xref: /llvm-project/llvm/lib/InterfaceStub/ELFObjHandler.cpp (revision b6b0fd6a940b7006ced344736decccffaa583b8a) 
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 
21 using llvm::object::ELFObjectFile;
22 
23 using namespace llvm;
24 using namespace llvm::object;
25 using namespace llvm::ELF;
26 
27 namespace llvm {
28 namespace ifs {
29 
30 // Simple struct to hold relevant .dynamic entries.
31 struct DynamicEntries {
32   uint64_t StrTabAddr = 0;
33   uint64_t StrSize = 0;
34   Optional<uint64_t> SONameOffset;
35   std::vector<uint64_t> NeededLibNames;
36   // Symbol table:
37   uint64_t DynSymAddr = 0;
38   // Hash tables:
39   Optional<uint64_t> ElfHash;
40   Optional<uint64_t> GnuHash;
41 };
42 
43 /// This initializes an ELF file header with information specific to a binary
44 /// dynamic shared object.
45 /// Offsets, indexes, links, etc. for section and program headers are just
46 /// zero-initialized as they will be updated elsewhere.
47 ///
48 /// @param ElfHeader Target ELFT::Ehdr to populate.
49 /// @param Machine Target architecture (e_machine from ELF specifications).
50 template <class ELFT>
51 static void initELFHeader(typename ELFT::Ehdr &ElfHeader, uint16_t Machine) {
52   memset(&ElfHeader, 0, sizeof(ElfHeader));
53   // ELF identification.
54   ElfHeader.e_ident[EI_MAG0] = ElfMagic[EI_MAG0];
55   ElfHeader.e_ident[EI_MAG1] = ElfMagic[EI_MAG1];
56   ElfHeader.e_ident[EI_MAG2] = ElfMagic[EI_MAG2];
57   ElfHeader.e_ident[EI_MAG3] = ElfMagic[EI_MAG3];
58   ElfHeader.e_ident[EI_CLASS] = ELFT::Is64Bits ? ELFCLASS64 : ELFCLASS32;
59   bool IsLittleEndian = ELFT::TargetEndianness == support::little;
60   ElfHeader.e_ident[EI_DATA] = IsLittleEndian ? ELFDATA2LSB : ELFDATA2MSB;
61   ElfHeader.e_ident[EI_VERSION] = EV_CURRENT;
62   ElfHeader.e_ident[EI_OSABI] = ELFOSABI_NONE;
63 
64   // Remainder of ELF header.
65   ElfHeader.e_type = ET_DYN;
66   ElfHeader.e_machine = Machine;
67   ElfHeader.e_version = EV_CURRENT;
68   ElfHeader.e_ehsize = sizeof(typename ELFT::Ehdr);
69   ElfHeader.e_phentsize = sizeof(typename ELFT::Phdr);
70   ElfHeader.e_shentsize = sizeof(typename ELFT::Shdr);
71 }
72 
73 namespace {
74 template <class ELFT> struct OutputSection {
75   using Elf_Shdr = typename ELFT::Shdr;
76   std::string Name;
77   Elf_Shdr Shdr;
78   uint64_t Addr;
79   uint64_t Offset;
80   uint64_t Size;
81   uint64_t Align;
82   uint32_t Index;
83   bool NoBits = true;
84 };
85 
86 template <class T, class ELFT>
87 struct ContentSection : public OutputSection<ELFT> {
88   T Content;
89   ContentSection() { this->NoBits = false; }
90 };
91 
92 // This class just wraps StringTableBuilder for the purpose of adding a
93 // default constructor.
