1 //===- Relocations.cpp ----------------------------------------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file contains platform-independent functions to process relocations.
11 // I'll describe the overview of this file here.
13 // Simple relocations are easy to handle for the linker. For example,
14 // for R_X86_64_PC64 relocs, the linker just has to fix up locations
15 // with the relative offsets to the target symbols. It would just be
16 // reading records from relocation sections and applying them to output.
18 // But not all relocations are that easy to handle. For example, for
19 // R_386_GOTOFF relocs, the linker has to create new GOT entries for
20 // symbols if they don't exist, and fix up locations with GOT entry
21 // offsets from the beginning of GOT section. So there is more than
22 // fixing addresses in relocation processing.
24 // ELF defines a large number of complex relocations.
26 // The functions in this file analyze relocations and do whatever needs
27 // to be done. It includes, but not limited to, the following.
29 // - create GOT/PLT entries
30 // - create new relocations in .dynsym to let the dynamic linker resolve
31 // them at runtime (since ELF supports dynamic linking, not all
32 // relocations can be resolved at link-time)
33 // - create COPY relocs and reserve space in .bss
34 // - replace expensive relocs (in terms of runtime cost) with cheap ones
35 // - error out infeasible combinations such as PIC and non-relative relocs
37 // Note that the functions in this file don't actually apply relocations
38 // because it doesn't know about the output file nor the output file buffer.
39 // It instead stores Relocation objects to InputSection's Relocations
40 // vector to let it apply later in InputSection::writeTo.
42 //===----------------------------------------------------------------------===//
44 #include "Relocations.h"
47 #include "OutputSections.h"
49 #include "SymbolTable.h"
50 #include "SyntheticSections.h"
54 #include "llvm/Support/Endian.h"
55 #include "llvm/Support/raw_ostream.h"
59 using namespace llvm::ELF;
60 using namespace llvm::object;
61 using namespace llvm::support::endian;
64 using namespace lld::elf;
66 // Construct a message in the following format.
68 // >>> defined in /home/alice/src/foo.o
69 // >>> referenced by bar.c:12 (/home/alice/src/bar.c:12)
70 // >>> /home/alice/src/bar.o:(.text+0x1)
72 static std::string getLocation(InputSectionBase &S, const SymbolBody &Sym,
75 "\n>>> defined in " + toString(Sym.File) + "\n>>> referenced by ";
76 std::string Src = S.getSrcMsg<ELFT>(Off);
78 Msg += Src + "\n>>> ";
79 return Msg + S.getObjMsg<ELFT>(Off);
82 static bool isPreemptible(const SymbolBody &Body, uint32_t Type) {
83 // In case of MIPS GP-relative relocations always resolve to a definition
84 // in a regular input file, ignoring the one-definition rule. So we,
85 // for example, should not attempt to create a dynamic relocation even
86 // if the target symbol is preemptible. There are two two MIPS GP-relative
87 // relocations R_MIPS_GPREL16 and R_MIPS_GPREL32. But only R_MIPS_GPREL16
88 // can be against a preemptible symbol.
89 // To get MIPS relocation type we apply 0xff mask. In case of O32 ABI all
90 // relocation types occupy eight bit. In case of N64 ABI we extract first
91 // relocation from 3-in-1 packet because only the first relocation can
92 // be against a real symbol.
93 if (Config->EMachine == EM_MIPS && (Type & 0xff) == R_MIPS_GPREL16)
95 return Body.isPreemptible();
98 // This function is similar to the `handleTlsRelocation`. MIPS does not
99 // support any relaxations for TLS relocations so by factoring out MIPS
100 // handling in to the separate function we can simplify the code and do not
101 // pollute other `handleTlsRelocation` by MIPS `ifs` statements.
102 // Mips has a custom MipsGotSection that handles the writing of GOT entries
103 // without dynamic relocations.
