lld/MachO/Symbols.h

   1 //===- Symbols.h ------------------------------------------------*- C++ -*-===//
   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 #ifndef LLD_MACHO_SYMBOLS_H
  10 #define LLD_MACHO_SYMBOLS_H
  11
  12 #include "Config.h"
  13 #include "InputFiles.h"
  14 #include "Target.h"
  15
  16 #include "llvm/Object/Archive.h"
  17 #include "llvm/Support/MathExtras.h"
  18
  19 namespace lld {
  20 namespace macho {
  21
  22 class MachHeaderSection;
  23
  24 struct StringRefZ {
  25   StringRefZ(const char *s) : data(s), size(-1) {}
  26   StringRefZ(StringRef s) : data(s.data()), size(s.size()) {}
  27
  28   const char *data;
  29   const uint32_t size;
  30 };
  31
  32 class Symbol {
  33 public:
  34   enum Kind {
  35     DefinedKind,
  36     UndefinedKind,
  37     CommonKind,
  38     DylibKind,
  39     LazyArchiveKind,
  40     LazyObjectKind,
  41     AliasKind,
  42   };
  43
  44   virtual ~Symbol() {}
  45
  46   Kind kind() const { return symbolKind; }
  47
  48   StringRef getName() const {
  49     if (nameSize == (uint32_t)-1)
  50       nameSize = strlen(nameData);
  51     return {nameData, nameSize};
  52   }
  53
  54   bool isLive() const { return used; }
  55   bool isLazy() const {
  56     return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind;
  57   }
  58
  59   virtual uint64_t getVA() const { return 0; }
  60
  61   virtual bool isWeakDef() const { return false; }
  62
  63   // Only undefined or dylib symbols can be weak references. A weak reference
  64   // need not be satisfied at runtime, e.g. due to the symbol not being
  65   // available on a given target platform.
  66   virtual bool isWeakRef() const { return false; }
  67
  68   virtual bool isTlv() const { return false; }
  69
  70   // Whether this symbol is in the GOT or TLVPointer sections.
  71   bool isInGot() const { return gotIndex != UINT32_MAX; }
  72
  73   // Whether this symbol is in the StubsSection.
  74   bool isInStubs() const { return stubsIndex != UINT32_MAX; }
  75
  76   uint64_t getStubVA() const;
  77   uint64_t getLazyPtrVA() const;
  78   uint64_t getGotVA() const;
  79   uint64_t getTlvVA() const;
  80   uint64_t resolveBranchVA() const {
  81     assert(isa<Defined>(this) || isa<DylibSymbol>(this));
  82     return isInStubs() ? getStubVA() : getVA();
  83   }
  84   uint64_t resolveGotVA() const { return isInGot() ? getGotVA() : getVA(); }
  85   uint64_t resolveTlvVA() const { return isInGot() ? getTlvVA() : getVA(); }
  86
  87   // The index of this symbol in the GOT or the TLVPointer section, depending
  88   // on whether it is a thread-local. A given symbol cannot be referenced by
  89   // both these sections at once.
  90   uint32_t gotIndex = UINT32_MAX;
  91   uint32_t lazyBindOffset = UINT32_MAX;
  92   uint32_t stubsHelperIndex = UINT32_MAX;
  93   uint32_t stubsIndex = UINT32_MAX;
  94   uint32_t symtabIndex = UINT32_MAX;
  95
  96   InputFile *getFile() const { return file; }
  97
  98 protected:
  99   Symbol(Kind k, StringRefZ name, InputFile *file)
 100       : symbolKind(k), nameData(name.data), file(file), nameSize(name.size),
 101         isUsedInRegularObj(!file || isa<ObjFile>(file)),
 102         used(!config->deadStrip) {}
 103
 104   Kind symbolKind;
 105   const char *nameData;
 106   InputFile *file;
 107   mutable uint32_t nameSize;
 108
 109 public:
 110   // True if this symbol was referenced by a regular (non-bitcode) object.
 111   bool isUsedInRegularObj : 1;
 112
 113   // True if this symbol is used from a live section.
