llvm/lib/InterfaceStub/ELFObjHandler.cpp

   1 //===- ELFObjHandler.cpp --------------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===-----------------------------------------------------------------------===/
   8
   9 #include "llvm/InterfaceStub/ELFObjHandler.h"
  10 #include "llvm/InterfaceStub/IFSStub.h"
  11 #include "llvm/MC/StringTableBuilder.h"
  12 #include "llvm/Object/Binary.h"
  13 #include "llvm/Object/ELFObjectFile.h"
  14 #include "llvm/Object/ELFTypes.h"
  15 #include "llvm/Support/Errc.h"
  16 #include "llvm/Support/Error.h"
  17 #include "llvm/Support/FileOutputBuffer.h"
  18 #include "llvm/Support/MathExtras.h"
  19 #include "llvm/Support/MemoryBuffer.h"
  20 #include "llvm/Support/Process.h"
  21
  22 using llvm::MemoryBufferRef;
  23 using llvm::object::ELFObjectFile;
  24
  25 using namespace llvm;
  26 using namespace llvm::object;
  27 using namespace llvm::ELF;
  28
  29 namespace llvm {
  30 namespace ifs {
  31
  32 // Simple struct to hold relevant .dynamic entries.
  33 struct DynamicEntries {
  34   uint64_t StrTabAddr = 0;
  35   uint64_t StrSize = 0;
  36   Optional<uint64_t> SONameOffset;
  37   std::vector<uint64_t> NeededLibNames;
  38   // Symbol table:
  39   uint64_t DynSymAddr = 0;
  40   // Hash tables:
  41   Optional<uint64_t> ElfHash;
  42   Optional<uint64_t> GnuHash;
  43 };
  44
  45 /// This initializes an ELF file header with information specific to a binary
  46 /// dynamic shared object.
  47 /// Offsets, indexes, links, etc. for section and program headers are just
  48 /// zero-initialized as they will be updated elsewhere.
  49 ///
  50 /// @param ElfHeader Target ELFT::Ehdr to populate.
  51 /// @param Machine Target architecture (e_machine from ELF specifications).
  52 template <class ELFT>
  53 static void initELFHeader(typename ELFT::Ehdr &ElfHeader, uint16_t Machine) {
  54   memset(&ElfHeader, 0, sizeof(ElfHeader));
  55   // ELF identification.
  56   ElfHeader.e_ident[EI_MAG0] = ElfMagic[EI_MAG0];
  57   ElfHeader.e_ident[EI_MAG1] = ElfMagic[EI_MAG1];
  58   ElfHeader.e_ident[EI_MAG2] = ElfMagic[EI_MAG2];
  59   ElfHeader.e_ident[EI_MAG3] = ElfMagic[EI_MAG3];
  60   ElfHeader.e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
  61   bool IsLittleEndian = ELFT::TargetEndianness == support::little;
  62   ElfHeader.e_ident[EI_DATA] = IsLittleEndian ? ELFDATA2LSB : ELFDATA2MSB;
  63   ElfHeader.e_ident[EI_VERSION] = EV_CURRENT;
  64   ElfHeader.e_ident[EI_OSABI] = ELFOSABI_NONE;
  65
  66   // Remainder of ELF header.
  67   ElfHeader.e_type = ET_DYN;
  68   ElfHeader.e_machine = Machine;
  69   ElfHeader.e_version = EV_CURRENT;
  70   ElfHeader.e_ehsize = sizeof(typename ELFT::Ehdr);
  71   ElfHeader.e_phentsize = sizeof(typename ELFT::Phdr);
  72   ElfHeader.e_shentsize = sizeof(typename ELFT::Shdr);
  73 }
  74
  75 namespace {
  76 template <class ELFT> struct OutputSection {
  77   using Elf_Shdr = typename ELFT::Shdr;
  78   std::string Name;
  79   Elf_Shdr Shdr;
  80   uint64_t Addr;
  81   uint64_t Offset;
  82   uint64_t Size;
  83   uint64_t Align;
  84   uint32_t Index;
  85   bool NoBits = true;
  86 };
  87
  88 template <class T, class ELFT>
  89 struct ContentSection : public OutputSection<ELFT> {
  90   T Content;
  91   ContentSection() { this->NoBits = false; }
  92 };
  93
  94 // This class just wraps StringTableBuilder for the purpose of adding a
  95 // default constructor.
