elf_builder.h revision 197160d47f34238cb5e7444fa4c2de300db8e2c6
1/* 2 * Copyright (C) 2015 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17#ifndef ART_COMPILER_ELF_BUILDER_H_ 18#define ART_COMPILER_ELF_BUILDER_H_ 19 20#include <vector> 21 22#include "arch/instruction_set.h" 23#include "arch/mips/instruction_set_features_mips.h" 24#include "base/bit_utils.h" 25#include "base/casts.h" 26#include "base/unix_file/fd_file.h" 27#include "elf_utils.h" 28#include "leb128.h" 29#include "linker/error_delaying_output_stream.h" 30#include "utils/array_ref.h" 31 32namespace art { 33 34// Writes ELF file. 35// 36// The basic layout of the elf file: 37// Elf_Ehdr - The ELF header. 38// Elf_Phdr[] - Program headers for the linker. 39// .rodata - DEX files and oat metadata. 40// .text - Compiled code. 41// .bss - Zero-initialized writeable section. 42// .dynstr - Names for .dynsym. 43// .dynsym - A few oat-specific dynamic symbols. 44// .hash - Hash-table for .dynsym. 45// .dynamic - Tags which let the linker locate .dynsym. 46// .strtab - Names for .symtab. 47// .symtab - Debug symbols. 48// .eh_frame - Unwind information (CFI). 49// .eh_frame_hdr - Index of .eh_frame. 50// .debug_frame - Unwind information (CFI). 51// .debug_frame.oat_patches - Addresses for relocation. 52// .debug_info - Debug information. 53// .debug_info.oat_patches - Addresses for relocation. 54// .debug_abbrev - Decoding information for .debug_info. 55// .debug_str - Strings for .debug_info. 56// .debug_line - Line number tables. 57// .debug_line.oat_patches - Addresses for relocation. 58// .text.oat_patches - Addresses for relocation. 59// .shstrtab - Names of ELF sections. 60// Elf_Shdr[] - Section headers. 61// 62// Some section are optional (the debug sections in particular). 63// 64// We try write the section data directly into the file without much 65// in-memory buffering. This means we generally write sections based on the 66// dependency order (e.g. .dynamic points to .dynsym which points to .text). 67// 68// In the cases where we need to buffer, we write the larger section first 69// and buffer the smaller one (e.g. .strtab is bigger than .symtab). 70// 71// The debug sections are written last for easier stripping. 72// 73template <typename ElfTypes> 74class ElfBuilder FINAL { 75 public: 76 static constexpr size_t kMaxProgramHeaders = 16; 77 using Elf_Addr = typename ElfTypes::Addr; 78 using Elf_Off = typename ElfTypes::Off; 79 using Elf_Word = typename ElfTypes::Word; 80 using Elf_Sword = typename ElfTypes::Sword; 81 using Elf_Ehdr = typename ElfTypes::Ehdr; 82 using Elf_Shdr = typename ElfTypes::Shdr; 83 using Elf_Sym = typename ElfTypes::Sym; 84 using Elf_Phdr = typename ElfTypes::Phdr; 85 using Elf_Dyn = typename ElfTypes::Dyn; 86 87 // Base class of all sections. 88 class Section : public OutputStream { 89 public: 90 Section(ElfBuilder<ElfTypes>* owner, 91 const std::string& name, 92 Elf_Word type, 93 Elf_Word flags, 94 const Section* link, 95 Elf_Word info, 96 Elf_Word align, 97 Elf_Word entsize) 98 : OutputStream(name), 99 owner_(owner), 100 header_(), 101 section_index_(0), 102 name_(name), 103 link_(link), 104 started_(false), 105 finished_(false), 106 phdr_flags_(PF_R), 107 phdr_type_(0) { 108 DCHECK_GE(align, 1u); 109 header_.sh_type = type; 110 header_.sh_flags = flags; 111 header_.sh_info = info; 112 header_.sh_addralign = align; 113 header_.sh_entsize = entsize; 114 } 115 116 // Start writing of this section. 117 void Start() { 118 CHECK(!started_); 119 CHECK(!finished_); 120 started_ = true; 121 auto& sections = owner_->sections_; 122 // Check that the previous section is complete. 