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/ELFStub.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 elfabi {
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 ELFStub &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 ELFSymbol &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 ELFSymbol &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                          (uint8_t)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, Stub.Arch);
254     ElfHeader.e_shstrndx = ShStrTab.Index;
255     ElfHeader.e_shnum = LastSection->Index + 1;
256     ElfHeader.e_shoff =
257         alignTo(LastSection->Offset + LastSection->Size, sizeof(Elf_Addr));
258   }
259 
260   size_t getSize() const {
261     return ElfHeader.e_shoff + ElfHeader.e_shnum * sizeof(Elf_Shdr);
262   }
263 
264   void write(uint8_t *Data) const {
265     write(Data, ElfHeader);
266     DynSym.Content.write(Data + DynSym.Shdr.sh_offset);
267     DynStr.Content.write(Data + DynStr.Shdr.sh_offset);
268     DynTab.Content.write(Data + DynTab.Shdr.sh_offset);
269     ShStrTab.Content.write(Data + ShStrTab.Shdr.sh_offset);
270     writeShdr(Data, DynSym);
271     writeShdr(Data, DynStr);
272     writeShdr(Data, DynTab);
273     writeShdr(Data, ShStrTab);
274   }
275 
276 private:
277   Elf_Ehdr ElfHeader;
278   ContentSection<ELFStringTableBuilder, ELFT> DynStr;
279   ContentSection<ELFStringTableBuilder, ELFT> ShStrTab;
280   ContentSection<ELFSymbolTableBuilder<ELFT>, ELFT> DynSym;
281   ContentSection<ELFDynamicTableBuilder<ELFT>, ELFT> DynTab;
282 
283   template <class T> static void write(uint8_t *Data, const T &Value) {
284     *reinterpret_cast<T *>(Data) = Value;
285   }
286 
287   void fillStrTabShdr(ContentSection<ELFStringTableBuilder, ELFT> &StrTab,
288                       uint32_t ShFlags = 0) const {
289     StrTab.Shdr.sh_type = SHT_STRTAB;
290     StrTab.Shdr.sh_flags = ShFlags;
291     StrTab.Shdr.sh_addr = StrTab.Addr;
292     StrTab.Shdr.sh_offset = StrTab.Offset;
293     StrTab.Shdr.sh_info = 0;
294     StrTab.Shdr.sh_size = StrTab.Size;
295     StrTab.Shdr.sh_name = ShStrTab.Content.getOffset(StrTab.Name);
296     StrTab.Shdr.sh_addralign = StrTab.Align;
297     StrTab.Shdr.sh_entsize = 0;
298     StrTab.Shdr.sh_link = 0;
299   }
300   void fillSymTabShdr(ContentSection<ELFSymbolTableBuilder<ELFT>, ELFT> &SymTab,
301                       uint32_t ShType) const {
302     SymTab.Shdr.sh_type = ShType;
303     SymTab.Shdr.sh_flags = SHF_ALLOC;
304     SymTab.Shdr.sh_addr = SymTab.Addr;
305     SymTab.Shdr.sh_offset = SymTab.Offset;
306     // Only non-local symbols are included in the tbe file, so .dynsym only
307     // contains 1 local symbol (the undefined symbol at index 0). The sh_info
308     // should always be 1.