  96 class ELFStringTableBuilder : public StringTableBuilder {
  97 public:
  98   ELFStringTableBuilder() : StringTableBuilder(StringTableBuilder::ELF) {}
  99 };
 100
 101 template <class ELFT> class ELFSymbolTableBuilder {
 102 public:
 103   using Elf_Sym = typename ELFT::Sym;
 104
 105   ELFSymbolTableBuilder() { Symbols.push_back({}); }
 106
 107   void add(size_t StNameOffset, uint64_t StSize, uint8_t StBind, uint8_t StType,
 108            uint8_t StOther, uint16_t StShndx) {
 109     Elf_Sym S{};
 110     S.st_name = StNameOffset;
 111     S.st_size = StSize;
 112     S.st_info = (StBind << 4) | (StType & 0xf);
 113     S.st_other = StOther;
 114     S.st_shndx = StShndx;
 115     Symbols.push_back(S);
 116   }
 117
 118   size_t getSize() const { return Symbols.size() * sizeof(Elf_Sym); }
 119
 120   void write(uint8_t *Buf) const {
 121     memcpy(Buf, Symbols.data(), sizeof(Elf_Sym) * Symbols.size());
 122   }
 123
 124 private:
 125   llvm::SmallVector<Elf_Sym, 8> Symbols;
 126 };
 127
 128 template <class ELFT> class ELFDynamicTableBuilder {
 129 public:
 130   using Elf_Dyn = typename ELFT::Dyn;
 131
 132   size_t addAddr(uint64_t Tag, uint64_t Addr) {
 133     Elf_Dyn Entry;
 134     Entry.d_tag = Tag;
 135     Entry.d_un.d_ptr = Addr;
 136     Entries.push_back(Entry);
 137     return Entries.size() - 1;
 138   }
 139
 140   void modifyAddr(size_t Index, uint64_t Addr) {
 141     Entries[Index].d_un.d_ptr = Addr;
 142   }
 143
 144   size_t addValue(uint64_t Tag, uint64_t Value) {
 145     Elf_Dyn Entry;
 146     Entry.d_tag = Tag;
 147     Entry.d_un.d_val = Value;
 148     Entries.push_back(Entry);
 149     return Entries.size() - 1;
 150   }
 151
 152   void modifyValue(size_t Index, uint64_t Value) {
 153     Entries[Index].d_un.d_val = Value;
 154   }
 155
 156   size_t getSize() const {
 157     // Add DT_NULL entry at the end.
 158     return (Entries.size() + 1) * sizeof(Elf_Dyn);
 159   }
 160
 161   void write(uint8_t *Buf) const {
 162     memcpy(Buf, Entries.data(), sizeof(Elf_Dyn) * Entries.size());
 163     // Add DT_NULL entry at the end.
 164     memset(Buf + sizeof(Elf_Dyn) * Entries.size(), 0, sizeof(Elf_Dyn));
 165   }
 166
 167 private:
 168   llvm::SmallVector<Elf_Dyn, 8> Entries;
 169 };
 170
 171 template <class ELFT> class ELFStubBuilder {
 172 public:
 173   using Elf_Ehdr = typename ELFT::Ehdr;
 174   using Elf_Shdr = typename ELFT::Shdr;
 175   using Elf_Phdr = typename ELFT::Phdr;
 176   using Elf_Sym = typename ELFT::Sym;
 177   using Elf_Addr = typename ELFT::Addr;
 178   using Elf_Dyn = typename ELFT::Dyn;
 179
 180   ELFStubBuilder(const ELFStubBuilder &) = delete;
 181   ELFStubBuilder(ELFStubBuilder &&) = default;
 182
 183   explicit ELFStubBuilder(const IFSStub &Stub) {
 184     DynSym.Name = ".dynsym";
 185     DynSym.Align = sizeof(Elf_Addr);
 186     DynStr.Name = ".dynstr";
 187     DynStr.Align = 1;
 188     DynTab.Name = ".dynamic";
 189     DynTab.Align = sizeof(Elf_Addr);
 190     ShStrTab.Name = ".shstrtab";
 191     ShStrTab.Align = 1;
 192
 193     // Populate string tables.