123 CHECK(sections.empty() || sections.back()->finished_); 124 // The first ELF section index is 1. Index 0 is reserved for NULL. 125 section_index_ = sections.size() + 1; 126 // Page-align if we switch between allocated and non-allocated sections, 127 // or if we change the type of allocation (e.g. executable vs non-executable). 128 if (!sections.empty()) { 129 if (header_.sh_flags != sections.back()->header_.sh_flags) { 130 header_.sh_addralign = kPageSize; 131 } 132 } 133 // Align file position. 134 if (header_.sh_type != SHT_NOBITS) { 135 header_.sh_offset = owner_->AlignFileOffset(header_.sh_addralign); 136 } else { 137 header_.sh_offset = 0; 138 } 139 // Align virtual memory address. 140 if ((header_.sh_flags & SHF_ALLOC) != 0) { 141 header_.sh_addr = owner_->AlignVirtualAddress(header_.sh_addralign); 142 } else { 143 header_.sh_addr = 0; 144 } 145 // Push this section on the list of written sections. 146 sections.push_back(this); 147 } 148 149 // Finish writing of this section. 150 void End() { 151 CHECK(started_); 152 CHECK(!finished_); 153 finished_ = true; 154 if (header_.sh_type == SHT_NOBITS) { 155 CHECK_GT(header_.sh_size, 0u); 156 } else { 157 // Use the current file position to determine section size. 158 off_t file_offset = owner_->stream_.Seek(0, kSeekCurrent); 159 CHECK_GE(file_offset, (off_t)header_.sh_offset); 160 header_.sh_size = file_offset - header_.sh_offset; 161 } 162 if ((header_.sh_flags & SHF_ALLOC) != 0) { 163 owner_->virtual_address_ += header_.sh_size; 164 } 165 } 166 167 // Get the location of this section in virtual memory. 168 Elf_Addr GetAddress() const { 169 CHECK(started_); 170 return header_.sh_addr; 171 } 172 173 // Returns the size of the content of this section. 174 Elf_Word GetSize() const { 175 if (finished_) { 176 return header_.sh_size; 177 } else { 178 CHECK(started_); 179 CHECK_NE(header_.sh_type, (Elf_Word)SHT_NOBITS); 180 return owner_->stream_.Seek(0, kSeekCurrent) - header_.sh_offset; 181 } 182 } 183 184 // Write this section as "NOBITS" section. (used for the .bss section) 185 // This means that the ELF file does not contain the initial data for this section 186 // and it will be zero-initialized when the ELF file is loaded in the running program. 187 void WriteNoBitsSection(Elf_Word size) { 188 DCHECK_NE(header_.sh_flags & SHF_ALLOC, 0u); 189 header_.sh_type = SHT_NOBITS; 190 Start(); 191 header_.sh_size = size; 192 End(); 193 } 194 195 // This function always succeeds to simplify code. 196 // Use builder's Good() to check the actual status. 197 bool WriteFully(const void* buffer, size_t byte_count) OVERRIDE { 198 CHECK(started_); 199 CHECK(!finished_); 200 return owner_->stream_.WriteFully(buffer, byte_count); 201 } 202 203 // This function always succeeds to simplify code. 204 // Use builder's Good() to check the actual status. 205 off_t Seek(off_t offset, Whence whence) OVERRIDE { 206 // Forward the seek as-is and trust the caller to use it reasonably. 207 return owner_->stream_.Seek(offset, whence); 208 } 209 210 // This function flushes the output and returns whether it succeeded. 211 // If there was a previous failure, this does nothing and returns false, i.e. failed. 