elf_builder.h revision 5b1c2ca30dad519be285f0a1e839c23cc4e3a51d
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 "base/bit_utils.h" 24#include "base/casts.h" 25#include "base/unix_file/fd_file.h" 26#include "elf_utils.h" 27#include "leb128.h" 28#include "linker/error_delaying_output_stream.h" 29#include "utils/array_ref.h" 30 31namespace art { 32 33// Writes ELF file. 34// 35// The basic layout of the elf file: 36// Elf_Ehdr - The ELF header. 37// Elf_Phdr[] - Program headers for the linker. 38// .rodata - DEX files and oat metadata. 39// .text - Compiled code. 40// .bss - Zero-initialized writeable section. 41// .dynstr - Names for .dynsym. 42// .dynsym - A few oat-specific dynamic symbols. 43// .hash - Hash-table for .dynsym. 44// .dynamic - Tags which let the linker locate .dynsym. 45// .strtab - Names for .symtab. 46// .symtab - Debug symbols. 47// .eh_frame - Unwind information (CFI). 48// .eh_frame_hdr - Index of .eh_frame. 49// .debug_frame - Unwind information (CFI). 50// .debug_frame.oat_patches - Addresses for relocation. 51// .debug_info - Debug information. 52// .debug_info.oat_patches - Addresses for relocation. 53// .debug_abbrev - Decoding information for .debug_info. 54// .debug_str - Strings for .debug_info. 55// .debug_line - Line number tables. 56// .debug_line.oat_patches - Addresses for relocation. 57// .text.oat_patches - Addresses for relocation. 58// .shstrtab - Names of ELF sections. 59// Elf_Shdr[] - Section headers. 60// 61// Some section are optional (the debug sections in particular). 62// 63// We try write the section data directly into the file without much 64// in-memory buffering. This means we generally write sections based on the 65// dependency order (e.g. .dynamic points to .dynsym which points to .text). 66// 67// In the cases where we need to buffer, we write the larger section first 68// and buffer the smaller one (e.g. .strtab is bigger than .symtab). 69// 70// The debug sections are written last for easier stripping. 71// 72template <typename ElfTypes> 73class ElfBuilder FINAL { 74 public: 75 static constexpr size_t kMaxProgramHeaders = 16; 76 using Elf_Addr = typename ElfTypes::Addr; 77 using Elf_Off = typename ElfTypes::Off; 78 using Elf_Word = typename ElfTypes::Word; 79 using Elf_Sword = typename ElfTypes::Sword; 80 using Elf_Ehdr = typename ElfTypes::Ehdr; 81 using Elf_Shdr = typename ElfTypes::Shdr; 82 using Elf_Sym = typename ElfTypes::Sym; 83 using Elf_Phdr = typename ElfTypes::Phdr; 84 using Elf_Dyn = typename ElfTypes::Dyn; 85 86 // Base class of all sections. 87 class Section : public OutputStream { 88 public: 89 Section(ElfBuilder<ElfTypes>* owner, const std::string& name, 90 Elf_Word type, Elf_Word flags, const Section* link, 91 Elf_Word info, Elf_Word align, Elf_Word entsize) 92 : OutputStream(name), owner_(owner), header_(), 93 section_index_(0), name_(name), link_(link), 94 started_(false), finished_(false), phdr_flags_(PF_R), phdr_type_(0) { 95 DCHECK_GE(align, 1u); 96 header_.sh_type = type; 97 header_.sh_flags = flags; 98 header_.sh_info = info; 99 header_.sh_addralign = align; 100 header_.sh_entsize = entsize; 101 } 102 103 // Start writing of this section. 104 void Start() { 105 CHECK(!started_); 106 CHECK(!finished_); 107 started_ = true; 108 auto& sections = owner_->sections_; 109 // Check that the previous section is complete. 