 194     for (const IFSSymbol &Sym : Stub.Symbols)
 195       DynStr.Content.add(Sym.Name);
 196     for (const std::string &Lib : Stub.NeededLibs)
 197       DynStr.Content.add(Lib);
 198     if (Stub.SoName)
 199       DynStr.Content.add(Stub.SoName.getValue());
 200
 201     std::vector<OutputSection<ELFT> *> Sections = {&DynSym, &DynStr, &DynTab,
 202                                                    &ShStrTab};
 203     const OutputSection<ELFT> *LastSection = Sections.back();
 204     // Now set the Index and put sections names into ".shstrtab".
 205     uint64_t Index = 1;
 206     for (OutputSection<ELFT> *Sec : Sections) {
 207       Sec->Index = Index++;
 208       ShStrTab.Content.add(Sec->Name);
 209     }
 210     ShStrTab.Content.finalize();
 211     ShStrTab.Size = ShStrTab.Content.getSize();
 212     DynStr.Content.finalize();
 213     DynStr.Size = DynStr.Content.getSize();
 214
 215     // Populate dynamic symbol table.
 216     for (const IFSSymbol &Sym : Stub.Symbols) {
 217       uint8_t Bind = Sym.Weak ? STB_WEAK : STB_GLOBAL;
 218       // For non-undefined symbols, value of the shndx is not relevant at link
 219       // time as long as it is not SHN_UNDEF. Set shndx to 1, which
 220       // points to ".dynsym".
 221       uint16_t Shndx = Sym.Undefined ? SHN_UNDEF : 1;
 222       DynSym.Content.add(DynStr.Content.getOffset(Sym.Name), Sym.Size, Bind,
 223                          convertIFSSymbolTypeToELF(Sym.Type), 0, Shndx);
 224     }
 225     DynSym.Size = DynSym.Content.getSize();
 226
 227     // Poplulate dynamic table.
 228     size_t DynSymIndex = DynTab.Content.addAddr(DT_SYMTAB, 0);
 229     size_t DynStrIndex = DynTab.Content.addAddr(DT_STRTAB, 0);
 230     for (const std::string &Lib : Stub.NeededLibs)
 231       DynTab.Content.addValue(DT_NEEDED, DynStr.Content.getOffset(Lib));
 232     if (Stub.SoName)
 233       DynTab.Content.addValue(DT_SONAME,
 234                               DynStr.Content.getOffset(Stub.SoName.getValue()));
 235     DynTab.Size = DynTab.Content.getSize();
 236     // Calculate sections' addresses and offsets.
 237     uint64_t CurrentOffset = sizeof(Elf_Ehdr);
 238     for (OutputSection<ELFT> *Sec : Sections) {
 239       Sec->Offset = alignTo(CurrentOffset, Sec->Align);
 240       Sec->Addr = Sec->Offset;
 241       CurrentOffset = Sec->Offset + Sec->Size;
 242     }
 243     // Fill Addr back to dynamic table.
 244     DynTab.Content.modifyAddr(DynSymIndex, DynSym.Addr);
 245     DynTab.Content.modifyAddr(DynStrIndex, DynStr.Addr);
 246     // Write section headers of string tables.
 247     fillSymTabShdr(DynSym, SHT_DYNSYM);
 248     fillStrTabShdr(DynStr, SHF_ALLOC);
 249     fillDynTabShdr(DynTab);
 250     fillStrTabShdr(ShStrTab);
 251
 252     // Finish initializing the ELF header.