309     SymTab.Shdr.sh_info = 1;
310     SymTab.Shdr.sh_size = SymTab.Size;
311     SymTab.Shdr.sh_name = this->ShStrTab.Content.getOffset(SymTab.Name);
312     SymTab.Shdr.sh_addralign = SymTab.Align;
313     SymTab.Shdr.sh_entsize = sizeof(Elf_Sym);
314     SymTab.Shdr.sh_link = this->DynStr.Index;
315   }
316   void fillDynTabShdr(
317       ContentSection<ELFDynamicTableBuilder<ELFT>, ELFT> &DynTab) const {
318     DynTab.Shdr.sh_type = SHT_DYNAMIC;
319     DynTab.Shdr.sh_flags = SHF_ALLOC;
320     DynTab.Shdr.sh_addr = DynTab.Addr;
321     DynTab.Shdr.sh_offset = DynTab.Offset;
322     DynTab.Shdr.sh_info = 0;
323     DynTab.Shdr.sh_size = DynTab.Size;
324     DynTab.Shdr.sh_name = this->ShStrTab.Content.getOffset(DynTab.Name);
325     DynTab.Shdr.sh_addralign = DynTab.Align;
326     DynTab.Shdr.sh_entsize = sizeof(Elf_Dyn);
327     DynTab.Shdr.sh_link = this->DynStr.Index;
328   }
329   uint64_t shdrOffset(const OutputSection<ELFT> &Sec) const {
330     return ElfHeader.e_shoff + Sec.Index * sizeof(Elf_Shdr);
331   }
332 
333   void writeShdr(uint8_t *Data, const OutputSection<ELFT> &Sec) const {
334     write(Data + shdrOffset(Sec), Sec.Shdr);
335   }
336 };
337 } // end anonymous namespace
338 
339 /// This function behaves similarly to StringRef::substr(), but attempts to
340 /// terminate the returned StringRef at the first null terminator. If no null
341 /// terminator is found, an error is returned.
342 ///
343 /// @param Str Source string to create a substring from.
344 /// @param Offset The start index of the desired substring.
345 static Expected<StringRef> terminatedSubstr(StringRef Str, size_t Offset) {
346   size_t StrEnd = Str.find('\0', Offset);
347   if (StrEnd == StringLiteral::npos) {
348     return createError(
349         "String overran bounds of string table (no null terminator)");
350   }
351 
352   size_t StrLen = StrEnd - Offset;
353   return Str.substr(Offset, StrLen);
354 }
355 
356 /// This function takes an error, and appends a string of text to the end of
357 /// that error. Since "appending" to an Error isn't supported behavior of an
358 /// Error, this function technically creates a new error with the combined
359 /// message and consumes the old error.
360 ///
361 /// @param Err Source error.
362 /// @param After Text to append at the end of Err's error message.
363 Error appendToError(Error Err, StringRef After) {
364   std::string Message;
365   raw_string_ostream Stream(Message);
366   Stream << Err;
367   Stream << " " << After;
368   consumeError(std::move(Err));
369   return createError(Stream.str().c_str());
370 }
371 
372 /// This function populates a DynamicEntries struct using an ELFT::DynRange.
373 /// After populating the struct, the members are validated with
374 /// some basic sanity checks.
375 ///
376 /// @param Dyn Target DynamicEntries struct to populate.
377 /// @param DynTable Source dynamic table.
378 template <class ELFT>
379 static Error populateDynamic(DynamicEntries &Dyn,
380                              typename ELFT::DynRange DynTable) {
381   if (DynTable.empty())
382     return createError("No .dynamic section found");
383 
384   // Search .dynamic for relevant entries.
385   bool FoundDynStr = false;
386   bool FoundDynStrSz = false;
387   bool FoundDynSym = false;
388   for (auto &Entry : DynTable) {
389     switch (Entry.d_tag) {
390     case DT_SONAME:
391       Dyn.SONameOffset = Entry.d_un.d_val;
392       break;
393     case DT_STRTAB:
394       Dyn.StrTabAddr = Entry.d_un.d_ptr;
395       FoundDynStr = true;
396       break;
397     case DT_STRSZ:
398       Dyn.StrSize = Entry.d_un.d_val;
399       FoundDynStrSz = true;
400       break;
401     case DT_NEEDED:
402       Dyn.NeededLibNames.push_back(Entry.d_un.d_val);
403       break;
404     case DT_SYMTAB:
405       Dyn.DynSymAddr = Entry.d_un.d_ptr;
406       FoundDynSym = true;
407       break;
408     case DT_HASH:
409       Dyn.ElfHash = Entry.d_un.d_ptr;
410       break;
411     case DT_GNU_HASH:
412       Dyn.GnuHash = Entry.d_un.d_ptr;
413     }
414   }
415 
416   if (!FoundDynStr) {
417     return createError(
418         "Couldn't locate dynamic string table (no DT_STRTAB entry)");
419   }
420   if (!FoundDynStrSz) {
421     return createError(
422         "Couldn't determine dynamic string table size (no DT_STRSZ entry)");
423   }
424   if (!FoundDynSym) {
425     return createError(
426         "Couldn't locate dynamic symbol table (no DT_SYMTAB entry)");
427   }
428   if (Dyn.SONameOffset.hasValue() && *Dyn.SONameOffset >= Dyn.StrSize) {
429     return createStringError(object_error::parse_failed,
430                              "DT_SONAME string offset (0x%016" PRIx64
431                              ") outside of dynamic string table",
432                              *Dyn.SONameOffset);
433   }
434   for (uint64_t Offset : Dyn.NeededLibNames) {
435     if (Offset >= Dyn.StrSize) {
436       return createStringError(object_error::parse_failed,
437                                "DT_NEEDED string offset (0x%016" PRIx64
438                                ") outside of dynamic string table",
439                                Offset);
440     }
441   }
442 
443   return Error::success();
444 }
445 
446 /// This function extracts symbol type from a symbol's st_info member and
447 /// maps it to an ELFSymbolType enum.