110 CHECK(sections.empty() || sections.back()->finished_); 111 // The first ELF section index is 1. Index 0 is reserved for NULL. 112 section_index_ = sections.size() + 1; 113 // Push this section on the list of written sections. 114 sections.push_back(this); 115 // Align file position. 116 if (header_.sh_type != SHT_NOBITS) { 117 header_.sh_offset = RoundUp(owner_->stream_.Seek(0, kSeekCurrent), header_.sh_addralign); 118 owner_->stream_.Seek(header_.sh_offset, kSeekSet); 119 } 120 // Align virtual memory address. 121 if ((header_.sh_flags & SHF_ALLOC) != 0) { 122 header_.sh_addr = RoundUp(owner_->virtual_address_, header_.sh_addralign); 123 owner_->virtual_address_ = header_.sh_addr; 124 } 125 } 126 127 // Finish writing of this section. 128 void End() { 129 CHECK(started_); 130 CHECK(!finished_); 131 finished_ = true; 132 if (header_.sh_type == SHT_NOBITS) { 133 CHECK_GT(header_.sh_size, 0u); 134 } else { 135 // Use the current file position to determine section size. 136 off_t file_offset = owner_->stream_.Seek(0, kSeekCurrent); 137 CHECK_GE(file_offset, (off_t)header_.sh_offset); 138 header_.sh_size = file_offset - header_.sh_offset; 139 } 140 if ((header_.sh_flags & SHF_ALLOC) != 0) { 141 owner_->virtual_address_ += header_.sh_size; 142 } 143 } 144 145 // Returns true if the section was written to disk. 146 // (Used to check whether we have .text when writing JIT debug info) 147 bool Exists() const { 148 return finished_; 149 } 150 151 // Get the location of this section in virtual memory. 152 Elf_Addr GetAddress() const { 153 CHECK(started_); 154 return header_.sh_addr; 155 } 156 157 // Returns the size of the content of this section. 158 Elf_Word GetSize() const { 159 if (finished_) { 160 return header_.sh_size; 161 } else { 162 CHECK(started_); 163 CHECK_NE(header_.sh_type, (Elf_Word)SHT_NOBITS); 164 return owner_->stream_.Seek(0, kSeekCurrent) - header_.sh_offset; 165 } 166 } 167 168 // Write this section as "NOBITS" section. (used for the .bss section) 169 // This means that the ELF file does not contain the initial data for this section 170 // and it will be zero-initialized when the ELF file is loaded in the running program. 171 void WriteNoBitsSection(Elf_Word size) { 172 DCHECK_NE(header_.sh_flags & SHF_ALLOC, 0u); 173 Start(); 174 header_.sh_type = SHT_NOBITS; 175 header_.sh_size = size; 176 End(); 177 } 178 179 // This function always succeeds to simplify code. 180 // Use builder's Good() to check the actual status. 181 bool WriteFully(const void* buffer, size_t byte_count) OVERRIDE { 182 CHECK(started_); 183 CHECK(!finished_); 184 return owner_->stream_.WriteFully(buffer, byte_count); 185 } 186 187 // This function always succeeds to simplify code. 188 // Use builder's Good() to check the actual status. 189 off_t Seek(off_t offset, Whence whence) OVERRIDE { 190 // Forward the seek as-is and trust the caller to use it reasonably. 191 return owner_->stream_.Seek(offset, whence); 192 } 193 194 // This function flushes the output and returns whether it succeeded. 195 // If there was a previous failure, this does nothing and returns false, i.e. failed. 196 bool Flush() OVERRIDE { 197 return owner_->stream_.Flush(); 198 } 199 200 Elf_Word GetSectionIndex() const { 201 DCHECK(started_); 202 DCHECK_NE(section_index_, 0u); 203 return section_index_; 204 } 205 206 private: 207 ElfBuilder<ElfTypes>* owner_; 208 Elf_Shdr header_; 209 Elf_Word section_index_; 210 const std::string name_; 211 const Section* const link_; 212 bool started_; 213 bool finished_; 214 Elf_Word phdr_flags_; 215 Elf_Word phdr_type_; 216 217 friend class ElfBuilder; 218 219 DISALLOW_COPY_AND_ASSIGN(Section); 220 }; 221 222 // Writer of .dynstr .strtab and .shstrtab sections. 