94 class ELFStringTableBuilder : public StringTableBuilder {
95 public:
96   ELFStringTableBuilder() : StringTableBuilder(StringTableBuilder::ELF) {}
97 };
98 
99 template <class ELFT> class ELFSymbolTableBuilder {
100 public:
101   using Elf_Sym = typename ELFT::Sym;
102 
103   ELFSymbolTableBuilder() { Symbols.push_back({}); }
104 
105   void add(size_t StNameOffset, uint64_t StSize, uint8_t StBind, uint8_t StType,
106            uint8_t StOther, uint16_t StShndx) {
107     Elf_Sym S{};
108     S.st_name = StNameOffset;
109     S.st_size = StSize;
110     S.st_info = (StBind << 4) | (StType & 0xf);
111     S.st_other = StOther;
112     S.st_shndx = StShndx;
113     Symbols.push_back(S);
114   }
115 
116   size_t getSize() const { return Symbols.size() * sizeof(Elf_Sym); }
117 
118   void write(uint8_t *Buf) const {
119     memcpy(Buf, Symbols.data(), sizeof(Elf_Sym) * Symbols.size());
120   }
121 
122 private:
123   llvm::SmallVector<Elf_Sym, 8> Symbols;
124 };
125 
126 template <class ELFT> class ELFDynamicTableBuilder {
127 public:
128   using Elf_Dyn = typename ELFT::Dyn;
129 
130   size_t addAddr(uint64_t Tag, uint64_t Addr) {
131     Elf_Dyn Entry;
132     Entry.d_tag = Tag;
133     Entry.d_un.d_ptr = Addr;
134     Entries.push_back(Entry);
135     return Entries.size() - 1;
136   }
137 
138   void modifyAddr(size_t Index, uint64_t Addr) {
139     Entries[Index].d_un.d_ptr = Addr;
140   }
141 
142   size_t addValue(uint64_t Tag, uint64_t Value) {
143     Elf_Dyn Entry;
144     Entry.d_tag = Tag;
145     Entry.d_un.d_val = Value;
146     Entries.push_back(Entry);
147     return Entries.size() - 1;
148   }
149 
150   void modifyValue(size_t Index, uint64_t Value) {
151     Entries[Index].d_un.d_val = Value;
152   }
153 
154   size_t getSize() const {
155     // Add DT_NULL entry at the end.
156     return (Entries.size() + 1) * sizeof(Elf_Dyn);
157   }
158 
159   void write(uint8_t *Buf) const {
160     memcpy(Buf, Entries.data(), sizeof(Elf_Dyn) * Entries.size());
161     // Add DT_NULL entry at the end.
162     memset(Buf + sizeof(Elf_Dyn) * Entries.size(), 0, sizeof(Elf_Dyn));
163   }
164 
165 private:
166   llvm::SmallVector<Elf_Dyn, 8> Entries;
167 };
168 
169 template <class ELFT> class ELFStubBuilder {
170 public:
171   using Elf_Ehdr = typename ELFT::Ehdr;
172   using Elf_Shdr = typename ELFT::Shdr;
173   using Elf_Phdr = typename ELFT::Phdr;
174   using Elf_Sym = typename ELFT::Sym;
175   using Elf_Addr = typename ELFT::Addr;
176   using Elf_Dyn = typename ELFT::Dyn;
177 
178   ELFStubBuilder(const ELFStubBuilder &) = delete;
179   ELFStubBuilder(ELFStubBuilder &&) = default;
180 
181   explicit ELFStubBuilder(const IFSStub &Stub) {
182     DynSym.Name = ".dynsym";
183     DynSym.Align = sizeof(Elf_Addr);
184     DynStr.Name = ".dynstr";
185     DynStr.Align = 1;
186     DynTab.Name = ".dynamic";
187     DynTab.Align = sizeof(Elf_Addr);
188     ShStrTab.Name = ".shstrtab";
189     ShStrTab.Align = 1;
190 
191     // Populate string tables.
192     for (const IFSSymbol &Sym : Stub.Symbols)
193       DynStr.Content.add(Sym.Name);
194     for (const std::string &Lib : Stub.NeededLibs)
195       DynStr.Content.add(Lib);
196     if (Stub.SoName)
197       DynStr.Content.add(Stub.SoName.getValue());
198 
199     std::vector<OutputSection<ELFT> *> Sections = {&DynSym, &DynStr, &DynTab,
200                                                    &ShStrTab};
201     const OutputSection<ELFT> *LastSection = Sections.back();
202     // Now set the Index and put sections names into ".shstrtab".