 114   bool used : 1;
 115 };
 116
 117 class Defined : public Symbol {
 118 public:
 119   Defined(StringRefZ name, InputFile *file, InputSection *isec, uint64_t value,
 120           uint64_t size, bool isWeakDef, bool isExternal, bool isPrivateExtern,
 121           bool includeInSymtab, bool isReferencedDynamically, bool noDeadStrip,
 122           bool canOverrideWeakDef = false, bool isWeakDefCanBeHidden = false,
 123           bool interposable = false);
 124
 125   bool isWeakDef() const override { return weakDef; }
 126   bool isExternalWeakDef() const {
 127     return isWeakDef() && isExternal() && !privateExtern;
 128   }
 129   bool isTlv() const override;
 130
 131   bool isExternal() const { return external; }
 132   bool isAbsolute() const { return isec == nullptr; }
 133
 134   uint64_t getVA() const override;
 135
 136   // Returns the object file that this symbol was defined in. This value differs
 137   // from `getFile()` if the symbol originated from a bitcode file.
 138   ObjFile *getObjectFile() const;
 139
 140   std::string getSourceLocation();
 141
 142   // Ensure this symbol's pointers to InputSections point to their canonical
 143   // copies.
 144   void canonicalize();
 145
 146   static bool classof(const Symbol *s) { return s->kind() == DefinedKind; }
 147
 148   // Place the bitfields first so that they can get placed in the tail padding
 149   // of the parent class, on platforms which support it.
 150   bool overridesWeakDef : 1;
 151   // Whether this symbol should appear in the output binary's export trie.
 152   bool privateExtern : 1;
 153   // Whether this symbol should appear in the output symbol table.
 154   bool includeInSymtab : 1;
 155   // Whether this symbol was folded into a different symbol during ICF.
 156   bool wasIdenticalCodeFolded : 1;
 157   // Symbols marked referencedDynamically won't be removed from the output's
 158   // symbol table by tools like strip. In theory, this could be set on arbitrary
 159   // symbols in input object files. In practice, it's used solely for the
 160   // synthetic __mh_execute_header symbol.
 161   // This is information for the static linker, and it's also written to the
 162   // output file's symbol table for tools running later (such as `strip`).
 163   bool referencedDynamically : 1;
 164   // Set on symbols that should not be removed by dead code stripping.
 165   // Set for example on `__attribute__((used))` globals, or on some Objective-C
 166   // metadata. This is information only for the static linker and not written
 167   // to the output.
 168   bool noDeadStrip : 1;
 169   // Whether references to this symbol can be interposed at runtime to point to
 170   // a different symbol definition (with the same name). For example, if both
 171   // dylib A and B define an interposable symbol _foo, and we load A before B at
 172   // runtime, then all references to _foo within dylib B will point to the
 173   // definition in dylib A.
 174   //
 175   // Only extern symbols may be interposable.
 176   bool interposable : 1;
 177
 178   bool weakDefCanBeHidden : 1;
 179
 180 private:
 181   const bool weakDef : 1;
 182   const bool external : 1;
 183
 184 public:
 185   InputSection *isec;
 186   // Contains the offset from the containing subsection. Note that this is
 187   // different from nlist::n_value, which is the absolute address of the symbol.
 188   uint64_t value;
 189   // size is only calculated for regular (non-bitcode) symbols.
 190   uint64_t size;
 191   // This can be a subsection of either __compact_unwind or __eh_frame.
 192   ConcatInputSection *unwindEntry = nullptr;
 193 };
 194
 195 // This enum does double-duty: as a symbol property, it indicates whether & how
 196 // a dylib symbol is referenced. As a DylibFile property, it indicates the kind
 197 // of referenced symbols contained within the file. If there are both weak
 198 // and strong references to the same file, we will count the file as
 199 // strongly-referenced.