223 class StringSection FINAL : public Section { 224 public: 225 StringSection(ElfBuilder<ElfTypes>* owner, const std::string& name, 226 Elf_Word flags, Elf_Word align) 227 : Section(owner, name, SHT_STRTAB, flags, nullptr, 0, align, 0), 228 current_offset_(0) { 229 } 230 231 Elf_Word Write(const std::string& name) { 232 if (current_offset_ == 0) { 233 DCHECK(name.empty()); 234 } 235 Elf_Word offset = current_offset_; 236 this->WriteFully(name.c_str(), name.length() + 1); 237 current_offset_ += name.length() + 1; 238 return offset; 239 } 240 241 private: 242 Elf_Word current_offset_; 243 }; 244 245 // Writer of .dynsym and .symtab sections. 246 class SymbolSection FINAL : public Section { 247 public: 248 SymbolSection(ElfBuilder<ElfTypes>* owner, const std::string& name, 249 Elf_Word type, Elf_Word flags, StringSection* strtab) 250 : Section(owner, name, type, flags, strtab, 0, 251 sizeof(Elf_Off), sizeof(Elf_Sym)) { 252 } 253 254 // Buffer symbol for this section. It will be written later. 255 // If the symbol's section is null, it will be considered absolute (SHN_ABS). 256 // (we use this in JIT to reference code which is stored outside the debug ELF file) 257 void Add(Elf_Word name, const Section* section, 258 Elf_Addr addr, bool is_relative, Elf_Word size, 259 uint8_t binding, uint8_t type, uint8_t other = 0) { 260 Elf_Sym sym = Elf_Sym(); 261 sym.st_name = name; 262 sym.st_value = addr + (is_relative ? section->GetAddress() : 0); 263 sym.st_size = size; 264 sym.st_other = other; 265 sym.st_shndx = (section != nullptr ? section->GetSectionIndex() 266 : static_cast<Elf_Word>(SHN_ABS)); 267 sym.st_info = (binding << 4) + (type & 0xf); 268 symbols_.push_back(sym); 269 } 270 271 void Write() { 272 // The symbol table always has to start with NULL symbol. 273 Elf_Sym null_symbol = Elf_Sym(); 274 this->WriteFully(&null_symbol, sizeof(null_symbol)); 275 this->WriteFully(symbols_.data(), symbols_.size() * sizeof(symbols_[0])); 276 symbols_.clear(); 277 symbols_.shrink_to_fit(); 278 } 279 280 private: 281 std::vector<Elf_Sym> symbols_; 282 }; 283 284 ElfBuilder(InstructionSet isa, OutputStream* output) 285 : isa_(isa), 286 stream_(output), 287 rodata_(this, ".rodata", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 288 text_(this, ".text", SHT_PROGBITS, SHF_ALLOC | SHF_EXECINSTR, nullptr, 0, kPageSize, 0), 289 bss_(this, ".bss", SHT_NOBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 290 dynstr_(this, ".dynstr", SHF_ALLOC, kPageSize), 291 dynsym_(this, ".dynsym", SHT_DYNSYM, SHF_ALLOC, &dynstr_), 292 hash_(this, ".hash", SHT_HASH, SHF_ALLOC, &dynsym_, 0, sizeof(Elf_Word), sizeof(Elf_Word)), 293 dynamic_(this, ".dynamic", SHT_DYNAMIC, SHF_ALLOC, &dynstr_, 0, kPageSize, sizeof(Elf_Dyn)), 294 eh_frame_(this, ".eh_frame", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, kPageSize, 0), 295 eh_frame_hdr_(this, ".eh_frame_hdr", SHT_PROGBITS, SHF_ALLOC, nullptr, 0, 4, 0), 296 strtab_(this, ".strtab", 0, kPageSize), 297 symtab_(this, ".symtab", SHT_SYMTAB, 0, &strtab_), 298 debug_frame_(this, ".debug_frame", SHT_PROGBITS, 0, nullptr, 0, sizeof(Elf_Addr), 0), 299 