104 template <class ELFT>
105 static unsigned handleMipsTlsRelocation(uint32_t Type, SymbolBody &Body,
106 InputSectionBase &C, uint64_t Offset,
107 int64_t Addend, RelExpr Expr) {
108 if (Expr == R_MIPS_TLSLD) {
109 if (In<ELFT>::MipsGot->addTlsIndex() && Config->Pic)
110 In<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, In<ELFT>::MipsGot,
111 In<ELFT>::MipsGot->getTlsIndexOff(), false,
113 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
117 if (Expr == R_MIPS_TLSGD) {
118 if (In<ELFT>::MipsGot->addDynTlsEntry(Body) && Body.isPreemptible()) {
119 uint64_t Off = In<ELFT>::MipsGot->getGlobalDynOffset(Body);
120 In<ELFT>::RelaDyn->addReloc(
121 {Target->TlsModuleIndexRel, In<ELFT>::MipsGot, Off, false, &Body, 0});
122 if (Body.isPreemptible())
123 In<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, In<ELFT>::MipsGot,
124 Off + Config->Wordsize, false, &Body, 0});
126 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
132 // This function is similar to the `handleMipsTlsRelocation`. ARM also does not
133 // support any relaxations for TLS relocations. ARM is logically similar to Mips
134 // in how it handles TLS, but Mips uses its own custom GOT which handles some
135 // of the cases that ARM uses GOT relocations for.
137 // We look for TLS global dynamic and local dynamic relocations, these may
138 // require the generation of a pair of GOT entries that have associated
139 // dynamic relocations. When the results of the dynamic relocations can be
140 // resolved at static link time we do so. This is necessary for static linking
141 // as there will be no dynamic loader to resolve them at load-time.
143 // The pair of GOT entries created are of the form
144 // GOT[e0] Module Index (Used to find pointer to TLS block at run-time)
145 // GOT[e1] Offset of symbol in TLS block
146 template <class ELFT>
147 static unsigned handleARMTlsRelocation(uint32_t Type, SymbolBody &Body,
148 InputSectionBase &C, uint64_t Offset,
149 int64_t Addend, RelExpr Expr) {
150 // The Dynamic TLS Module Index Relocation for a symbol defined in an
151 // executable is always 1. If the target Symbol is not preemtible then
152 // we know the offset into the TLS block at static link time.
153 bool NeedDynId = Body.isPreemptible() || Config->Shared;
154 bool NeedDynOff = Body.isPreemptible();
156 auto AddTlsReloc = [&](uint64_t Off, uint32_t Type, SymbolBody *Dest,
159 In<ELFT>::RelaDyn->addReloc({Type, In<ELFT>::Got, Off, false, Dest, 0});
161 In<ELFT>::Got->Relocations.push_back({R_ABS, Type, Off, 0, Dest});
164 // Local Dynamic is for access to module local TLS variables, while still
165 // being suitable for being dynamically loaded via dlopen.
166 // GOT[e0] is the module index, with a special value of 0 for the current
167 // module. GOT[e1] is unused. There only needs to be one module index entry.
168 if (Expr == R_TLSLD_PC && In<ELFT>::Got->addTlsIndex()) {
169 AddTlsReloc(In<ELFT>::Got->getTlsIndexOff(), Target->TlsModuleIndexRel,
170 NeedDynId ? nullptr : &Body, NeedDynId);
171 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
175 // Global Dynamic is the most general purpose access model. When we know
176 // the module index and offset of symbol in TLS block we can fill these in
177 // using static GOT relocations.
178 if (Expr == R_TLSGD_PC) {
179 if (In<ELFT>::Got->addDynTlsEntry(Body)) {
180 uint64_t Off = In<ELFT>::Got->getGlobalDynOffset(Body);
181 AddTlsReloc(Off, Target->TlsModuleIndexRel, &Body, NeedDynId);
182 AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Body,
185 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
191 // Returns the number of relocations processed.
192 template <class ELFT>
194 handleTlsRelocation(uint32_t Type, SymbolBody &Body, InputSectionBase &C,
195 typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
196 if (!(C.Flags & SHF_ALLOC))
202 if (Config->EMachine == EM_ARM)
203 return handleARMTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr);
204 if (Config->EMachine == EM_MIPS)
205 return handleMipsTlsRelocation<ELFT>(Type, Body, C, Offset, Addend, Expr);
207 bool IsPreemptible = isPreemptible(Body, Type);
208 if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL>(Expr) &&
210 if (In<ELFT>::Got->addDynTlsEntry(Body)) {
211 uint64_t Off = In<ELFT>::Got->getGlobalDynOffset(Body);
212 In<ELFT>::RelaDyn->addReloc({Target->TlsDescRel, In<ELFT>::Got, Off,
213 !IsPreemptible, &Body, 0});
215 if (Expr != R_TLSDESC_CALL)
216 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
220 if (isRelExprOneOf<R_TLSLD_PC, R_TLSLD>(Expr)) {
221 // Local-Dynamic relocs can be relaxed to Local-Exec.