 253     initELFHeader<ELFT>(ElfHeader,
 254                         static_cast<uint16_t>(Stub.Target.Arch.getValue()));
 255     ElfHeader.e_shstrndx = ShStrTab.Index;
 256     ElfHeader.e_shnum = LastSection->Index + 1;
 257     ElfHeader.e_shoff =
 258         alignTo(LastSection->Offset + LastSection->Size, sizeof(Elf_Addr));
 259   }
 260
 261   size_t getSize() const {
 262     return ElfHeader.e_shoff + ElfHeader.e_shnum * sizeof(Elf_Shdr);
 263   }
 264
 265   void write(uint8_t *Data) const {
 266     write(Data, ElfHeader);
 267     DynSym.Content.write(Data + DynSym.Shdr.sh_offset);
 268     DynStr.Content.write(Data + DynStr.Shdr.sh_offset);
 269     DynTab.Content.write(Data + DynTab.Shdr.sh_offset);
 270     ShStrTab.Content.write(Data + ShStrTab.Shdr.sh_offset);
 271     writeShdr(Data, DynSym);
 272     writeShdr(Data, DynStr);
 273     writeShdr(Data, DynTab);
 274     writeShdr(Data, ShStrTab);
 275   }
 276
 277 private:
 278   Elf_Ehdr ElfHeader;
 279   ContentSection<ELFStringTableBuilder, ELFT> DynStr;
 280   ContentSection<ELFStringTableBuilder, ELFT> ShStrTab;
 281   ContentSection<ELFSymbolTableBuilder<ELFT>, ELFT> DynSym;
 282   ContentSection<ELFDynamicTableBuilder<ELFT>, ELFT> DynTab;
 283
 284   template <class T> static void write(uint8_t *Data, const T &Value) {
 285     *reinterpret_cast<T *>(Data) = Value;
 286   }
 287
 288   void fillStrTabShdr(ContentSection<ELFStringTableBuilder, ELFT> &StrTab,
 289                       uint32_t ShFlags = 0) const {
 290     StrTab.Shdr.sh_type = SHT_STRTAB;
 291     StrTab.Shdr.sh_flags = ShFlags;
 292     StrTab.Shdr.sh_addr = StrTab.Addr;
 293     StrTab.Shdr.sh_offset = StrTab.Offset;
 294     StrTab.Shdr.sh_info = 0;
 295     StrTab.Shdr.sh_size = StrTab.Size;
 296     StrTab.Shdr.sh_name = ShStrTab.Content.getOffset(StrTab.Name);
 297     StrTab.Shdr.sh_addralign = StrTab.Align;
 298     StrTab.Shdr.sh_entsize = 0;
 299     StrTab.Shdr.sh_link = 0;
 300   }
 301   void fillSymTabShdr(ContentSection<ELFSymbolTableBuilder<ELFT>, ELFT> &SymTab,
 302                       uint32_t ShType) const {
 303     SymTab.Shdr.sh_type = ShType;
 304     SymTab.Shdr.sh_flags = SHF_ALLOC;
 305     SymTab.Shdr.sh_addr = SymTab.Addr;
 306     SymTab.Shdr.sh_offset = SymTab.Offset;
 307     // Only non-local symbols are included in the tbe file, so .dynsym only
 308     // contains 1 local symbol (the undefined symbol at index 0). The sh_info
 309     // should always be 1.
 310     SymTab.Shdr.sh_info = 1;
 311     SymTab.Shdr.sh_size = SymTab.Size;
 312     SymTab.Shdr.sh_name = this->ShStrTab.Content.getOffset(SymTab.Name);
 313     SymTab.Shdr.sh_addralign = SymTab.Align;
 314     SymTab.Shdr.sh_entsize = sizeof(Elf_Sym);
 315     SymTab.Shdr.sh_link = this->DynStr.Index;
 316   }
 317   void fillDynTabShdr(
 318       ContentSection<ELFDynamicTableBuilder<ELFT>, ELFT> &DynTab) const {
 319     DynTab.Shdr.sh_type = SHT_DYNAMIC;
 320     DynTab.Shdr.sh_flags = SHF_ALLOC;
 321     DynTab.Shdr.sh_addr = DynTab.Addr;
 322     DynTab.Shdr.sh_offset = DynTab.Offset;
 323     DynTab.Shdr.sh_info = 0;
 324     DynTab.Shdr.sh_size = DynTab.Size;
 325     DynTab.Shdr.sh_name = this->ShStrTab.Content.getOffset(DynTab.Name);
 326     DynTab.Shdr.sh_addralign = DynTab.Align;
 327     DynTab.Shdr.sh_entsize = sizeof(Elf_Dyn);
 328     DynTab.Shdr.sh_link = this->DynStr.Index;
 329   }
 330   uint64_t shdrOffset(const OutputSection<ELFT> &Sec) const {
 331     return ElfHeader.e_shoff + Sec.Index * sizeof(Elf_Shdr);
 332   }
 333
 334   void writeShdr(uint8_t *Data, const OutputSection<ELFT> &Sec) const {
 335     write(Data + shdrOffset(Sec), Sec.Shdr);
 336   }
 337 };
 338 } // end anonymous namespace
 339
 340 /// This function behaves similarly to StringRef::substr(), but attempts to
 341 /// terminate the returned StringRef at the first null terminator. If no null
 342 /// terminator is found, an error is returned.