debug_info_(this, ".debug_info", SHT_PROGBITS, 0, nullptr, 0, 1, 0), 300 debug_line_(this, ".debug_line", SHT_PROGBITS, 0, nullptr, 0, 1, 0), 301 shstrtab_(this, ".shstrtab", 0, 1), 302 virtual_address_(0) { 303 text_.phdr_flags_ = PF_R | PF_X; 304 bss_.phdr_flags_ = PF_R | PF_W; 305 dynamic_.phdr_flags_ = PF_R | PF_W; 306 dynamic_.phdr_type_ = PT_DYNAMIC; 307 eh_frame_hdr_.phdr_type_ = PT_GNU_EH_FRAME; 308 } 309 ~ElfBuilder() {} 310 311 InstructionSet GetIsa() { return isa_; } 312 Section* GetRoData() { return &rodata_; } 313 Section* GetText() { return &text_; } 314 Section* GetBss() { return &bss_; } 315 StringSection* GetStrTab() { return &strtab_; } 316 SymbolSection* GetSymTab() { return &symtab_; } 317 Section* GetEhFrame() { return &eh_frame_; } 318 Section* GetEhFrameHdr() { return &eh_frame_hdr_; } 319 Section* GetDebugFrame() { return &debug_frame_; } 320 Section* GetDebugInfo() { return &debug_info_; } 321 Section* GetDebugLine() { return &debug_line_; } 322 323 // Encode patch locations as LEB128 list of deltas between consecutive addresses. 324 // (exposed publicly for tests) 325 static void EncodeOatPatches(const ArrayRef<const uintptr_t>& locations, 326 std::vector<uint8_t>* buffer) { 327 buffer->reserve(buffer->size() + locations.size() * 2); // guess 2 bytes per ULEB128. 328 uintptr_t address = 0; // relative to start of section. 329 for (uintptr_t location : locations) { 330 DCHECK_GE(location, address) << "Patch locations are not in sorted order"; 331 EncodeUnsignedLeb128(buffer, dchecked_integral_cast<uint32_t>(location - address)); 332 address = location; 333 } 334 } 335 336 void WritePatches(const char* name, const ArrayRef<const uintptr_t>& patch_locations) { 337 std::vector<uint8_t> buffer; 338 EncodeOatPatches(patch_locations, &buffer); 339 std::unique_ptr<Section> s(new Section(this, name, SHT_OAT_PATCH, 0, nullptr, 0, 1, 0)); 340 s->Start(); 341 s->WriteFully(buffer.data(), buffer.size()); 342 s->End(); 343 other_sections_.push_back(std::move(s)); 344 } 345 346 void WriteSection(const char* name, const std::vector<uint8_t>* buffer) { 347 std::unique_ptr<Section> s(new Section(this, name, SHT_PROGBITS, 0, nullptr, 0, 1, 0)); 348 s->Start(); 349 s->WriteFully(buffer->data(), buffer->size()); 350 s->End(); 351 other_sections_.push_back(std::move(s)); 352 } 353 354 // Set where the next section will be allocated in the virtual address space. 355 void SetVirtualAddress(Elf_Addr address) { 356 DCHECK_GE(address, virtual_address_); 357 virtual_address_ = address; 358 } 359 360 void Start() { 361 // Reserve space for ELF header and program headers. 362 // We do not know the number of headers until later, so 363 // it is easiest to just reserve a fixed amount of space. 364 int size = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * kMaxProgramHeaders; 365 stream_.Seek(size, kSeekSet); 366 virtual_address_ += size; 367 } 368 369 void End() { 370 // Write section names and finish the section headers. 371 shstrtab_.Start(); 372 shstrtab_.Write(""); 373 for (auto* section : sections_) { 374 section->header_.sh_name = shstrtab_.Write(section->name_); 375 if (section->link_ != nullptr) { 376 section->header_.sh_link = section->link_->GetSectionIndex(); 377 } 378 } 379 shstrtab_.End(); 380 381 // Write section headers at the end of the ELF file. 382 std::vector<Elf_Shdr> shdrs; 383 shdrs.reserve(1u + sections_.size()); 384 shdrs.push_back(Elf_Shdr()); // NULL at index 0. 