212 bool Flush() OVERRIDE { 213 return owner_->stream_.Flush(); 214 } 215 216 Elf_Word GetSectionIndex() const { 217 DCHECK(started_); 218 DCHECK_NE(section_index_, 0u); 219 return section_index_; 220 } 221 222 private: 223 ElfBuilder<ElfTypes>* owner_; 224 Elf_Shdr header_; 225 Elf_Word section_index_; 226 const std::string name_; 227 const Section* const link_; 228 bool started_; 229 bool finished_; 230 Elf_Word phdr_flags_; 231 Elf_Word phdr_type_; 232 233 friend class ElfBuilder; 234 235 DISALLOW_COPY_AND_ASSIGN(Section); 236 }; 237 238 class CachedSection : public Section { 239 public: 240 CachedSection(ElfBuilder<ElfTypes>* owner, 241 const std::string& name, 242 Elf_Word type, 243 Elf_Word flags, 244 const Section* link, 245 Elf_Word info, 246 Elf_Word align, 247 Elf_Word entsize) 248 : Section(owner, name, type, flags, link, info, align, entsize), cache_() { } 249 250 Elf_Word Add(const void* data, size_t length) { 251 Elf_Word offset = cache_.size(); 252 const uint8_t* d = reinterpret_cast<const uint8_t*>(data); 253 cache_.insert(cache_.end(), d, d + length); 254 return offset; 255 } 256 257 Elf_Word GetCacheSize() { 258 return cache_.size(); 259 } 260 261 void Write() { 262 this->WriteFully(cache_.data(), cache_.size()); 263 cache_.clear(); 264 cache_.shrink_to_fit(); 265 } 266 267 void WriteCachedSection() { 268 this->Start(); 269 Write(); 270 this->End(); 271 } 272 273 private: 274 std::vector<uint8_t> cache_; 275 }; 276 277 // Writer of .dynstr section. 278 class CachedStringSection FINAL : public CachedSection { 279 public: 280 CachedStringSection(ElfBuilder<ElfTypes>* owner, 281 const std::string& name, 282 Elf_Word flags, 283 Elf_Word align) 284 : CachedSection(owner, 285 name, 286 SHT_STRTAB, 287 flags, 288 /* link */ nullptr, 289 /* info */ 0, 290 align, 291 /* entsize */ 0) { } 292 293 Elf_Word Add(const std::string& name) { 294 if (CachedSection::GetCacheSize() == 0u) { 295 DCHECK(name.empty()); 296 } 297 return CachedSection::Add(name.c_str(), name.length() + 1); 298 } 299 }; 300 301 // Writer of .strtab and .shstrtab sections. 302 class StringSection FINAL : public Section { 303 public: 304 StringSection(ElfBuilder<ElfTypes>* owner, 305 const std::string& name, 306 Elf_Word flags, 307 Elf_Word align) 308 : Section(owner, 309 name, 310 SHT_STRTAB, 311 flags, 312 /* link */ nullptr, 313 /* info */ 0, 314 align, 315 /* entsize */ 0), 316 current_offset_(0) { 317 } 318 319 Elf_Word Write(const std::string& name) { 320 if (current_offset_ == 0) { 321 DCHECK(name.empty()); 322 } 323 Elf_Word offset = current_offset_; 324 this->WriteFully(name.c_str(), name.length() + 1); 325 current_offset_ += name.length() + 1; 326 return offset; 327 } 328 329 private: 330 Elf_Word current_offset_; 331 }; 332 333 // Writer of .dynsym and .symtab sections. 334 class SymbolSection FINAL : public CachedSection { 335 public: 336 SymbolSection(ElfBuilder<ElfTypes>* owner, 337 const std::string& name, 338 Elf_Word type, 339 Elf_Word flags, 340 Section* strtab) 341 : CachedSection(owner, 342 name, 343 type, 344 flags, 345 strtab, 346 /* info */ 0, 347 sizeof(Elf_Off), 348 sizeof(Elf_Sym)) { 349 // The symbol table always has to start with NULL symbol. 350 Elf_Sym null_symbol = Elf_Sym(); 351 CachedSection::Add(&null_symbol, sizeof(null_symbol)); 352 } 353 354 // Buffer symbol for this section. It will be written later. 355 // If the symbol's section is null, it will be considered absolute (SHN_ABS). 356 // (we use this in JIT to reference code which is stored outside the debug ELF file) 357 void Add(Elf_Word name, 358 const Section* section, 359 Elf_Addr addr, 360 Elf_Word size, 361 uint8_t binding, 362 uint8_t type) { 363 Elf_Word section_index; 364 if (section != nullptr) { 365 DCHECK_LE(section->GetAddress(), addr); 366 DCHECK_LE(addr, section->GetAddress() + section->GetSize()); 367 section_index = section->GetSectionIndex(); 368 } else { 369 section_index = static_cast<Elf_Word>(SHN_ABS); 370 } 371 Add(name, section_index, addr, size, binding, type); 372 } 373 374 void Add(Elf_Word name, 375 Elf_Word section_index, 376 Elf_Addr addr, 377 Elf_Word size, 378 uint8_t binding, 379 uint8_t type) { 380 Elf_Sym sym = Elf_Sym(); 