xref: /freebsd/contrib/llvm-project/llvm/lib/InterfaceStub/ELFObjHandler.cpp (revision 5e801ac66d24704442eba426ed13c3effb8a34e7) 
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());
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
676