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 {
StringRefZStringRefZ25 StringRefZ(const char *s) : data(s), size(-1) {}
StringRefZStringRefZ26 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
~Symbol()44 virtual ~Symbol() {}
45
kind()46 Kind kind() const { return symbolKind; }
47
getName()48 StringRef getName() const {
49 if (nameSize == (uint32_t)-1)
50 nameSize = strlen(nameData);
51 return {nameData, nameSize};
52 }
53
isLive()54 bool isLive() const { return used; }
isLazy()55 bool isLazy() const {
56 return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind;
57 }
58
getVA()59 virtual uint64_t getVA() const { return 0; }
60
isWeakDef()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.
isWeakRef()66 virtual bool isWeakRef() const { return false; }
67
isTlv()68 virtual bool isTlv() const { return false; }
69
70 // Whether this symbol is in the GOT or TLVPointer sections.
isInGot()71 bool isInGot() const { return gotIndex != UINT32_MAX; }
72
73 // Whether this symbol is in the StubsSection.
isInStubs()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;
resolveBranchVA()80 uint64_t resolveBranchVA() const {
81 assert(isa<Defined>(this) || isa<DylibSymbol>(this));
82 return isInStubs() ? getStubVA() : getVA();
83 }
resolveGotVA()84 uint64_t resolveGotVA() const { return isInGot() ? getGotVA() : getVA(); }
resolveTlvVA()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
getFile()96 InputFile *getFile() const { return file; }
97
98 protected:
Symbol(Kind k,StringRefZ name,InputFile * file)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
isWeakDef()125 bool isWeakDef() const override { return weakDef; }
isExternalWeakDef()126 bool isExternalWeakDef() const {
127 return isWeakDef() && isExternal() && !privateExtern;
128 }
129 bool isTlv() const override;
130
isExternal()131 bool isExternal() const { return external; }
isAbsolute()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
classof(const Symbol * s)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:
Undefined(StringRefZ name,InputFile * file,RefState refState,bool wasBitcodeSymbol)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
isWeakRef()211 bool isWeakRef() const override { return refState == RefState::Weak; }
212
classof(const Symbol * s)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:
CommonSymbol(StringRefZ name,InputFile * file,uint64_t size,uint32_t align,bool isPrivateExtern)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
classof(const Symbol * s)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:
DylibSymbol(DylibFile * file,StringRefZ name,bool isWeakDef,RefState refState,bool isTlv)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;
isWeakDef()262 bool isWeakDef() const override { return weakDef; }
263
264 // Symbols from weak libraries/frameworks are also weakly-referenced.
isWeakRef()265 bool isWeakRef() const override {
266 return refState == RefState::Weak ||
267 (file && getFile()->umbrella->forceWeakImport);
268 }
isReferenced()269 bool isReferenced() const { return refState != RefState::Unreferenced; }
isTlv()270 bool isTlv() const override { return tlv; }
isDynamicLookup()271 bool isDynamicLookup() const { return file == nullptr; }
hasStubsHelper()272 bool hasStubsHelper() const { return stubsHelperIndex != UINT32_MAX; }
273
getFile()274 DylibFile *getFile() const {
275 assert(!isDynamicLookup());
276 return cast<DylibFile>(file);
277 }
278
classof(const Symbol * s)279 static bool classof(const Symbol *s) { return s->kind() == DylibKind; }
280
getRefState()281 RefState getRefState() const { return refState; }
282
reference(RefState newState)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
unreference()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:
LazyArchive(ArchiveFile * file,const llvm::object::Archive::Symbol & sym)307 LazyArchive(ArchiveFile *file, const llvm::object::Archive::Symbol &sym)
308 : Symbol(LazyArchiveKind, sym.getName(), file), sym(sym) {}
309
getFile()310 ArchiveFile *getFile() const { return cast<ArchiveFile>(file); }
311 void fetchArchiveMember();
312
classof(const Symbol * s)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:
LazyObject(InputFile & file,StringRef name)323 LazyObject(InputFile &file, StringRef name)
324 : Symbol(LazyObjectKind, name, &file) {
325 isUsedInRegularObj = false;
326 }
327
classof(const Symbol * s)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:
AliasSymbol(InputFile * file,StringRef name,StringRef aliasedName,bool isPrivateExtern)336 AliasSymbol(InputFile *file, StringRef name, StringRef aliasedName,
337 bool isPrivateExtern)
338 : Symbol(AliasKind, name, file), privateExtern(isPrivateExtern),
339 aliasedName(aliasedName) {}
340
getAliasedName()341 StringRef getAliasedName() const { return aliasedName; }
342
classof(const Symbol * s)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>
replaceSymbol(Symbol * s,ArgT &&...arg)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?
needsBinding(const Symbol * sym)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.
isPrivateLabel(StringRef name)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
399