381 sym.st_name = name; 382 sym.st_value = addr; 383 sym.st_size = size; 384 sym.st_other = 0; 385 sym.st_shndx = section_index; 386 sym.st_info = (binding << 4) + (type & 0xf); 387 CachedSection::Add(&sym, sizeof(sym)); 388 } 389 }; 390 391 ElfBuilder(InstructionSet isa, const InstructionSetFeatures* features, OutputStream* output) 392 : isa_(isa), 393 features_(features), 394 stream_(output), 395 rodata_(this, ".rodata", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 396 text_(this, ".text", SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, nullptr, 0, kPageSize, 0), 397 bss_(this, ".bss", SHT_NOBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 398 dynstr_(this, ".dynstr", SHF_ALLOC, kPageSize), 399 dynsym_(this, ".dynsym", SHT_DYNSYM, SHF_ALLOC, &dynstr_), 400 hash_(this, ".hash", SHT_HASH, SHF_ALLOC, &dynsym_, 0, sizeof(Elf_Word), sizeof(Elf_Word)), 401 dynamic_(this, ".dynamic", SHT_DYNAMIC, SHF_ALLOC, &dynstr_, 0, kPageSize, sizeof(Elf_Dyn)), 402 eh_frame_(this, ".eh_frame", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 403 eh_frame_hdr_(this, ".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, 4, 0), 404 strtab_(this, ".strtab", 0, 1), 405 symtab_(this, ".symtab", SHT_SYMTAB, 0, &strtab_), 406 debug_frame_(this, ".debug_frame", SHT_PROGBITS, 0, nullptr, 0, sizeof(Elf_Addr), 0), 407 debug_info_(this, ".debug_info", SHT_PROGBITS, 0, nullptr, 0, 1, 0), 408 debug_line_(this, ".debug_line", SHT_PROGBITS, 0, nullptr, 0, 1, 0), 409 shstrtab_(this, ".shstrtab", 0, 1), 410 started_(false), 411 write_program_headers_(false), 412 loaded_size_(0u), 413 virtual_address_(0) { 414 text_.phdr_flags_ = PF_R | PF_X; 415 bss_.phdr_flags_ = PF_R | PF_W; 416 dynamic_.phdr_flags_ = PF_R | PF_W; 417 dynamic_.phdr_type_ = PT_DYNAMIC; 418 eh_frame_hdr_.phdr_type_ = PT_GNU_EH_FRAME; 419 } 420 ~ElfBuilder() {} 421 422 InstructionSet GetIsa() { return isa_; } 423 Section* GetRoData() { return &rodata_; } 424 Section* GetText() { return &text_; } 425 Section* GetBss() { return &bss_; } 426 StringSection* GetStrTab() { return &strtab_; } 427 SymbolSection* GetSymTab() { return &symtab_; } 428 Section* GetEhFrame() { return &eh_frame_; } 429 Section* GetEhFrameHdr() { return &eh_frame_hdr_; } 430 Section* GetDebugFrame() { return &debug_frame_; } 431 Section* GetDebugInfo() { return &debug_info_; } 432 Section* GetDebugLine() { return &debug_line_; } 433 434 // Encode patch locations as LEB128 list of deltas between consecutive addresses. 435 // (exposed publicly for tests) 436 static void EncodeOatPatches(const ArrayRef<const uintptr_t>& locations, 437 std::vector<uint8_t>* buffer) { 438 buffer->reserve(buffer->size() + locations.size() * 2); // guess 2 bytes per ULEB128. 439 uintptr_t address = 0; // relative to start of section. 440 for (uintptr_t location : locations) { 441 DCHECK_GE(location, address) << "Patch locations are not in sorted order"; 442 EncodeUnsignedLeb128(buffer, dchecked_integral_cast<uint32_t>(location - address)); 443 address = location; 444 } 445 } 446 447 void WritePatches(const char* name, const ArrayRef<const uintptr_t>& patch_locations) { 448 std::vector<uint8_t> buffer; 449 EncodeOatPatches(patch_locations, &buffer); 450 std::unique_ptr<Section> s(new Section(this, name, SHT_OAT_PATCH, 0, nullptr, 0, 1, 0)); 451 s->Start(); 452 s->WriteFully(buffer.data(), buffer.size()); 453 s->End(); 454 other_sections_.push_back(std::move(s)); 455 } 456 457 void WriteSection(const char* name, const std::vector<uint8_t>* buffer) { 458 std::unique_ptr<Section> s(new Section(this, name, SHT_PROGBITS, 0, nullptr, 0, 1, 0)); 459 s->Start(); 460 s->WriteFully(buffer->data(), buffer->size()); 461 s->End(); 462 other_sections_.push_back(std::move(s)); 463 } 464 465 // Reserve space for ELF header and program headers. 