203     uint64_t Index = 1;
204     for (OutputSection<ELFT> *Sec : Sections) {
205       Sec->Index = Index++;
206       ShStrTab.Content.add(Sec->Name);
207     }
208     ShStrTab.Content.finalize();
209     ShStrTab.Size = ShStrTab.Content.getSize();
210     DynStr.Content.finalize();
211     DynStr.Size = DynStr.Content.getSize();
212 
213     // Populate dynamic symbol table.
214     for (const IFSSymbol &Sym : Stub.Symbols) {
215       uint8_t Bind = Sym.Weak ? STB_WEAK : STB_GLOBAL;
216       // For non-undefined symbols, value of the shndx is not relevant at link
217       // time as long as it is not SHN_UNDEF. Set shndx to 1, which
218       // points to ".dynsym".
219       uint16_t Shndx = Sym.Undefined ? SHN_UNDEF : 1;
220       uint64_t Size = Sym.Size ? *Sym.Size : 0;
221       DynSym.Content.add(DynStr.Content.getOffset(Sym.Name), Size, Bind,
222                          convertIFSSymbolTypeToELF(Sym.Type), 0, Shndx);
223     }
224     DynSym.Size = DynSym.Content.getSize();
225 
226     // Poplulate dynamic table.
227     size_t DynSymIndex = DynTab.Content.addAddr(DT_SYMTAB, 0);
228     size_t DynStrIndex = DynTab.Content.addAddr(DT_STRTAB, 0);
229     DynTab.Content.addValue(DT_STRSZ, DynSym.Size);
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 
339 /// This function takes an error, and appends a string of text to the end of
340 /// that error. Since "appending" to an Error isn't supported behavior of an
341 /// Error, this function technically creates a new error with the combined
342 /// message and consumes the old error.
343 ///
344 /// @param Err Source error.
345 /// @param After Text to append at the end of Err's error message.
346 Error appendToError(Error Err, StringRef After) {
347   std::string Message;
348   raw_string_ostream Stream(Message);
349   Stream << Err;
350   Stream << " " << After;
351   consumeError(std::move(Err));
352   return createError(Stream.str());
353 }
354 
355 template <class ELFT> class DynSym {
356   using Elf_Shdr_Range = typename ELFT::ShdrRange;
357   using Elf_Shdr = typename ELFT::Shdr;
358 
359 public:
360   static Expected<DynSym> create(const ELFFile<ELFT> &ElfFile,
361                                  const DynamicEntries &DynEnt) {
362     Expected<Elf_Shdr_Range> Shdrs = ElfFile.sections();
363     if (!Shdrs)
364       return Shdrs.takeError();
365     return DynSym(ElfFile, DynEnt, *Shdrs);
366   }
367 
368   Expected<const uint8_t *> getDynSym() {
369     if (DynSymHdr)
370       return ElfFile.base() + DynSymHdr->sh_offset;
371     return getDynamicData(DynEnt.DynSymAddr, "dynamic symbol table");
372   }
373 
374   Expected<StringRef> getDynStr() {
375     if (DynSymHdr)
376       return ElfFile.getStringTableForSymtab(*DynSymHdr, Shdrs);
377     Expected<const uint8_t *> DataOrErr = getDynamicData(
378         DynEnt.StrTabAddr, "dynamic string table", DynEnt.StrSize);
379     if (!DataOrErr)
380       return DataOrErr.takeError();
381     return StringRef(reinterpret_cast<const char *>(*DataOrErr),
382                      DynEnt.StrSize);
383   }
384 
385 private:
386   DynSym(const ELFFile<ELFT> &ElfFile, const DynamicEntries &DynEnt,
387          Elf_Shdr_Range Shdrs)
388       : ElfFile(ElfFile), DynEnt(DynEnt), Shdrs(Shdrs),
389         DynSymHdr(findDynSymHdr()) {}
390 
391   const Elf_Shdr *findDynSymHdr() {
392     for (const Elf_Shdr &Sec : Shdrs)
393       if (Sec.sh_type == SHT_DYNSYM) {
394         // If multiple .dynsym are present, use the first one.