 200 enum class RefState : uint8_t { Unreferenced = 0, Weak = 1, Strong = 2 };
 201
 202 class Undefined : public Symbol {
 203 public:
 204   Undefined(StringRefZ name, InputFile *file, RefState refState,
 205             bool wasBitcodeSymbol)
 206       : Symbol(UndefinedKind, name, file), refState(refState),
 207         wasBitcodeSymbol(wasBitcodeSymbol) {
 208     assert(refState != RefState::Unreferenced);
 209   }
 210
 211   bool isWeakRef() const override { return refState == RefState::Weak; }
 212
 213   static bool classof(const Symbol *s) { return s->kind() == UndefinedKind; }
 214
 215   RefState refState : 2;
 216   bool wasBitcodeSymbol;
 217 };
 218
 219 // On Unix, it is traditionally allowed to write variable definitions without
 220 // initialization expressions (such as "int foo;") to header files. These are
 221 // called tentative definitions.
 222 //
 223 // Using tentative definitions is usually considered a bad practice; you should
 224 // write only declarations (such as "extern int foo;") to header files.
 225 // Nevertheless, the linker and the compiler have to do something to support
 226 // bad code by allowing duplicate definitions for this particular case.
 227 //
 228 // The compiler creates common symbols when it sees tentative definitions.
 229 // (You can suppress this behavior and let the compiler create a regular
 230 // defined symbol by passing -fno-common. -fno-common is the default in clang
 231 // as of LLVM 11.0.) When linking the final binary, if there are remaining
 232 // common symbols after name resolution is complete, the linker converts them
 233 // to regular defined symbols in a __common section.
 234 class CommonSymbol : public Symbol {
 235 public:
 236   CommonSymbol(StringRefZ name, InputFile *file, uint64_t size, uint32_t align,
 237                bool isPrivateExtern)
 238       : Symbol(CommonKind, name, file), size(size),
 239         align(align != 1 ? align : llvm::PowerOf2Ceil(size)),
 240         privateExtern(isPrivateExtern) {
 241     // TODO: cap maximum alignment
 242   }
 243
 244   static bool classof(const Symbol *s) { return s->kind() == CommonKind; }
 245
 246   const uint64_t size;
 247   const uint32_t align;
 248   const bool privateExtern;
 249 };
 250
 251 class DylibSymbol : public Symbol {
 252 public:
 253   DylibSymbol(DylibFile *file, StringRefZ name, bool isWeakDef,
 254               RefState refState, bool isTlv)
 255       : Symbol(DylibKind, name, file), shouldReexport(false),
 256         refState(refState), weakDef(isWeakDef), tlv(isTlv) {
 257     if (file && refState > RefState::Unreferenced)
 258       file->numReferencedSymbols++;
 259   }
 260
 261   uint64_t getVA() const override;
 262   bool isWeakDef() const override { return weakDef; }
 263
 264   // Symbols from weak libraries/frameworks are also weakly-referenced.
 265   bool isWeakRef() const override {
 266     return refState == RefState::Weak ||
 267            (file && getFile()->umbrella->forceWeakImport);
 268   }
 269   bool isReferenced() const { return refState != RefState::Unreferenced; }
 270   bool isTlv() const override { return tlv; }
 271   bool isDynamicLookup() const { return file == nullptr; }
 272   bool hasStubsHelper() const { return stubsHelperIndex != UINT32_MAX; }
 273
 274   DylibFile *getFile() const {
 275     assert(!isDynamicLookup());
 276     return cast<DylibFile>(file);
 277   }
 278
 279   static bool classof(const Symbol *s) { return s->kind() == DylibKind; }
 280
 281   RefState getRefState() const { return refState; }
 282
 283   void reference(RefState newState) {
 284     assert(newState > RefState::Unreferenced);
 285     if (refState == RefState::Unreferenced && file)
 286       getFile()->numReferencedSymbols++;
 287     refState = std::max(refState, newState);
 288   }
 289
 290   void unreference() {
 291     // dynamic_lookup symbols have no file.