466 // We do not know the number of headers until later, so 467 // it is easiest to just reserve a fixed amount of space. 468 // Program headers are required for loading by the linker. 469 // It is possible to omit them for ELF files used for debugging. 470 void Start(bool write_program_headers = true) { 471 int size = sizeof(Elf_Ehdr); 472 if (write_program_headers) { 473 size += sizeof(Elf_Phdr) * kMaxProgramHeaders; 474 } 475 stream_.Seek(size, kSeekSet); 476 started_ = true; 477 virtual_address_ += size; 478 write_program_headers_ = write_program_headers; 479 } 480 481 void End() { 482 DCHECK(started_); 483 484 // Note: loaded_size_ == 0 for tests that don't write .rodata, .text, .bss, 485 // .dynstr, dynsym, .hash and .dynamic. These tests should not read loaded_size_. 486 // TODO: Either refactor the .eh_frame creation so that it counts towards loaded_size_, 487 // or remove all support for .eh_frame. (The currently unused .eh_frame counts towards 488 // the virtual_address_ but we don't consider it for loaded_size_.) 489 CHECK(loaded_size_ == 0 || loaded_size_ == RoundUp(virtual_address_, kPageSize)) 490 << loaded_size_ << " " << virtual_address_; 491 492 // Write section names and finish the section headers. 493 shstrtab_.Start(); 494 shstrtab_.Write(""); 495 for (auto* section : sections_) { 496 section->header_.sh_name = shstrtab_.Write(section->name_); 497 if (section->link_ != nullptr) { 498 section->header_.sh_link = section->link_->GetSectionIndex(); 499 } 500 } 501 shstrtab_.End(); 502 503 // Write section headers at the end of the ELF file. 504 std::vector<Elf_Shdr> shdrs; 505 shdrs.reserve(1u + sections_.size()); 506 shdrs.push_back(Elf_Shdr()); // NULL at index 0. 507 for (auto* section : sections_) { 508 shdrs.push_back(section->header_); 509 } 510 Elf_Off section_headers_offset; 511 section_headers_offset = AlignFileOffset(sizeof(Elf_Off)); 512 stream_.WriteFully(shdrs.data(), shdrs.size() * sizeof(shdrs[0])); 513 514 // Flush everything else before writing the program headers. This should prevent 515 // the OS from reordering writes, so that we don't end up with valid headers 516 // and partially written data if we suddenly lose power, for example. 517 stream_.Flush(); 518 519 // The main ELF header. 520 Elf_Ehdr elf_header = MakeElfHeader(isa_); 521 elf_header.e_shoff = section_headers_offset; 522 elf_header.e_shnum = shdrs.size(); 523 elf_header.e_shstrndx = shstrtab_.GetSectionIndex(); 524 525 // Program headers (i.e. mmap instructions). 526 std::vector<Elf_Phdr> phdrs; 527 if (write_program_headers_) { 528 phdrs = MakeProgramHeaders(); 529 CHECK_LE(phdrs.size(), kMaxProgramHeaders); 530 elf_header.e_phoff = sizeof(Elf_Ehdr); 531 elf_header.e_phnum = phdrs.size(); 532 } 533 534 stream_.Seek(0, kSeekSet); 535 stream_.WriteFully(&elf_header, sizeof(elf_header)); 536 stream_.WriteFully(phdrs.data(), phdrs.size() * sizeof(phdrs[0])); 537 stream_.Flush(); 538 } 539 540 // The running program does not have access to section headers 541 // and the loader is not supposed to use them either. 542 // The dynamic sections therefore replicates some of the layout 543 // information like the address and size of .rodata and .text. 544 // It also contains other metadata like the SONAME. 545 // The .dynamic section is found using the PT_DYNAMIC program header. 546 void PrepareDynamicSection(const std::string& elf_file_path, 547 Elf_Word rodata_size, 548 Elf_Word text_size, 549 Elf_Word bss_size) { 550 std::string soname(elf_file_path); 551 size_t directory_separator_pos = soname.rfind('/'); 552 if (directory_separator_pos != std::string::npos) { 553 soname = soname.substr(directory_separator_pos + 1); 554 } 555 556 // Calculate addresses of .text, .bss and .dynstr. 