 343 ///
 344 /// @param Str Source string to create a substring from.
 345 /// @param Offset The start index of the desired substring.
 346 static Expected<StringRef> terminatedSubstr(StringRef Str, size_t Offset) {
 347   size_t StrEnd = Str.find('\0', Offset);
 348   if (StrEnd == StringLiteral::npos) {
 349     return createError(
 350         "String overran bounds of string table (no null terminator)");
 351   }
 352
 353   size_t StrLen = StrEnd - Offset;
 354   return Str.substr(Offset, StrLen);
 355 }
 356
 357 /// This function takes an error, and appends a string of text to the end of
 358 /// that error. Since "appending" to an Error isn't supported behavior of an
 359 /// Error, this function technically creates a new error with the combined
 360 /// message and consumes the old error.
 361 ///
 362 /// @param Err Source error.
 363 /// @param After Text to append at the end of Err's error message.
 364 Error appendToError(Error Err, StringRef After) {
 365   std::string Message;
 366   raw_string_ostream Stream(Message);
 367   Stream << Err;
 368   Stream << " " << After;
 369   consumeError(std::move(Err));
 370   return createError(Stream.str().c_str());
 371 }
 372
 373 /// This function populates a DynamicEntries struct using an ELFT::DynRange.
 374 /// After populating the struct, the members are validated with
 375 /// some basic sanity checks.
 376 ///
 377 /// @param Dyn Target DynamicEntries struct to populate.
 378 /// @param DynTable Source dynamic table.
 379 template <class ELFT>
 380 static Error populateDynamic(DynamicEntries &Dyn,
 381                              typename ELFT::DynRange DynTable) {
 382   if (DynTable.empty())
 383     return createError("No .dynamic section found");
 384
 385   // Search .dynamic for relevant entries.
 386   bool FoundDynStr = false;
 387   bool FoundDynStrSz = false;
 388   bool FoundDynSym = false;
 389   for (auto &Entry : DynTable) {
 390     switch (Entry.d_tag) {
 391     case DT_SONAME:
 392       Dyn.SONameOffset = Entry.d_un.d_val;
 393       break;
 394     case DT_STRTAB:
 395       Dyn.StrTabAddr = Entry.d_un.d_ptr;
 396       FoundDynStr = true;
 397       break;
 398     case DT_STRSZ:
 399       Dyn.StrSize = Entry.d_un.d_val;
 400       FoundDynStrSz = true;
 401       break;
 402     case DT_NEEDED:
 403       Dyn.NeededLibNames.push_back(Entry.d_un.d_val);
 404       break;
 405     case DT_SYMTAB:
 406       Dyn.DynSymAddr = Entry.d_un.d_ptr;
 407       FoundDynSym = true;
 408       break;
 409     case DT_HASH:
 410       Dyn.ElfHash = Entry.d_un.d_ptr;
 411       break;
 412     case DT_GNU_HASH:
 413       Dyn.GnuHash = Entry.d_un.d_ptr;
 414     }
 415   }
 416
 417   if (!FoundDynStr) {
 418     return createError(
 419         "Couldn't locate dynamic string table (no DT_STRTAB entry)");
 420   }
 421   if (!FoundDynStrSz) {
 422     return createError(
 423         "Couldn't determine dynamic string table size (no DT_STRSZ entry)");
 424   }
 425   if (!FoundDynSym) {
 426     return createError(
 427         "Couldn't locate dynamic symbol table (no DT_SYMTAB entry)");
 428   }
 429   if (Dyn.SONameOffset.hasValue() && *Dyn.SONameOffset >= Dyn.StrSize) {
 430     return createStringError(object_error::parse_failed,
 431                              "DT_SONAME string offset (0x%016" PRIx64
 432                              ") outside of dynamic string table",
 433                              *Dyn.SONameOffset);
 434   }
 435   for (uint64_t Offset : Dyn.NeededLibNames) {
 436     if (Offset >= Dyn.StrSize) {
 437       return createStringError(object_error::parse_failed,
 438                                "DT_NEEDED string offset (0x%016" PRIx64
 439                                ") outside of dynamic string table",
 440                                Offset);
 441     }
 442   }
 443
 444   return Error::success();
 445 }
 446
 447 /// This function creates an IFSSymbol and populates all members using
 448 /// information from a binary ELFT::Sym.