222 if (!Config->Shared) {
223 C.Relocations.push_back(
224 {R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Body});
227 if (In<ELFT>::Got->addTlsIndex())
228 In<ELFT>::RelaDyn->addReloc({Target->TlsModuleIndexRel, In<ELFT>::Got,
229 In<ELFT>::Got->getTlsIndexOff(), false,
231 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
235 // Local-Dynamic relocs can be relaxed to Local-Exec.
236 if (Target->isTlsLocalDynamicRel(Type) && !Config->Shared) {
237 C.Relocations.push_back(
238 {R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Body});
242 if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL, R_TLSGD,
244 if (Config->Shared) {
245 if (In<ELFT>::Got->addDynTlsEntry(Body)) {
246 uint64_t Off = In<ELFT>::Got->getGlobalDynOffset(Body);
247 In<ELFT>::RelaDyn->addReloc(
248 {Target->TlsModuleIndexRel, In<ELFT>::Got, Off, false, &Body, 0});
250 // If the symbol is preemptible we need the dynamic linker to write
252 uint64_t OffsetOff = Off + Config->Wordsize;
254 In<ELFT>::RelaDyn->addReloc({Target->TlsOffsetRel, In<ELFT>::Got,
255 OffsetOff, false, &Body, 0});
257 In<ELFT>::Got->Relocations.push_back(
258 {R_ABS, Target->TlsOffsetRel, OffsetOff, 0, &Body});
260 C.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
264 // Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec
265 // depending on the symbol being locally defined or not.
267 C.Relocations.push_back(
268 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type,
269 Offset, Addend, &Body});
270 if (!Body.isInGot()) {
271 In<ELFT>::Got->addEntry(Body);
272 In<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, In<ELFT>::Got,
273 Body.getGotOffset(), false, &Body, 0});
276 C.Relocations.push_back(
277 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type,
278 Offset, Addend, &Body});
280 return Target->TlsGdRelaxSkip;
283 // Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally
285 if (Target->isTlsInitialExecRel(Type) && !Config->Shared && !IsPreemptible) {
286 C.Relocations.push_back(
287 {R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Body});
291 if (Expr == R_TLSDESC_CALL)
296 static uint32_t getMipsPairType(uint32_t Type, const SymbolBody &Sym) {
301 return Sym.isLocal() ? R_MIPS_LO16 : R_MIPS_NONE;
303 return R_MIPS_PCLO16;
304 case R_MICROMIPS_HI16:
305 return R_MICROMIPS_LO16;
311 // True if non-preemptable symbol always has the same value regardless of where
312 // the DSO is loaded.
313 static bool isAbsolute(const SymbolBody &Body) {
314 if (Body.isUndefined())
315 return !Body.isLocal() && Body.symbol()->isWeak();
316 if (const auto *DR = dyn_cast<DefinedRegular>(&Body))
317 return DR->Section == nullptr; // Absolute symbol.
321 static bool isAbsoluteValue(const SymbolBody &Body) {
322 return isAbsolute(Body) || Body.isTls();
325 // Returns true if Expr refers a PLT entry.
326 static bool needsPlt(RelExpr Expr) {
327 return isRelExprOneOf<R_PLT_PC, R_PPC_PLT_OPD, R_PLT, R_PLT_PAGE_PC>(Expr);
330 // Returns true if Expr refers a GOT entry. Note that this function
331 // returns false for TLS variables even though they need GOT, because
332 // TLS variables uses GOT differently than the regular variables.
333 static bool needsGot(RelExpr Expr) {
334 return isRelExprOneOf<R_GOT, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOT_OFF,
335 R_MIPS_GOT_OFF32, R_GOT_PAGE_PC, R_GOT_PC,
336 R_GOT_FROM_END>(Expr);
339 // True if this expression is of the form Sym - X, where X is a position in the
340 // file (PC, or GOT for example).
341 static bool isRelExpr(RelExpr Expr) {
342 return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
343 R_PAGE_PC, R_RELAX_GOT_PC>(Expr);
346 // Returns true if a given relocation can be computed at link-time.
348 // For instance, we know the offset from a relocation to its target at
349 // link-time if the relocation is PC-relative and refers a
350 // non-interposable function in the same executable. This function
351 // will return true for such relocation.
353 // If this function returns false, that means we need to emit a
354 // dynamic relocation so that the relocation will be fixed at load-time.