557 DCHECK_EQ(rodata_.header_.sh_addralign, static_cast<Elf_Word>(kPageSize)); 558 DCHECK_EQ(text_.header_.sh_addralign, static_cast<Elf_Word>(kPageSize)); 559 DCHECK_EQ(bss_.header_.sh_addralign, static_cast<Elf_Word>(kPageSize)); 560 DCHECK_EQ(dynstr_.header_.sh_addralign, static_cast<Elf_Word>(kPageSize)); 561 Elf_Word rodata_address = rodata_.GetAddress(); 562 Elf_Word text_address = RoundUp(rodata_address + rodata_size, kPageSize); 563 Elf_Word bss_address = RoundUp(text_address + text_size, kPageSize); 564 Elf_Word dynstr_address = RoundUp(bss_address + bss_size, kPageSize); 565 566 // Cache .dynstr, .dynsym and .hash data. 567 dynstr_.Add(""); // dynstr should start with empty string. 568 Elf_Word rodata_index = rodata_.GetSectionIndex(); 569 Elf_Word oatdata = dynstr_.Add("oatdata"); 570 dynsym_.Add(oatdata, rodata_index, rodata_address, rodata_size, STB_GLOBAL, STT_OBJECT); 571 if (text_size != 0u) { 572 Elf_Word text_index = rodata_index + 1u; 573 Elf_Word oatexec = dynstr_.Add("oatexec"); 574 dynsym_.Add(oatexec, text_index, text_address, text_size, STB_GLOBAL, STT_OBJECT); 575 Elf_Word oatlastword = dynstr_.Add("oatlastword"); 576 Elf_Word oatlastword_address = text_address + text_size - 4; 577 dynsym_.Add(oatlastword, text_index, oatlastword_address, 4, STB_GLOBAL, STT_OBJECT); 578 } else if (rodata_size != 0) { 579 // rodata_ can be size 0 for dwarf_test. 580 Elf_Word oatlastword = dynstr_.Add("oatlastword"); 581 Elf_Word oatlastword_address = rodata_address + rodata_size - 4; 582 dynsym_.Add(oatlastword, rodata_index, oatlastword_address, 4, STB_GLOBAL, STT_OBJECT); 583 } 584 if (bss_size != 0u) { 585 Elf_Word bss_index = rodata_index + 1u + (text_size != 0 ? 1u : 0u); 586 Elf_Word oatbss = dynstr_.Add("oatbss"); 587 dynsym_.Add(oatbss, bss_index, bss_address, bss_size, STB_GLOBAL, STT_OBJECT); 588 Elf_Word oatbsslastword = dynstr_.Add("oatbsslastword"); 589 Elf_Word bsslastword_address = bss_address + bss_size - 4; 590 dynsym_.Add(oatbsslastword, bss_index, bsslastword_address, 4, STB_GLOBAL, STT_OBJECT); 591 } 592 Elf_Word soname_offset = dynstr_.Add(soname); 593 594 // We do not really need a hash-table since there is so few entries. 595 // However, the hash-table is the only way the linker can actually 596 // determine the number of symbols in .dynsym so it is required. 597 int count = dynsym_.GetCacheSize() / sizeof(Elf_Sym); // Includes NULL. 598 std::vector<Elf_Word> hash; 599 hash.push_back(1); // Number of buckets. 600 hash.push_back(count); // Number of chains. 601 // Buckets. Having just one makes it linear search. 602 hash.push_back(1); // Point to first non-NULL symbol. 603 // Chains. This creates linked list of symbols. 604 hash.push_back(0); // Dummy entry for the NULL symbol. 605 for (int i = 1; i < count - 1; i++) { 606 hash.push_back(i + 1); // Each symbol points to the next one. 607 } 608 hash.push_back(0); // Last symbol terminates the chain. 609 hash_.Add(hash.data(), hash.size() * sizeof(hash[0])); 610 611 // Calculate addresses of .dynsym, .hash and .dynamic. 