448 /// Currently, STT_NOTYPE, STT_OBJECT, STT_FUNC, and STT_TLS are supported.
449 /// Other symbol types are mapped to ELFSymbolType::Unknown.
450 ///
451 /// @param Info Binary symbol st_info to extract symbol type from.
452 static ELFSymbolType convertInfoToType(uint8_t Info) {
453   Info = Info & 0xf;
454   switch (Info) {
455   case ELF::STT_NOTYPE:
456     return ELFSymbolType::NoType;
457   case ELF::STT_OBJECT:
458     return ELFSymbolType::Object;
459   case ELF::STT_FUNC:
460     return ELFSymbolType::Func;
461   case ELF::STT_TLS:
462     return ELFSymbolType::TLS;
463   default:
464     return ELFSymbolType::Unknown;
465   }
466 }
467 
468 /// This function creates an ELFSymbol and populates all members using
469 /// information from a binary ELFT::Sym.
470 ///
471 /// @param SymName The desired name of the ELFSymbol.
472 /// @param RawSym ELFT::Sym to extract symbol information from.
473 template <class ELFT>
474 static ELFSymbol createELFSym(StringRef SymName,
475                               const typename ELFT::Sym &RawSym) {
476   ELFSymbol TargetSym{std::string(SymName)};
477   uint8_t Binding = RawSym.getBinding();
478   if (Binding == STB_WEAK)
479     TargetSym.Weak = true;
480   else
481     TargetSym.Weak = false;
482 
483   TargetSym.Undefined = RawSym.isUndefined();
484   TargetSym.Type = convertInfoToType(RawSym.st_info);
485 
486   if (TargetSym.Type == ELFSymbolType::Func) {
487     TargetSym.Size = 0;
488   } else {
489     TargetSym.Size = RawSym.st_size;
490   }
491   return TargetSym;
492 }
493 
494 /// This function populates an ELFStub with symbols using information read
495 /// from an ELF binary.
496 ///
497 /// @param TargetStub ELFStub to add symbols to.
498 /// @param DynSym Range of dynamic symbols to add to TargetStub.
499 /// @param DynStr StringRef to the dynamic string table.
500 template <class ELFT>
501 static Error populateSymbols(ELFStub &TargetStub,
502                              const typename ELFT::SymRange DynSym,
503                              StringRef DynStr) {
504   // Skips the first symbol since it's the NULL symbol.
505   for (auto RawSym : DynSym.drop_front(1)) {
506     // If a symbol does not have global or weak binding, ignore it.
507     uint8_t Binding = RawSym.getBinding();
508     if (!(Binding == STB_GLOBAL || Binding == STB_WEAK))
509       continue;
510     // If a symbol doesn't have default or protected visibility, ignore it.
511     uint8_t Visibility = RawSym.getVisibility();
512     if (!(Visibility == STV_DEFAULT || Visibility == STV_PROTECTED))
513       continue;
514     // Create an ELFSymbol and populate it with information from the symbol
515     // table entry.