385 for (auto* section : sections_) { 386 shdrs.push_back(section->header_); 387 } 388 Elf_Off section_headers_offset; 389 section_headers_offset = RoundUp(stream_.Seek(0, kSeekCurrent), sizeof(Elf_Off)); 390 stream_.Seek(section_headers_offset, kSeekSet); 391 stream_.WriteFully(shdrs.data(), shdrs.size() * sizeof(shdrs[0])); 392 393 // Flush everything else before writing the program headers. This should prevent 394 // the OS from reordering writes, so that we don't end up with valid headers 395 // and partially written data if we suddenly lose power, for example. 396 stream_.Flush(); 397 398 // Write the initial file headers. 399 std::vector<Elf_Phdr> phdrs = MakeProgramHeaders(); 400 Elf_Ehdr elf_header = MakeElfHeader(isa_); 401 elf_header.e_phoff = sizeof(Elf_Ehdr); 402 elf_header.e_shoff = section_headers_offset; 403 elf_header.e_phnum = phdrs.size(); 404 elf_header.e_shnum = shdrs.size(); 405 elf_header.e_shstrndx = shstrtab_.GetSectionIndex(); 406 stream_.Seek(0, kSeekSet); 407 stream_.WriteFully(&elf_header, sizeof(elf_header)); 408 stream_.WriteFully(phdrs.data(), phdrs.size() * sizeof(phdrs[0])); 409 stream_.Flush(); 410 } 411 412 // The running program does not have access to section headers 413 // and the loader is not supposed to use them either. 414 // The dynamic sections therefore replicates some of the layout 415 // information like the address and size of .rodata and .text. 416 // It also contains other metadata like the SONAME. 417 // The .dynamic section is found using the PT_DYNAMIC program header. 418 void WriteDynamicSection(const std::string& elf_file_path) { 419 std::string soname(elf_file_path); 420 size_t directory_separator_pos = soname.rfind('/'); 421 if (directory_separator_pos != std::string::npos) { 422 soname = soname.substr(directory_separator_pos + 1); 423 } 424 425 dynstr_.Start(); 426 dynstr_.Write(""); // dynstr should start with empty string. 427 dynsym_.Add(dynstr_.Write("oatdata"), &rodata_, 0, true, 428 rodata_.GetSize(), STB_GLOBAL, STT_OBJECT); 429 if (text_.GetSize() != 0u) { 430 dynsym_.Add(dynstr_.Write("oatexec"), &text_, 0, true, 431 text_.GetSize(), STB_GLOBAL, STT_OBJECT); 432 dynsym_.Add(dynstr_.Write("oatlastword"), &text_, text_.GetSize() - 4, 433 true, 4, STB_GLOBAL, STT_OBJECT); 434 } else if (rodata_.GetSize() != 0) { 435 // rodata_ can be size 0 for dwarf_test. 436 dynsym_.Add(dynstr_.Write("oatlastword"), &rodata_, rodata_.GetSize() - 4, 437 true, 4, STB_GLOBAL, STT_OBJECT); 438 } 439 if (bss_.finished_) { 440 dynsym_.Add(dynstr_.Write("oatbss"), &bss_, 441 0, true, bss_.GetSize(), STB_GLOBAL, STT_OBJECT); 442 dynsym_.Add(dynstr_.Write("oatbsslastword"), &bss_, 443 bss_.GetSize() - 4, true, 4, STB_GLOBAL, STT_OBJECT); 444 } 445 Elf_Word soname_offset = dynstr_.Write(soname); 446 dynstr_.End(); 447 448 dynsym_.Start(); 449 dynsym_.Write(); 450 dynsym_.End(); 451 452 // We do not really need a hash-table since there is so few entries. 453 // However, the hash-table is the only way the linker can actually 454 // determine the number of symbols in .dynsym so it is required. 455 hash_.Start(); 456 int count = dynsym_.GetSize() / sizeof(Elf_Sym); // Includes NULL. 457 std::vector<Elf_Word> hash; 458 hash.push_back(1); // Number of buckets. 459 hash.push_back(count); // Number of chains. 460 // Buckets. Having just one makes it linear search. 