno)); 81 return false; 82 } 83 84 path_ = lib_path; 85 86 if (!ReadElfHeader(error) || !ReadProgramHeader(error) || 87 !ReserveAddressSpace(error)) { 88 return false; 89 } 90 91 if (!LoadSegments(error) || !FindPhdr(error)) { 92 // An error occured, cleanup the address space by un-mapping the 93 // range that was reserved by ReserveAddressSpace(). 94 if (load_start_ && load_size_) 95 munmap(load_start_, load_size_); 96 97 return false; 98 } 99 100 return true; 101 } 102 ReadElfHeader(Error * error)103 bool ElfLoader::ReadElfHeader(Error* error) { 104 int ret = fd_.Read(&header_, sizeof(header_)); 105 if (ret < 0) { 106 error->Format("Can't read file: %s", strerror(errno)); 107 return false; 108 } 109 if (ret != static_cast<int>(sizeof(header_))) { 110 error->Set("File too small to be ELF"); 111 return false; 112 } 113 114 if (memcmp(header_.e_ident, ELFMAG, SELFMAG) != 0) { 115 error->Set("Bad ELF magic"); 116 return false; 117 } 118 119 if (header_.e_ident[EI_CLASS] != ELF::kElfClass) { 120 error->Format("Not a %d-bit class: %d", 121 ELF::kElfBits, 122 header_.e_ident[EI_CLASS]); 123 return false; 124 } 125 126 if (header_.e_ident[EI_DATA] != ELFDATA2LSB) { 127 error->Format("Not little-endian class: %d", header_.e_ident[EI_DATA]); 128 return false; 129 } 130 131 if (header_.e_type != ET_DYN) { 132 error->Format("Not a shared library type: %d", header_.e_type); 133 return false; 134 } 135 136 if (header_.e_version != EV_CURRENT) { 137 error->Format("Unexpected ELF version: %d", header_.e_version); 138 return false; 139 } 140 141 if (header_.e_machine != ELF_MACHINE) { 142 error->Format("Unexpected ELF machine type: %d", header_.e_machine); 143 return false; 144 } 145 146 return true; 147 } 148 149 // Loads the program header table from an ELF file into a read-only private 150 // anonymous mmap-ed block. ReadProgramHeader(Error * error)151 bool ElfLoader::ReadProgramHeader(Error* error) { 152 phdr_num_ = header_.e_phnum; 153 154 // Like the kernel, only accept program header tables smaller than 64 KB. 155 if (phdr_num_ < 1 || phdr_num_ > 65536 / sizeof(ELF::Phdr)) { 156 error->Format("Invalid program header count: %d", phdr_num_); 157 return false; 158 } 159 160 ELF::Addr page_min = PAGE_START(header_.e_phoff); 161 ELF::Addr page_max = 162 PAGE_END(header_.e_phoff + (phdr_num_ * sizeof(ELF::Phdr))); 163 ELF::Addr page_offset = PAGE_OFFSET(header_.e_phoff); 164 165 phdr_size_ = page_max - page_min; 166 167 void* mmap_result = fd_.Map( 168 NULL, phdr_size_, PROT_READ, MAP_PRIVATE, page_min + file_offset_); 169 if (mmap_result == MAP_FAILED) { 170 error->Format("Phdr mmap failed: %s", strerror(errno)); 171 return false; 172 } 173 174 phdr_mmap_ = mmap_result; 175 phdr_table_ = reinterpret_cast<ELF::Phdr*>( 176 reinterpret_cast<char*>(mmap_result) + page_offset); 177 return true; 178 } 179 180 // Reserve a virtual address range big enough to hold all loadable 181 // segments of a program header table. This is done by creating a 182 // private anonymous mmap() with PROT_NONE. 183 // 184 // This will use the wanted_load_address_ value, ReserveAddressSpace(Error * error)185 bool ElfLoader::ReserveAddressSpace(Error* error) { 186 ELF::Addr min_vaddr; 187 load_size_ = 188 phdr_table_get_load_size(phdr_table_, phdr_num_, &min_vaddr, NULL); 189 if (load_size_ == 0) { 190 error->Set("No loadable segments"); 191 return false; 192 } 193 194 uint8_t* addr = reinterpret_cast<uint8_t*>(min_vaddr); 195 int mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS; 196 197 // Support loading at a fixed address. 198 if (wanted_load_address_) { 199 addr = static_cast<uint8_t*>(wanted_load_address_); 200 mmap_flags |= MAP_FIXED; 201 } 202 203 LOG("%s: address=%p size=%p\n", __FUNCTION__, addr, load_size_); 204 void* start = mmap(addr, load_size_, PROT_NONE, mmap_flags, -1, 0); 205 if (start == MAP_FAILED) { 206 error->Format("Could not reserve %d bytes of address space", load_size_); 207 return false; 208 } 209 210 load_start_ = start; 211 load_bias_ = reinterpret_cast<ELF::Addr>(start) - min_vaddr; 212 return true; 213 } 214 215 // Returns the address of the program header table as it appears in the loaded 216 // segments in memory. This is in contrast with 'phdr_table_' which 217 // is temporary and will be released before the library is relocated. FindPhdr(Error * error)218 bool ElfLoader::FindPhdr(Error* error) { 219 const ELF::Phdr* phdr_limit = phdr_table_ + phdr_num_; 220 221 // If there is a PT_PHDR, use it directly. 