 292     if (refState > RefState::Unreferenced && file) {
 293       assert(getFile()->numReferencedSymbols > 0);
 294       getFile()->numReferencedSymbols--;
 295     }
 296   }
 297
 298   bool shouldReexport : 1;
 299 private:
 300   RefState refState : 2;
 301   const bool weakDef : 1;
 302   const bool tlv : 1;
 303 };
 304
 305 class LazyArchive : public Symbol {
 306 public:
 307   LazyArchive(ArchiveFile *file, const llvm::object::Archive::Symbol &sym)
 308       : Symbol(LazyArchiveKind, sym.getName(), file), sym(sym) {}
 309
 310   ArchiveFile *getFile() const { return cast<ArchiveFile>(file); }
 311   void fetchArchiveMember();
 312
 313   static bool classof(const Symbol *s) { return s->kind() == LazyArchiveKind; }
 314
 315 private:
 316   const llvm::object::Archive::Symbol sym;
 317 };
 318
 319 // A defined symbol in an ObjFile/BitcodeFile surrounded by --start-lib and
 320 // --end-lib.
 321 class LazyObject : public Symbol {
 322 public:
 323   LazyObject(InputFile &file, StringRef name)
 324       : Symbol(LazyObjectKind, name, &file) {
 325     isUsedInRegularObj = false;
 326   }
 327
 328   static bool classof(const Symbol *s) { return s->kind() == LazyObjectKind; }
 329 };
 330
 331 // Represents N_INDR symbols. Note that if we are given valid, linkable inputs,
 332 // then all AliasSymbol instances will be converted into one of the other Symbol
 333 // types after `createAliases()` runs.
 334 class AliasSymbol final : public Symbol {
 335 public:
 336   AliasSymbol(InputFile *file, StringRef name, StringRef aliasedName,
 337               bool isPrivateExtern)
 338       : Symbol(AliasKind, name, file), privateExtern(isPrivateExtern),
 339         aliasedName(aliasedName) {}
 340
 341   StringRef getAliasedName() const { return aliasedName; }
 342
 343   static bool classof(const Symbol *s) { return s->kind() == AliasKind; }
 344
 345   const bool privateExtern;
 346
 347 private:
 348   StringRef aliasedName;
 349 };
 350
 351 union SymbolUnion {
 352   alignas(Defined) char a[sizeof(Defined)];
 353   alignas(Undefined) char b[sizeof(Undefined)];
 354   alignas(CommonSymbol) char c[sizeof(CommonSymbol)];
 355   alignas(DylibSymbol) char d[sizeof(DylibSymbol)];
 356   alignas(LazyArchive) char e[sizeof(LazyArchive)];
 357   alignas(LazyObject) char f[sizeof(LazyObject)];
 358   alignas(AliasSymbol) char g[sizeof(AliasSymbol)];
 359 };
 360
 361 template <typename T, typename... ArgT>
 362 T *replaceSymbol(Symbol *s, ArgT &&...arg) {
 363   static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
 364   static_assert(alignof(T) <= alignof(SymbolUnion),
 365                 "SymbolUnion not aligned enough");
 366   assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
 367          "Not a Symbol");
 368
 369   bool isUsedInRegularObj = s->isUsedInRegularObj;
 370   bool used = s->used;
 371   T *sym = new (s) T(std::forward<ArgT>(arg)...);
 372   sym->isUsedInRegularObj |= isUsedInRegularObj;
 373   sym->used |= used;
 374   return sym;
 375 }
 376
 377 // Can a symbol's address only be resolved at runtime?
 378 inline bool needsBinding(const Symbol *sym) {
 379   if (isa<DylibSymbol>(sym))
 380     return true;
 381   if (const auto *defined = dyn_cast<Defined>(sym))
 382     return defined->isExternalWeakDef() || defined->interposable;
 383   return false;
 384 }
 385
 386 // Symbols with `l` or `L` as a prefix are linker-private and never appear in
 387 // the output.
 388 inline bool isPrivateLabel(StringRef name) {
 389   return name.starts_with("l") || name.starts_with("L");
 390 }
 391 } // namespace macho
 392
 393 std::string toString(const macho::Symbol &);
 394 std::string toMachOString(const llvm::object::Archive::Symbol &);
 395
 396 } // namespace lld
 397
 398 #endif