395         // This behavior aligns with llvm::object::ELFFile::getDynSymtabSize()
396         return &Sec;
397       }
398     return nullptr;
399   }
400 
401   Expected<const uint8_t *> getDynamicData(uint64_t EntAddr, StringRef Name,
402                                            uint64_t Size = 0) {
403     Expected<const uint8_t *> SecPtr = ElfFile.toMappedAddr(EntAddr);
404     if (!SecPtr)
405       return appendToError(
406           SecPtr.takeError(),
407           ("when locating " + Name + " section contents").str());
408     Expected<const uint8_t *> SecEndPtr = ElfFile.toMappedAddr(EntAddr + Size);
409     if (!SecEndPtr)
410       return appendToError(
411           SecEndPtr.takeError(),
412           ("when locating " + Name + " section contents").str());
413     return *SecPtr;
414   }
415 
416   const ELFFile<ELFT> &ElfFile;
417   const DynamicEntries &DynEnt;
418   Elf_Shdr_Range Shdrs;
419   const Elf_Shdr *DynSymHdr;
420 };
421 } // end anonymous namespace
422 
423 /// This function behaves similarly to StringRef::substr(), but attempts to
424 /// terminate the returned StringRef at the first null terminator. If no null
425 /// terminator is found, an error is returned.
426 ///
427 /// @param Str Source string to create a substring from.
428 /// @param Offset The start index of the desired substring.
429 static Expected<StringRef> terminatedSubstr(StringRef Str, size_t Offset) {
430   size_t StrEnd = Str.find('\0', Offset);
431   if (StrEnd == StringLiteral::npos) {
432     return createError(
433         "String overran bounds of string table (no null terminator)");
434   }
435 
436   size_t StrLen = StrEnd - Offset;
437   return Str.substr(Offset, StrLen);
438 }
439 
440 /// This function populates a DynamicEntries struct using an ELFT::DynRange.
441 /// After populating the struct, the members are validated with
442 /// some basic correctness checks.
443 ///
444 /// @param Dyn Target DynamicEntries struct to populate.
445 /// @param DynTable Source dynamic table.
446 template <class ELFT>
447 static Error populateDynamic(DynamicEntries &Dyn,
448                              typename ELFT::DynRange DynTable) {
449   if (DynTable.empty())
450     return createError("No .dynamic section found");
451 
452   // Search .dynamic for relevant entries.