 449 ///
 450 /// @param SymName The desired name of the IFSSymbol.
 451 /// @param RawSym ELFT::Sym to extract symbol information from.
 452 template <class ELFT>
 453 static IFSSymbol createELFSym(StringRef SymName,
 454                               const typename ELFT::Sym &RawSym) {
 455   IFSSymbol TargetSym{std::string(SymName)};
 456   uint8_t Binding = RawSym.getBinding();
 457   if (Binding == STB_WEAK)
 458     TargetSym.Weak = true;
 459   else
 460     TargetSym.Weak = false;
 461
 462   TargetSym.Undefined = RawSym.isUndefined();
 463   TargetSym.Type = convertELFSymbolTypeToIFS(RawSym.st_info);
 464
 465   if (TargetSym.Type == IFSSymbolType::Func) {
 466     TargetSym.Size = 0;
 467   } else {
 468     TargetSym.Size = RawSym.st_size;
 469   }
 470   return TargetSym;
 471 }
 472
 473 /// This function populates an IFSStub with symbols using information read
 474 /// from an ELF binary.
 475 ///
 476 /// @param TargetStub IFSStub to add symbols to.
 477 /// @param DynSym Range of dynamic symbols to add to TargetStub.
 478 /// @param DynStr StringRef to the dynamic string table.
 479 template <class ELFT>
 480 static Error populateSymbols(IFSStub &TargetStub,
 481                              const typename ELFT::SymRange DynSym,
 482                              StringRef DynStr) {
 483   // Skips the first symbol since it's the NULL symbol.
 484   for (auto RawSym : DynSym.drop_front(1)) {
 485     // If a symbol does not have global or weak binding, ignore it.
 486     uint8_t Binding = RawSym.getBinding();
 487     if (!(Binding == STB_GLOBAL || Binding == STB_WEAK))
 488       continue;
 489     // If a symbol doesn't have default or protected visibility, ignore it.
 490     uint8_t Visibility = RawSym.getVisibility();
 491     if (!(Visibility == STV_DEFAULT || Visibility == STV_PROTECTED))
 492       continue;
 493     // Create an IFSSymbol and populate it with information from the symbol
 494     // table entry.
 495     Expected<StringRef> SymName = terminatedSubstr(DynStr, RawSym.st_name);
 496     if (!SymName)
 497       return SymName.takeError();
 498     IFSSymbol Sym = createELFSym<ELFT>(*SymName, RawSym);
 499     TargetStub.Symbols.push_back(std::move(Sym));
 500     // TODO: Populate symbol warning.
 501   }
 502   return Error::success();
 503 }
 504
 505 /// Returns a new IFSStub with all members populated from an ELFObjectFile.
 506 /// @param ElfObj Source ELFObjectFile.