612 DCHECK_EQ(dynstr_.header_.sh_flags, dynsym_.header_.sh_flags); 613 DCHECK_EQ(dynsym_.header_.sh_flags, hash_.header_.sh_flags); 614 Elf_Word dynsym_address = 615 RoundUp(dynstr_address + dynstr_.GetCacheSize(), dynsym_.header_.sh_addralign); 616 Elf_Word hash_address = 617 RoundUp(dynsym_address + dynsym_.GetCacheSize(), hash_.header_.sh_addralign); 618 DCHECK_EQ(dynamic_.header_.sh_addralign, static_cast<Elf_Word>(kPageSize)); 619 Elf_Word dynamic_address = RoundUp(hash_address + dynsym_.GetCacheSize(), kPageSize); 620 621 Elf_Dyn dyns[] = { 622 { DT_HASH, { hash_address } }, 623 { DT_STRTAB, { dynstr_address } }, 624 { DT_SYMTAB, { dynsym_address } }, 625 { DT_SYMENT, { sizeof(Elf_Sym) } }, 626 { DT_STRSZ, { dynstr_.GetCacheSize() } }, 627 { DT_SONAME, { soname_offset } }, 628 { DT_NULL, { 0 } }, 629 }; 630 dynamic_.Add(&dyns, sizeof(dyns)); 631 632 loaded_size_ = RoundUp(dynamic_address + dynamic_.GetCacheSize(), kPageSize); 633 } 634 635 void WriteDynamicSection() { 636 dynstr_.WriteCachedSection(); 637 dynsym_.WriteCachedSection(); 638 hash_.WriteCachedSection(); 639 dynamic_.WriteCachedSection(); 640 641 CHECK_EQ(loaded_size_, RoundUp(dynamic_.GetAddress() + dynamic_.GetSize(), kPageSize)); 642 } 643 644 Elf_Word GetLoadedSize() { 645 CHECK_NE(loaded_size_, 0u); 646 return loaded_size_; 647 } 648 649 // Returns true if all writes and seeks on the output stream succeeded. 650 bool Good() { 651 return stream_.Good(); 652 } 653 654 // Returns the builder's internal stream. 655 OutputStream* GetStream() { 656 return &stream_; 657 } 658 659 off_t AlignFileOffset(size_t alignment) { 660 return stream_.Seek(RoundUp(stream_.Seek(0, kSeekCurrent), alignment), kSeekSet); 661 } 662 663 Elf_Addr AlignVirtualAddress(size_t alignment) { 664 return virtual_address_ = RoundUp(virtual_address_, alignment); 665 } 666 667 private: 668 static Elf_Ehdr MakeElfHeader(InstructionSet isa) { 669 Elf_Ehdr elf_header = Elf_Ehdr(); 670 switch (isa) { 671 case kArm: 672 // Fall through. 673 case kThumb2: { 674 elf_header.e_machine = EM_ARM; 675 elf_header.e_flags = EF_ARM_EABI_VER5; 676 break; 677 } 678 case kArm64: { 679 elf_header.e_machine = EM_AARCH64; 680 elf_header.e_flags = 0; 681 break; 682 } 683 case kX86: { 684 elf_header.e_machine = EM_386; 685 elf_header.e_flags = 0; 686 break; 687 } 688 case kX86_64: { 689 elf_header.e_machine = EM_X86_64; 690 elf_header.e_flags = 0; 691 break; 692 } 693 case kMips: { 694 elf_header.e_machine = EM_MIPS; 695 elf_header.e_flags = (EF_MIPS_NOREORDER | 696 EF_MIPS_PIC | 697 EF_MIPS_CPIC | 698 EF_MIPS_ABI_O32 | 699 EF_MIPS_ARCH_32R2); 700 break; 701 } 702 case kMips64: { 703 elf_header.e_machine = EM_MIPS; 704 elf_header.e_flags = (EF_MIPS_NOREORDER | 705 EF_MIPS_PIC | 706 EF_MIPS_CPIC | 707 EF_MIPS_ARCH_64R6); 708 break; 709 } 710 case kNone: { 711 LOG(FATAL) << "No instruction set"; 712 break; 713 } 714 default: { 715 LOG(FATAL) << "Unknown instruction set " << isa; 716 } 717 } 718 719 elf_header.e_ident[EI_MAG0] = ELFMAG0; 720 elf_header.e_ident[EI_MAG1] = ELFMAG1; 721 elf_header.e_ident[EI_MAG2] = ELFMAG2; 722 elf_header.e_ident[EI_MAG3] = ELFMAG3; 723 elf_header.e_ident[EI_CLASS] = (sizeof(Elf_Addr) == sizeof(Elf32_Addr)) 724 ? ELFCLASS32 : ELFCLASS64;; 725 elf_header.e_ident[EI_DATA] = ELFDATA2LSB; 726 elf_header.e_ident[EI_VERSION] = EV_CURRENT; 727 elf_header.e_ident[EI_OSABI] = ELFOSABI_LINUX; 728 elf_header.e_ident[EI_ABIVERSION] = 0; 729 elf_header.e_type = ET_DYN; 730 elf_header.e_version = 1; 731 elf_header.e_entry = 0; 732 elf_header.e_ehsize = sizeof(Elf_Ehdr); 733 elf_header.e_phentsize = sizeof(Elf_Phdr); 734 elf_header.e_shentsize = sizeof(Elf_Shdr); 735 elf_header.e_phoff = sizeof(Elf_Ehdr); 736 return elf_header; 737 } 738 739 // Create program headers based on written sections. 740 std::vector<Elf_Phdr> MakeProgramHeaders() { 741 CHECK(!sections_.empty()); 742 std::vector<Elf_Phdr> phdrs; 743 { 744 // The program headers must start with PT_PHDR which is used in 745 // loaded process to determine the number of program headers. 