516     Expected<StringRef> SymName = terminatedSubstr(DynStr, RawSym.st_name);
517     if (!SymName)
518       return SymName.takeError();
519     ELFSymbol Sym = createELFSym<ELFT>(*SymName, RawSym);
520     TargetStub.Symbols.insert(std::move(Sym));
521     // TODO: Populate symbol warning.
522   }
523   return Error::success();
524 }
525 
526 /// Returns a new ELFStub with all members populated from an ELFObjectFile.
527 /// @param ElfObj Source ELFObjectFile.
528 template <class ELFT>
529 static Expected<std::unique_ptr<ELFStub>>
530 buildStub(const ELFObjectFile<ELFT> &ElfObj) {
531   using Elf_Dyn_Range = typename ELFT::DynRange;
532   using Elf_Phdr_Range = typename ELFT::PhdrRange;
533   using Elf_Sym_Range = typename ELFT::SymRange;
534   using Elf_Sym = typename ELFT::Sym;
535   std::unique_ptr<ELFStub> DestStub = std::make_unique<ELFStub>();
536   const ELFFile<ELFT> &ElfFile = ElfObj.getELFFile();
537   // Fetch .dynamic table.
538   Expected<Elf_Dyn_Range> DynTable = ElfFile.dynamicEntries();
539   if (!DynTable) {
540     return DynTable.takeError();
541   }
542 
543   // Fetch program headers.
544   Expected<Elf_Phdr_Range> PHdrs = ElfFile.program_headers();
545   if (!PHdrs) {
546     return PHdrs.takeError();
547   }
548 
549   // Collect relevant .dynamic entries.
550   DynamicEntries DynEnt;
551   if (Error Err = populateDynamic<ELFT>(DynEnt, *DynTable))
552     return std::move(Err);
553 
554   // Get pointer to in-memory location of .dynstr section.
555   Expected<const uint8_t *> DynStrPtr = ElfFile.toMappedAddr(DynEnt.StrTabAddr);
556   if (!DynStrPtr)
557     return appendToError(DynStrPtr.takeError(),
558                          "when locating .dynstr section contents");
559 
560   StringRef DynStr(reinterpret_cast<const char *>(DynStrPtr.get()),
561                    DynEnt.StrSize);
562 
563   // Populate Arch from ELF header.
564   DestStub->Arch = ElfFile.getHeader().e_machine;
565 
566   // Populate SoName from .dynamic entries and dynamic string table.
567   if (DynEnt.SONameOffset.hasValue()) {
568     Expected<StringRef> NameOrErr =
569         terminatedSubstr(DynStr, *DynEnt.SONameOffset);
570     if (!NameOrErr) {
571       return appendToError(NameOrErr.takeError(), "when reading DT_SONAME");
572     }
573     DestStub->SoName = std::string(*NameOrErr);
574   }
575 
576   // Populate NeededLibs from .dynamic entries and dynamic string table.
577   for (uint64_t NeededStrOffset : DynEnt.NeededLibNames) {
578     Expected<StringRef> LibNameOrErr =
579         terminatedSubstr(DynStr, NeededStrOffset);
580     if (!LibNameOrErr) {
581       return appendToError(LibNameOrErr.takeError(), "when reading DT_NEEDED");
582     }
583     DestStub->NeededLibs.push_back(std::string(*LibNameOrErr));
584   }
585 
586   // Populate Symbols from .dynsym table and dynamic string table.
587   Expected<uint64_t> SymCount = ElfFile.getDynSymtabSize();
588   if (!SymCount)
589     return SymCount.takeError();
590   if (*SymCount > 0) {
591     // Get pointer to in-memory location of .dynsym section.