453   bool FoundDynStr = false;
454   bool FoundDynStrSz = false;
455   bool FoundDynSym = false;
456   for (auto &Entry : DynTable) {
457     switch (Entry.d_tag) {
458     case DT_SONAME:
459       Dyn.SONameOffset = Entry.d_un.d_val;
460       break;
461     case DT_STRTAB:
462       Dyn.StrTabAddr = Entry.d_un.d_ptr;
463       FoundDynStr = true;
464       break;
465     case DT_STRSZ:
466       Dyn.StrSize = Entry.d_un.d_val;
467       FoundDynStrSz = true;
468       break;
469     case DT_NEEDED:
470       Dyn.NeededLibNames.push_back(Entry.d_un.d_val);
471       break;
472     case DT_SYMTAB:
473       Dyn.DynSymAddr = Entry.d_un.d_ptr;
474       FoundDynSym = true;
475       break;
476     case DT_HASH:
477       Dyn.ElfHash = Entry.d_un.d_ptr;
478       break;
479     case DT_GNU_HASH:
480       Dyn.GnuHash = Entry.d_un.d_ptr;
481     }
482   }
483 
484   if (!FoundDynStr) {
485     return createError(
486         "Couldn't locate dynamic string table (no DT_STRTAB entry)");
487   }
488   if (!FoundDynStrSz) {
489     return createError(
490         "Couldn't determine dynamic string table size (no DT_STRSZ entry)");
491   }
492   if (!FoundDynSym) {
493     return createError(
494         "Couldn't locate dynamic symbol table (no DT_SYMTAB entry)");
495   }
496   if (Dyn.SONameOffset.hasValue() && *Dyn.SONameOffset >= Dyn.StrSize) {
497     return createStringError(object_error::parse_failed,
498                              "DT_SONAME string offset (0x%016" PRIx64
499                              ") outside of dynamic string table",
500                              *Dyn.SONameOffset);
501   }
502   for (uint64_t Offset : Dyn.NeededLibNames) {
503     if (Offset >= Dyn.StrSize) {
504       return createStringError(object_error::parse_failed,
505                                "DT_NEEDED string offset (0x%016" PRIx64
506                                ") outside of dynamic string table",
507                                Offset);
508     }
509   }
510 
511   return Error::success();
512 }
513 
514 /// This function creates an IFSSymbol and populates all members using
515 /// information from a binary ELFT::Sym.
516 ///
517 /// @param SymName The desired name of the IFSSymbol.
518 /// @param RawSym ELFT::Sym to extract symbol information from.
519 template <class ELFT>
520 static IFSSymbol createELFSym(StringRef SymName,
521                               const typename ELFT::Sym &RawSym) {
522   IFSSymbol TargetSym{std::string(SymName)};
523   uint8_t Binding = RawSym.getBinding();
524   if (Binding == STB_WEAK)
525     TargetSym.Weak = true;
526   else
527     TargetSym.Weak = false;
528 
529   TargetSym.Undefined = RawSym.isUndefined();
530   TargetSym.Type = convertELFSymbolTypeToIFS(RawSym.st_info);
531 
532   if (TargetSym.Type == IFSSymbolType::Func) {
533     TargetSym.Size = 0;
534   } else {
535     TargetSym.Size = RawSym.st_size;
536   }
537   return TargetSym;
538 }
539 
540 /// This function populates an IFSStub with symbols using information read
541 /// from an ELF binary.
542 ///
543 /// @param TargetStub IFSStub to add symbols to.
544 /// @param DynSym Range of dynamic symbols to add to TargetStub.
545 /// @param DynStr StringRef to the dynamic string table.
546 template <class ELFT>
547 static Error populateSymbols(IFSStub &TargetStub,
548                              const typename ELFT::SymRange DynSym,
549                              StringRef DynStr) {
550   // Skips the first symbol since it's the NULL symbol.
551   for (auto RawSym : DynSym.drop_front(1)) {
552     // If a symbol does not have global or weak binding, ignore it.
553     uint8_t Binding = RawSym.getBinding();
554     if (!(Binding == STB_GLOBAL || Binding == STB_WEAK))
555       continue;
556     // If a symbol doesn't have default or protected visibility, ignore it.
557     uint8_t Visibility = RawSym.getVisibility();
558     if (!(Visibility == STV_DEFAULT || Visibility == STV_PROTECTED))
559       continue;
560     // Create an IFSSymbol and populate it with information from the symbol
561     // table entry.
562     Expected<StringRef> SymName = terminatedSubstr(DynStr, RawSym.st_name);
563     if (!SymName)
564       return SymName.takeError();
565     IFSSymbol Sym = createELFSym<ELFT>(*SymName, RawSym);
566     TargetStub.Symbols.push_back(std::move(Sym));
567     // TODO: Populate symbol warning.
568   }
569   return Error::success();
570 }
571 
572 /// Returns a new IFSStub with all members populated from an ELFObjectFile.