461 hash.push_back(1); // Point to first non-NULL symbol. 462 // Chains. This creates linked list of symbols. 463 hash.push_back(0); // Dummy entry for the NULL symbol. 464 for (int i = 1; i < count - 1; i++) { 465 hash.push_back(i + 1); // Each symbol points to the next one. 466 } 467 hash.push_back(0); // Last symbol terminates the chain. 468 hash_.WriteFully(hash.data(), hash.size() * sizeof(hash[0])); 469 hash_.End(); 470 471 dynamic_.Start(); 472 Elf_Dyn dyns[] = { 473 { DT_HASH, { hash_.GetAddress() } }, 474 { DT_STRTAB, { dynstr_.GetAddress() } }, 475 { DT_SYMTAB, { dynsym_.GetAddress() } }, 476 { DT_SYMENT, { sizeof(Elf_Sym) } }, 477 { DT_STRSZ, { dynstr_.GetSize() } }, 478 { DT_SONAME, { soname_offset } }, 479 { DT_NULL, { 0 } }, 480 }; 481 dynamic_.WriteFully(&dyns, sizeof(dyns)); 482 dynamic_.End(); 483 } 484 485 // Returns true if all writes and seeks on the output stream succeeded. 486 bool Good() { 487 return stream_.Good(); 488 } 489 490 // Returns the builder's internal stream. 491 OutputStream* GetStream() { 492 return &stream_; 493 } 494 495 private: 496 static Elf_Ehdr MakeElfHeader(InstructionSet isa) { 497 Elf_Ehdr elf_header = Elf_Ehdr(); 498 switch (isa) { 499 case kArm: 500 // Fall through. 501 case kThumb2: { 502 elf_header.e_machine = EM_ARM; 503 elf_header.e_flags = EF_ARM_EABI_VER5; 504 break; 505 } 506 case kArm64: { 507 elf_header.e_machine = EM_AARCH64; 508 elf_header.e_flags = 0; 509 break; 510 } 511 case kX86: { 512 elf_header.e_machine = EM_386; 513 elf_header.e_flags = 0; 514 break; 515 } 516 case kX86_64: { 517 elf_header.e_machine = EM_X86_64; 518 elf_header.e_flags = 0; 519 break; 520 } 521 case kMips: { 522 elf_header.e_machine = EM_MIPS; 523 elf_header.e_flags = (EF_MIPS_NOREORDER | 524 EF_MIPS_PIC | 525 EF_MIPS_CPIC | 526 EF_MIPS_ABI_O32 | 527 EF_MIPS_ARCH_32R2); 528 break; 529 } 530 case kMips64: { 531 elf_header.e_machine = EM_MIPS; 532 elf_header.e_flags = (EF_MIPS_NOREORDER | 533 EF_MIPS_PIC | 534 EF_MIPS_CPIC | 535 EF_MIPS_ARCH_64R6); 536 break; 537 } 538 case kNone: { 539 LOG(FATAL) << "No instruction set"; 540 break; 541 } 542 default: { 543 LOG(FATAL) << "Unknown instruction set " << isa; 544 } 545 } 546 547 elf_header.e_ident[EI_MAG0] = ELFMAG0; 548 elf_header.e_ident[EI_MAG1] = ELFMAG1; 549 elf_header.e_ident[EI_MAG2] = ELFMAG2; 550 elf_header.e_ident[EI_MAG3] = ELFMAG3; 551 elf_header.e_ident[EI_CLASS] = (sizeof(Elf_Addr) == sizeof(Elf32_Addr)) 552 ? ELFCLASS32 : ELFCLASS64;; 553 elf_header.e_ident[EI_DATA] = ELFDATA2LSB; 554 elf_header.e_ident[EI_VERSION] = EV_CURRENT; 555 elf_header.e_ident[EI_OSABI] = ELFOSABI_LINUX; 556 elf_header.e_ident[EI_ABIVERSION] = 0; 557 elf_header.e_type = ET_DYN; 558 elf_header.e_version = 1; 559 elf_header.e_entry = 0; 560 elf_header.e_ehsize = sizeof(Elf_Ehdr); 561 elf_header.e_phentsize = sizeof(Elf_Phdr); 562 elf_header.e_shentsize = sizeof(Elf_Shdr); 563 elf_header.e_phoff = sizeof(Elf_Ehdr); 564 return elf_header; 565 } 566 567 // Create program headers based on written sections. 568 std::vector<Elf_Phdr> MakeProgramHeaders() { 569 CHECK(!sections_.empty()); 570 std::vector<Elf_Phdr> phdrs; 571 { 572 // The program headers must start with PT_PHDR which is used in 573 // loaded process to determine the number of program headers. 574 Elf_Phdr phdr = Elf_Phdr(); 575 phdr.p_type = PT_PHDR; 576 phdr.p_flags = PF_R; 577 phdr.p_offset = phdr.p_vaddr = phdr.p_paddr = sizeof(Elf_Ehdr); 578 phdr.p_filesz = phdr.p_memsz = 0; // We need to fill this later. 