746 Elf_Phdr phdr = Elf_Phdr(); 747 phdr.p_type = PT_PHDR; 748 phdr.p_flags = PF_R; 749 phdr.p_offset = phdr.p_vaddr = phdr.p_paddr = sizeof(Elf_Ehdr); 750 phdr.p_filesz = phdr.p_memsz = 0; // We need to fill this later. 751 phdr.p_align = sizeof(Elf_Off); 752 phdrs.push_back(phdr); 753 // Tell the linker to mmap the start of file to memory. 754 Elf_Phdr load = Elf_Phdr(); 755 load.p_type = PT_LOAD; 756 load.p_flags = PF_R; 757 load.p_offset = load.p_vaddr = load.p_paddr = 0; 758 load.p_filesz = load.p_memsz = sections_[0]->header_.sh_offset; 759 load.p_align = kPageSize; 760 phdrs.push_back(load); 761 } 762 // Create program headers for sections. 763 for (auto* section : sections_) { 764 const Elf_Shdr& shdr = section->header_; 765 if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) { 766 // PT_LOAD tells the linker to mmap part of the file. 767 // The linker can only mmap page-aligned sections. 768 // Single PT_LOAD may contain several ELF sections. 769 Elf_Phdr& prev = phdrs.back(); 770 Elf_Phdr load = Elf_Phdr(); 771 load.p_type = PT_LOAD; 772 load.p_flags = section->phdr_flags_; 773 load.p_offset = shdr.sh_offset; 774 load.p_vaddr = load.p_paddr = shdr.sh_addr; 775 load.p_filesz = (shdr.sh_type != SHT_NOBITS ? shdr.sh_size : 0u); 776 load.p_memsz = shdr.sh_size; 777 load.p_align = shdr.sh_addralign; 778 if (prev.p_type == load.p_type && 779 prev.p_flags == load.p_flags && 780 prev.p_filesz == prev.p_memsz && // Do not merge .bss 781 load.p_filesz == load.p_memsz) { // Do not merge .bss 782 // Merge this PT_LOAD with the previous one. 783 Elf_Word size = shdr.sh_offset + shdr.sh_size - prev.p_offset; 784 prev.p_filesz = size; 785 prev.p_memsz = size; 786 } else { 787 // If we are adding new load, it must be aligned. 788 CHECK_EQ(shdr.sh_addralign, (Elf_Word)kPageSize); 789 phdrs.push_back(load); 790 } 791 } 792 } 793 for (auto* section : sections_) { 794 const Elf_Shdr& shdr = section->header_; 795 if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) { 796 // Other PT_* types allow the program to locate interesting 797 // parts of memory at runtime. They must overlap with PT_LOAD. 798 if (section->phdr_type_ != 0) { 799 Elf_Phdr phdr = Elf_Phdr(); 800 phdr.p_type = section->phdr_type_; 801 phdr.p_flags = section->phdr_flags_; 802 phdr.p_offset = shdr.sh_offset; 803 phdr.p_vaddr = phdr.p_paddr = shdr.sh_addr; 804 phdr.p_filesz = phdr.p_memsz = shdr.sh_size; 805 phdr.p_align = shdr.sh_addralign; 806 phdrs.push_back(phdr); 807 } 808 } 809 } 810 // Set the size of the initial PT_PHDR. 811 CHECK_EQ(phdrs[0].p_type, (Elf_Word)PT_PHDR); 812 phdrs[0].p_filesz = phdrs[0].p_memsz = phdrs.size() * sizeof(Elf_Phdr); 813 814 return phdrs; 815 } 816 817 InstructionSet isa_; 818 const InstructionSetFeatures* features_; 819 820 ErrorDelayingOutputStream stream_; 821 822 Section rodata_; 823 Section text_; 824 Section bss_; 825 CachedStringSection dynstr_; 826 SymbolSection dynsym_; 827 CachedSection hash_; 828 CachedSection dynamic_; 829 Section eh_frame_; 830 Section eh_frame_hdr_; 831 StringSection strtab_; 832 SymbolSection symtab_; 833 Section debug_frame_; 834 Section debug_info_; 835 Section debug_line_; 836 StringSection shstrtab_; 837 std::vector<std::unique_ptr<Section>> other_sections_; 838 839 // List of used section in the order in which they were written. 840 std::vector<Section*> sections_; 841 842 bool started_; 843 bool write_program_headers_; 844 845 // The size of the memory taken by the ELF file when loaded. 846 size_t loaded_size_; 847 848 // Used for allocation of virtual address space. 849 Elf_Addr virtual_address_; 850 851 DISALLOW_COPY_AND_ASSIGN(ElfBuilder); 852}; 853 854} // namespace art 855 856#endif // ART_COMPILER_ELF_BUILDER_H_ 857