573 /// @param ElfObj Source ELFObjectFile.
574 template <class ELFT>
575 static Expected<std::unique_ptr<IFSStub>>
576 buildStub(const ELFObjectFile<ELFT> &ElfObj) {
577   using Elf_Dyn_Range = typename ELFT::DynRange;
578   using Elf_Sym_Range = typename ELFT::SymRange;
579   using Elf_Sym = typename ELFT::Sym;
580   std::unique_ptr<IFSStub> DestStub = std::make_unique<IFSStub>();
581   const ELFFile<ELFT> &ElfFile = ElfObj.getELFFile();
582   // Fetch .dynamic table.
583   Expected<Elf_Dyn_Range> DynTable = ElfFile.dynamicEntries();
584   if (!DynTable) {
585     return DynTable.takeError();
586   }
587 
588   // Collect relevant .dynamic entries.
589   DynamicEntries DynEnt;
590   if (Error Err = populateDynamic<ELFT>(DynEnt, *DynTable))
591     return std::move(Err);
592   Expected<DynSym<ELFT>> EDynSym = DynSym<ELFT>::create(ElfFile, DynEnt);
593   if (!EDynSym)
594     return EDynSym.takeError();
595 
596   Expected<StringRef> EDynStr = EDynSym->getDynStr();
597   if (!EDynStr)
598     return EDynStr.takeError();
599 
600   StringRef DynStr = *EDynStr;
601 
602   // Populate Arch from ELF header.
603   DestStub->Target.Arch = static_cast<IFSArch>(ElfFile.getHeader().e_machine);
604   DestStub->Target.BitWidth =
605       convertELFBitWidthToIFS(ElfFile.getHeader().e_ident[EI_CLASS]);
606   DestStub->Target.Endianness =
607       convertELFEndiannessToIFS(ElfFile.getHeader().e_ident[EI_DATA]);
608   DestStub->Target.ObjectFormat = "ELF";
609 
610   // Populate SoName from .dynamic entries and dynamic string table.
611   if (DynEnt.SONameOffset.hasValue()) {
612     Expected<StringRef> NameOrErr =
613         terminatedSubstr(DynStr, *DynEnt.SONameOffset);
614     if (!NameOrErr) {
615       return appendToError(NameOrErr.takeError(), "when reading DT_SONAME");
616     }
617     DestStub->SoName = std::string(*NameOrErr);
618   }
619 
620   // Populate NeededLibs from .dynamic entries and dynamic string table.
621   for (uint64_t NeededStrOffset : DynEnt.NeededLibNames) {
622     Expected<StringRef> LibNameOrErr =
623         terminatedSubstr(DynStr, NeededStrOffset);
624     if (!LibNameOrErr) {
625       return appendToError(LibNameOrErr.takeError(), "when reading DT_NEEDED");
626     }
627     DestStub->NeededLibs.push_back(std::string(*LibNameOrErr));
628   }
629 
630   // Populate Symbols from .dynsym table and dynamic string table.
631   Expected<uint64_t> SymCount = ElfFile.getDynSymtabSize();
632   if (!SymCount)
633     return SymCount.takeError();
634   if (*SymCount > 0) {
635     // Get pointer to in-memory location of .dynsym section.
636     Expected<const uint8_t *> DynSymPtr = EDynSym->getDynSym();
637     if (!DynSymPtr)
638       return appendToError(DynSymPtr.takeError(),
639                            "when locating .dynsym section contents");
640     Elf_Sym_Range DynSyms = ArrayRef<Elf_Sym>(
641         reinterpret_cast<const Elf_Sym *>(*DynSymPtr), *SymCount);
642     Error SymReadError = populateSymbols<ELFT>(*DestStub, DynSyms, DynStr);
643     if (SymReadError)
644       return appendToError(std::move(SymReadError),
645                            "when reading dynamic symbols");
646   }
647 
648   return std::move(DestStub);
649 }
650 
651 /// This function opens a file for writing and then writes a binary ELF stub to
652 /// the file.