222 for (const ELF::Phdr* phdr = phdr_table_; phdr < phdr_limit; ++phdr) { 223 if (phdr->p_type == PT_PHDR) { 224 return CheckPhdr(load_bias_ + phdr->p_vaddr, error); 225 } 226 } 227 228 // Otherwise, check the first loadable segment. If its file offset 229 // is 0, it starts with the ELF header, and we can trivially find the 230 // loaded program header from it. 231 for (const ELF::Phdr* phdr = phdr_table_; phdr < phdr_limit; ++phdr) { 232 if (phdr->p_type == PT_LOAD) { 233 if (phdr->p_offset == 0) { 234 ELF::Addr elf_addr = load_bias_ + phdr->p_vaddr; 235 const ELF::Ehdr* ehdr = (const ELF::Ehdr*)(void*)elf_addr; 236 ELF::Addr offset = ehdr->e_phoff; 237 return CheckPhdr((ELF::Addr)ehdr + offset, error); 238 } 239 break; 240 } 241 } 242 243 error->Set("Can't find loaded program header"); 244 return false; 245 } 246 247 // Ensures that our program header is actually within a loadable 248 // segment. This should help catch badly-formed ELF files that 249 // would cause the linker to crash later when trying to access it. CheckPhdr(ELF::Addr loaded,Error * error)250 bool ElfLoader::CheckPhdr(ELF::Addr loaded, Error* error) { 251 const ELF::Phdr* phdr_limit = phdr_table_ + phdr_num_; 252 ELF::Addr loaded_end = loaded + (phdr_num_ * sizeof(ELF::Phdr)); 253 for (ELF::Phdr* phdr = phdr_table_; phdr < phdr_limit; ++phdr) { 254 if (phdr->p_type != PT_LOAD) { 255 continue; 256 } 257 ELF::Addr seg_start = phdr->p_vaddr + load_bias_; 258 ELF::Addr seg_end = phdr->p_filesz + seg_start; 259 if (seg_start <= loaded && loaded_end <= seg_end) { 260 loaded_phdr_ = reinterpret_cast<const ELF::Phdr*>(loaded); 261 return true; 262 } 263 } 264 error->Format("Loaded program header %x not in loadable segment", loaded); 265 return false; 266 } 267 268 // Map all loadable segments in process' address space. 269 // This assumes you already called phdr_table_reserve_memory to 270 // reserve the address space range for the library. LoadSegments(Error * error)271 bool ElfLoader::LoadSegments(Error* error) { 272 for (size_t i = 0; i < phdr_num_; ++i) { 273 const ELF::Phdr* phdr = &phdr_table_[i]; 274 275 if (phdr->p_type != PT_LOAD) { 276 continue; 277 } 278 279 // Segment addresses in memory. 280 ELF::Addr seg_start = phdr->p_vaddr + load_bias_; 281 ELF::Addr seg_end = seg_start + phdr->p_memsz; 282 283 ELF::Addr seg_page_start = PAGE_START(seg_start); 284 ELF::Addr seg_page_end = PAGE_END(seg_end); 285 286 ELF::Addr seg_file_end = seg_start + phdr->p_filesz; 287 288 // File offsets. 289 ELF::Addr file_start = phdr->p_offset; 290 ELF::Addr file_end = file_start + phdr->p_filesz; 291 292 ELF::Addr file_page_start = PAGE_START(file_start); 293 ELF::Addr file_length = file_end - file_page_start; 294 295 LOG("%s: file_offset=%p file_length=%p start_address=%p end_address=%p\n", 296 __FUNCTION__, 297 file_offset_ + file_page_start, 298 file_length, 299 seg_page_start, 300 seg_page_start + PAGE_END(file_length)); 301 302 if (file_length != 0) { 303 void* seg_addr = fd_.Map((void*)seg_page_start, 304 file_length, 305 PFLAGS_TO_PROT(phdr->p_flags), 306 MAP_FIXED | MAP_PRIVATE, 307 file_page_start + file_offset_); 308 if (seg_addr == MAP_FAILED) { 309 error->Format("Could not map segment %d: %s", i, strerror(errno)); 310 return false; 311 } 312 } 313 314 // if the segment is writable, and does not end on a page boundary, 315 // zero-fill it until the page limit. 316 if ((phdr->p_flags & PF_W) != 0 && PAGE_OFFSET(seg_file_end) > 0) { 317 memset((void*)seg_file_end, 0, PAGE_SIZE - PAGE_OFFSET(seg_file_end)); 318 } 319 320 seg_file_end = PAGE_END(seg_file_end); 321 322 // seg_file_end is now the first page address after the file 323 // content. If seg_end is larger, we need to zero anything 324 // between them. This is done by using a private anonymous 325 // map for all extra pages. 326 if (seg_page_end > seg_file_end) { 327 void* zeromap = mmap((void*)seg_file_end, 328 seg_page_end - seg_file_end, 329 PFLAGS_TO_PROT(phdr->p_flags), 330 MAP_FIXED | MAP_ANONYMOUS | MAP_PRIVATE, 331 -1, 332 0); 333 if (zeromap == MAP_FAILED) { 334 error->Format("Could not zero-fill gap: %s", strerror(errno)); 335 return false; 336 } 337 } 338 } 339 return true; 340 } 341 342 } // namespace crazy 343