579 phdr.p_align = sizeof(Elf_Off); 580 phdrs.push_back(phdr); 581 // Tell the linker to mmap the start of file to memory. 582 Elf_Phdr load = Elf_Phdr(); 583 load.p_type = PT_LOAD; 584 load.p_flags = PF_R; 585 load.p_offset = load.p_vaddr = load.p_paddr = 0; 586 load.p_filesz = load.p_memsz = sections_[0]->header_.sh_offset; 587 load.p_align = kPageSize; 588 phdrs.push_back(load); 589 } 590 // Create program headers for sections. 591 for (auto* section : sections_) { 592 const Elf_Shdr& shdr = section->header_; 593 if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) { 594 // PT_LOAD tells the linker to mmap part of the file. 595 // The linker can only mmap page-aligned sections. 596 // Single PT_LOAD may contain several ELF sections. 597 Elf_Phdr& prev = phdrs.back(); 598 Elf_Phdr load = Elf_Phdr(); 599 load.p_type = PT_LOAD; 600 load.p_flags = section->phdr_flags_; 601 load.p_offset = shdr.sh_offset; 602 load.p_vaddr = load.p_paddr = shdr.sh_addr; 603 load.p_filesz = (shdr.sh_type != SHT_NOBITS ? shdr.sh_size : 0u); 604 load.p_memsz = shdr.sh_size; 605 load.p_align = shdr.sh_addralign; 606 if (prev.p_type == load.p_type && 607 prev.p_flags == load.p_flags && 608 prev.p_filesz == prev.p_memsz && // Do not merge .bss 609 load.p_filesz == load.p_memsz) { // Do not merge .bss 610 // Merge this PT_LOAD with the previous one. 611 Elf_Word size = shdr.sh_offset + shdr.sh_size - prev.p_offset; 612 prev.p_filesz = size; 613 prev.p_memsz = size; 614 } else { 615 // If we are adding new load, it must be aligned. 616 CHECK_EQ(shdr.sh_addralign, (Elf_Word)kPageSize); 617 phdrs.push_back(load); 618 } 619 } 620 } 621 for (auto* section : sections_) { 622 const Elf_Shdr& shdr = section->header_; 623 if ((shdr.sh_flags & SHF_ALLOC) != 0 && shdr.sh_size != 0) { 624 // Other PT_* types allow the program to locate interesting 625 // parts of memory at runtime. They must overlap with PT_LOAD. 626 if (section->phdr_type_ != 0) { 627 Elf_Phdr phdr = Elf_Phdr(); 628 phdr.p_type = section->phdr_type_; 629 phdr.p_flags = section->phdr_flags_; 630 phdr.p_offset = shdr.sh_offset; 631 phdr.p_vaddr = phdr.p_paddr = shdr.sh_addr; 632 phdr.p_filesz = phdr.p_memsz = shdr.sh_size; 633 phdr.p_align = shdr.sh_addralign; 634 phdrs.push_back(phdr); 635 } 636 } 637 } 638 // Set the size of the initial PT_PHDR. 639 CHECK_EQ(phdrs[0].p_type, (Elf_Word)PT_PHDR); 640 phdrs[0].p_filesz = phdrs[0].p_memsz = phdrs.size() * sizeof(Elf_Phdr); 641 642 return phdrs; 643 } 644 645 InstructionSet isa_; 646 647 ErrorDelayingOutputStream stream_; 648 649 Section rodata_; 650 Section text_; 651 Section bss_; 652 StringSection dynstr_; 653 SymbolSection dynsym_; 654 Section hash_; 655 Section dynamic_; 656 Section eh_frame_; 657 Section eh_frame_hdr_; 658 StringSection strtab_; 659 SymbolSection symtab_; 660 Section debug_frame_; 661 Section debug_info_; 662 Section debug_line_; 663 StringSection shstrtab_; 664 std::vector<std::unique_ptr<Section>> other_sections_; 665 666 // List of used section in the order in which they were written. 667 std::vector<Section*> sections_; 668 669 // Used for allocation of virtual address space. 670 Elf_Addr virtual_address_; 671 672 DISALLOW_COPY_AND_ASSIGN(ElfBuilder); 673}; 674 675} // namespace art 676 677#endif // ART_COMPILER_ELF_BUILDER_H_ 678