653 ///
654 /// @param FilePath File path for writing the ELF binary.
655 /// @param Stub Source InterFace Stub to generate a binary ELF stub from.
656 template <class ELFT>
657 static Error writeELFBinaryToFile(StringRef FilePath, const IFSStub &Stub,
658                                   bool WriteIfChanged) {
659   ELFStubBuilder<ELFT> Builder{Stub};
660   // Write Stub to memory first.
661   std::vector<uint8_t> Buf(Builder.getSize());
662   Builder.write(Buf.data());
663 
664   if (WriteIfChanged) {
665     if (ErrorOr<std::unique_ptr<MemoryBuffer>> BufOrError =
666             MemoryBuffer::getFile(FilePath)) {
667       // Compare Stub output with existing Stub file.
668       // If Stub file unchanged, abort updating.
669       if ((*BufOrError)->getBufferSize() == Builder.getSize() &&
670           !memcmp((*BufOrError)->getBufferStart(), Buf.data(),
671                   Builder.getSize()))
672         return Error::success();
673     }
674   }
675 
676   Expected<std::unique_ptr<FileOutputBuffer>> BufOrError =
677       FileOutputBuffer::create(FilePath, Builder.getSize());
678   if (!BufOrError)
679     return createStringError(errc::invalid_argument,
680                              toString(BufOrError.takeError()) +
681                                  " when trying to open `" + FilePath +
682                                  "` for writing");
683 
684   // Write binary to file.
685   std::unique_ptr<FileOutputBuffer> FileBuf = std::move(*BufOrError);
686   memcpy(FileBuf->getBufferStart(), Buf.data(), Buf.size());
687 
688   return FileBuf->commit();
689 }
690 
691 Expected<std::unique_ptr<IFSStub>> readELFFile(MemoryBufferRef Buf) {
692   Expected<std::unique_ptr<Binary>> BinOrErr = createBinary(Buf);
693   if (!BinOrErr) {
694     return BinOrErr.takeError();
695   }
696 
697   Binary *Bin = BinOrErr->get();
698   if (auto Obj = dyn_cast<ELFObjectFile<ELF32LE>>(Bin)) {
699     return buildStub(*Obj);
700   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64LE>>(Bin)) {
701     return buildStub(*Obj);
702   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF32BE>>(Bin)) {
703     return buildStub(*Obj);
704   } else if (auto Obj = dyn_cast<ELFObjectFile<ELF64BE>>(Bin)) {
705     return buildStub(*Obj);
706   }
707   return createStringError(errc::not_supported, "unsupported binary format");
708 }
709 
710 // This function wraps the ELFT writeELFBinaryToFile() so writeBinaryStub()
711 // can be called without having to use ELFType templates directly.
712 Error writeBinaryStub(StringRef FilePath, const IFSStub &Stub,
713                       bool WriteIfChanged) {
714   assert(Stub.Target.Arch);
715   assert(Stub.Target.BitWidth);
716   assert(Stub.Target.Endianness);
717   if (Stub.Target.BitWidth == IFSBitWidthType::IFS32) {
718     if (Stub.Target.Endianness == IFSEndiannessType::Little) {
719       return writeELFBinaryToFile<ELF32LE>(FilePath, Stub, WriteIfChanged);
720     } else {
721       return writeELFBinaryToFile<ELF32BE>(FilePath, Stub, WriteIfChanged);
722     }
723   } else {
724     if (Stub.Target.Endianness == IFSEndiannessType::Little) {
725       return writeELFBinaryToFile<ELF64LE>(FilePath, Stub, WriteIfChanged);
726     } else {
727       return writeELFBinaryToFile<ELF64BE>(FilePath, Stub, WriteIfChanged);
728     }
729   }
730   llvm_unreachable("invalid binary output target");
731 }
732 
733 } // end namespace ifs
734 } // end namespace llvm
735