592     Expected<const uint8_t *> DynSymPtr =
593         ElfFile.toMappedAddr(DynEnt.DynSymAddr);
594     if (!DynSymPtr)
595       return appendToError(DynSymPtr.takeError(),
596                            "when locating .dynsym section contents");
597     Elf_Sym_Range DynSyms = ArrayRef<Elf_Sym>(
598         reinterpret_cast<const Elf_Sym *>(*DynSymPtr), *SymCount);
599     Error SymReadError = populateSymbols<ELFT>(*DestStub, DynSyms, DynStr);
600     if (SymReadError)
601       return appendToError(std::move(SymReadError),
602                            "when reading dynamic symbols");
603   }
604 
605   return std::move(DestStub);
606 }
607 
608 /// This function opens a file for writing and then writes a binary ELF stub to
609 /// the file.
610 ///
611 /// @param FilePath File path for writing the ELF binary.
612 /// @param Stub Source ELFStub to generate a binary ELF stub from.
613 template <class ELFT>
614 static Error writeELFBinaryToFile(StringRef FilePath, const ELFStub &Stub,
615                                   bool WriteIfChanged) {
616   ELFStubBuilder<ELFT> Builder{Stub};
617   // Write Stub to memory first.
618   std::vector<uint8_t> Buf(Builder.getSize());
619   Builder.write(Buf.data());
620 
621   if (WriteIfChanged) {
622     if (ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrError =
623             MemoryBuffer::getFile(FilePath)) {
624       // Compare Stub output with existing Stub file.
625       // If Stub file unchanged, abort updating.
626       if ((*BufOrError)->getBufferSize() == Builder.getSize() &&
627           !memcmp((*BufOrError)->getBufferStart(), Buf.data(),
628                   Builder.getSize()))
629         return Error::success();
630     }
631   }
632 
633   Expected<std::unique_ptr<FileOutputBuffer>> BufOrError =
634       FileOutputBuffer::create(FilePath, Builder.getSize());
635   if (!BufOrError)
636     return createStringError(errc::invalid_argument,
637                              toString(BufOrError.takeError()) +
638                                  " when trying to open `" + FilePath +
639                                  "` for writing");
640 
641   // Write binary to file.
642   std::unique_ptr<FileOutputBuffer> FileBuf = std::move(*BufOrError);
643   memcpy(FileBuf->getBufferStart(), Buf.data(), Buf.size());
644 
645   return FileBuf->commit();
646 }
647 
648 Expected<std::unique_ptr<ELFStub>> readELFFile(MemoryBufferRef Buf) {
649   Expected<std::unique_ptr<Binary>> BinOrErr = createBinary(Buf);
650   if (!BinOrErr) {
651     return BinOrErr.takeError();
652   }
653 
654   Binary *Bin = BinOrErr->get();
655   if (auto Obj = dyn_cast<ELFObjectFile<ELF32LE>>(Bin)) {
656     return buildStub(*Obj);
657   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64LE>>(Bin)) {
658     return buildStub(*Obj);
659   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF32BE>>(Bin)) {
660     return buildStub(*Obj);
661   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64BE>>(Bin)) {
662     return buildStub(*Obj);
663   }
664   return createStringError(errc::not_supported, "unsupported binary format");
665 }
666 
667 // This function wraps the ELFT writeELFBinaryToFile() so writeBinaryStub()
668 // can be called without having to use ELFType templates directly.
669 Error writeBinaryStub(StringRef FilePath, const ELFStub &Stub,
670                       ELFTarget OutputFormat, bool WriteIfChanged) {
671   if (OutputFormat == ELFTarget::ELF32LE)
672     return writeELFBinaryToFile<ELF32LE>(FilePath, Stub, WriteIfChanged);
673   if (OutputFormat == ELFTarget::ELF32BE)
674     return writeELFBinaryToFile<ELF32BE>(FilePath, Stub, WriteIfChanged);
675   if (OutputFormat == ELFTarget::ELF64LE)
676     return writeELFBinaryToFile<ELF64LE>(FilePath, Stub, WriteIfChanged);
677   if (OutputFormat == ELFTarget::ELF64BE)
678     return writeELFBinaryToFile<ELF64BE>(FilePath, Stub, WriteIfChanged);
679   llvm_unreachable("invalid binary output target");
680 }
681 
682 } // end namespace elfabi
683 } // end namespace llvm
684