 507 template <class ELFT>
 508 static Expected<std::unique_ptr<IFSStub>>
 509 buildStub(const ELFObjectFile<ELFT> &ElfObj) {
 510   using Elf_Dyn_Range = typename ELFT::DynRange;
 511   using Elf_Phdr_Range = typename ELFT::PhdrRange;
 512   using Elf_Sym_Range = typename ELFT::SymRange;
 513   using Elf_Sym = typename ELFT::Sym;
 514   std::unique_ptr<IFSStub> DestStub = std::make_unique<IFSStub>();
 515   const ELFFile<ELFT> &ElfFile = ElfObj.getELFFile();
 516   // Fetch .dynamic table.
 517   Expected<Elf_Dyn_Range> DynTable = ElfFile.dynamicEntries();
 518   if (!DynTable) {
 519     return DynTable.takeError();
 520   }
 521
 522   // Fetch program headers.
 523   Expected<Elf_Phdr_Range> PHdrs = ElfFile.program_headers();
 524   if (!PHdrs) {
 525     return PHdrs.takeError();
 526   }
 527
 528   // Collect relevant .dynamic entries.
 529   DynamicEntries DynEnt;
 530   if (Error Err = populateDynamic<ELFT>(DynEnt, *DynTable))
 531     return std::move(Err);
 532
 533   // Get pointer to in-memory location of .dynstr section.
 534   Expected<const uint8_t *> DynStrPtr = ElfFile.toMappedAddr(DynEnt.StrTabAddr);
 535   if (!DynStrPtr)
 536     return appendToError(DynStrPtr.takeError(),
 537                          "when locating .dynstr section contents");
 538
 539   StringRef DynStr(reinterpret_cast<const char *>(DynStrPtr.get()),
 540                    DynEnt.StrSize);
 541
 542   // Populate Arch from ELF header.
 543   DestStub->Target.Arch = static_cast<IFSArch>(ElfFile.getHeader().e_machine);
 544   DestStub->Target.BitWidth =
 545       convertELFBitWidthToIFS(ElfFile.getHeader().e_ident[EI_CLASS]);
 546   DestStub->Target.Endianness =
 547       convertELFEndiannessToIFS(ElfFile.getHeader().e_ident[EI_DATA]);
 548   DestStub->Target.ObjectFormat = "ELF";
 549
 550   // Populate SoName from .dynamic entries and dynamic string table.
 551   if (DynEnt.SONameOffset.hasValue()) {
 552     Expected<StringRef> NameOrErr =
 553         terminatedSubstr(DynStr, *DynEnt.SONameOffset);
 554     if (!NameOrErr) {
 555       return appendToError(NameOrErr.takeError(), "when reading DT_SONAME");
 556     }
 557     DestStub->SoName = std::string(*NameOrErr);
 558   }
 559
 560   // Populate NeededLibs from .dynamic entries and dynamic string table.
 561   for (uint64_t NeededStrOffset : DynEnt.NeededLibNames) {
 562     Expected<StringRef> LibNameOrErr =
 563         terminatedSubstr(DynStr, NeededStrOffset);
 564     if (!LibNameOrErr) {
 565       return appendToError(LibNameOrErr.takeError(), "when reading DT_NEEDED");
 566     }
 567     DestStub->NeededLibs.push_back(std::string(*LibNameOrErr));
 568   }
 569
 570   // Populate Symbols from .dynsym table and dynamic string table.
 571   Expected<uint64_t> SymCount = ElfFile.getDynSymtabSize();
 572   if (!SymCount)
 573     return SymCount.takeError();
 574   if (*SymCount > 0) {
 575     // Get pointer to in-memory location of .dynsym section.
 576     Expected<const uint8_t *> DynSymPtr =
 577         ElfFile.toMappedAddr(DynEnt.DynSymAddr);
 578     if (!DynSymPtr)
 579       return appendToError(DynSymPtr.takeError(),
 580                            "when locating .dynsym section contents");
 581     Elf_Sym_Range DynSyms = ArrayRef<Elf_Sym>(
 582         reinterpret_cast<const Elf_Sym *>(*DynSymPtr), *SymCount);
 583     Error SymReadError = populateSymbols<ELFT>(*DestStub, DynSyms, DynStr);
 584     if (SymReadError)
 585       return appendToError(std::move(SymReadError),
 586                            "when reading dynamic symbols");
 587   }
 588
 589   return std::move(DestStub);
 590 }
 591
 592 /// This function opens a file for writing and then writes a binary ELF stub to
 593 /// the file.
 594 ///
 595 /// @param FilePath File path for writing the ELF binary.
 596 /// @param Stub Source InterFace Stub to generate a binary ELF stub from.
 597 template <class ELFT>
 598 static Error writeELFBinaryToFile(StringRef FilePath, const IFSStub &Stub,
 599                                   bool WriteIfChanged) {
 600   ELFStubBuilder<ELFT> Builder{Stub};
 601   // Write Stub to memory first.
 602   std::vector<uint8_t> Buf(Builder.getSize());
 603   Builder.write(Buf.data());
 604
 605   if (WriteIfChanged) {
 606     if (ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrError =
 607             MemoryBuffer::getFile(FilePath)) {
 608       // Compare Stub output with existing Stub file.
 609       // If Stub file unchanged, abort updating.
 610       if ((*BufOrError)->getBufferSize() == Builder.getSize() &&
 611           !memcmp((*BufOrError)->getBufferStart(), Buf.data(),
 612                   Builder.getSize()))
 613         return Error::success();
 614     }
 615   }
 616
 617   Expected<std::unique_ptr<FileOutputBuffer>> BufOrError =
 618       FileOutputBuffer::create(FilePath, Builder.getSize());
 619   if (!BufOrError)
 620     return createStringError(errc::invalid_argument,
 621                              toString(BufOrError.takeError()) +
 622                                  " when trying to open `" + FilePath +
 623                                  "` for writing");
 624
 625   // Write binary to file.
 626   std::unique_ptr<FileOutputBuffer> FileBuf = std::move(*BufOrError);
 627   memcpy(FileBuf->getBufferStart(), Buf.data(), Buf.size());
 628
 629   return FileBuf->commit();
 630 }
 631
 632 Expected<std::unique_ptr<IFSStub>> readELFFile(MemoryBufferRef Buf) {
 633   Expected<std::unique_ptr<Binary>> BinOrErr = createBinary(Buf);
 634   if (!BinOrErr) {
 635     return BinOrErr.takeError();
 636   }
 637
 638   Binary *Bin = BinOrErr->get();
 639   if (auto Obj = dyn_cast<ELFObjectFile<ELF32LE>>(Bin)) {
 640     return buildStub(*Obj);
 641   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64LE>>(Bin)) {
 642     return buildStub(*Obj);
 643   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF32BE>>(Bin)) {
 644     return buildStub(*Obj);
 645   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64BE>>(Bin)) {
 646     return buildStub(*Obj);
 647   }
 648   return createStringError(errc::not_supported, "unsupported binary format");
 649 }
 650
 651 // This function wraps the ELFT writeELFBinaryToFile() so writeBinaryStub()
 652 // can be called without having to use ELFType templates directly.
 653 Error writeBinaryStub(StringRef FilePath, const IFSStub &Stub,
 654                       bool WriteIfChanged) {
 655   assert(Stub.Target.Arch);
 656   assert(Stub.Target.BitWidth);
 657   assert(Stub.Target.Endianness);
 658   if (Stub.Target.BitWidth == IFSBitWidthType::IFS32) {
 659     if (Stub.Target.Endianness == IFSEndiannessType::Little) {
 660       return writeELFBinaryToFile<ELF32LE>(FilePath, Stub, WriteIfChanged);
 661     } else {
 662       return writeELFBinaryToFile<ELF32BE>(FilePath, Stub, WriteIfChanged);
 663     }
 664   } else {
 665     if (Stub.Target.Endianness == IFSEndiannessType::Little) {
 666       return writeELFBinaryToFile<ELF64LE>(FilePath, Stub, WriteIfChanged);
 667     } else {
 668       return writeELFBinaryToFile<ELF64BE>(FilePath, Stub, WriteIfChanged);
 669     }
 670   }
 671   llvm_unreachable("invalid binary output target");
 672 }
 673
 674 } // end namespace ifs
 675 } // end namespace llvm