355 template <class ELFT>
356 static bool isStaticLinkTimeConstant(RelExpr E, uint32_t Type,
357 const SymbolBody &Body,
358 InputSectionBase &S, uint64_t RelOff) {
359 // These expressions always compute a constant
360 if (isRelExprOneOf<R_SIZE, R_GOT_FROM_END, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE,
361 R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC,
362 R_MIPS_TLSGD, R_GOT_PAGE_PC, R_GOT_PC, R_PLT_PC,
363 R_TLSGD_PC, R_TLSGD, R_PPC_PLT_OPD, R_TLSDESC_CALL,
364 R_TLSDESC_PAGE, R_HINT>(E))
367 // These never do, except if the entire file is position dependent or if
368 // only the low bits are used.
369 if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
370 return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
372 if (isPreemptible(Body, Type))
377 // For the target and the relocation, we want to know if they are
378 // absolute or relative.
379 bool AbsVal = isAbsoluteValue(Body);
380 bool RelE = isRelExpr(E);
385 if (!AbsVal && !RelE)
386 return Target->usesOnlyLowPageBits(Type);
388 // Relative relocation to an absolute value. This is normally unrepresentable,
389 // but if the relocation refers to a weak undefined symbol, we allow it to
390 // resolve to the image base. This is a little strange, but it allows us to
391 // link function calls to such symbols. Normally such a call will be guarded
392 // with a comparison, which will load a zero from the GOT.
393 // Another special case is MIPS _gp_disp symbol which represents offset
394 // between start of a function and '_gp' value and defined as absolute just
395 // to simplify the code.
396 assert(AbsVal && RelE);
397 if (Body.isUndefined() && !Body.isLocal() && Body.symbol()->isWeak())
400 error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
401 toString(Body) + getLocation<ELFT>(S, Body, RelOff));
405 static RelExpr toPlt(RelExpr Expr) {
406 if (Expr == R_PPC_OPD)
407 return R_PPC_PLT_OPD;
410 if (Expr == R_PAGE_PC)
411 return R_PLT_PAGE_PC;
417 static RelExpr fromPlt(RelExpr Expr) {
418 // We decided not to use a plt. Optimize a reference to the plt to a
419 // reference to the symbol itself.
420 if (Expr == R_PLT_PC)
422 if (Expr == R_PPC_PLT_OPD)
429 // Returns true if a given shared symbol is in a read-only segment in a DSO.
430 template <class ELFT> static bool isReadOnly(SharedSymbol *SS) {
431 typedef typename ELFT::Phdr Elf_Phdr;
432 uint64_t Value = SS->getValue<ELFT>();
434 // Determine if the symbol is read-only by scanning the DSO's program headers.
435 auto *File = cast<SharedFile<ELFT>>(SS->File);
436 for (const Elf_Phdr &Phdr : check(File->getObj().program_headers()))
437 if ((Phdr.p_type == ELF::PT_LOAD || Phdr.p_type == ELF::PT_GNU_RELRO) &&
438 !(Phdr.p_flags & ELF::PF_W) && Value >= Phdr.p_vaddr &&
439 Value < Phdr.p_vaddr + Phdr.p_memsz)
444 // Returns symbols at the same offset as a given symbol, including SS itself.
446 // If two or more symbols are at the same offset, and at least one of
447 // them are copied by a copy relocation, all of them need to be copied.
448 // Otherwise, they would refer different places at runtime.
449 template <class ELFT>
450 static std::vector<SharedSymbol *> getSymbolsAt(SharedSymbol *SS) {
451 typedef typename ELFT::Sym Elf_Sym;
453 auto *File = cast<SharedFile<ELFT>>(SS->File);
454 uint64_t Shndx = SS->getShndx<ELFT>();
455 uint64_t Value = SS->getValue<ELFT>();
457 std::vector<SharedSymbol *> Ret;
458 for (const Elf_Sym &S : File->getGlobalSymbols()) {
459 if (S.st_shndx != Shndx || S.st_value != Value)
461 StringRef Name = check(S.getName(File->getStringTable()));
462 SymbolBody *Sym = Symtab<ELFT>::X->find(Name);
463 if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
464 Ret.push_back(Alias);
469 // Reserve space in .bss or .bss.rel.ro for copy relocation.
471 // The copy relocation is pretty much a hack. If you use a copy relocation
472 // in your program, not only the symbol name but the symbol's size, RW/RO
473 // bit and alignment become part of the ABI. In addition to that, if the
474 // symbol has aliases, the aliases become part of the ABI. That's subtle,
475 // but if you violate that implicit ABI, that can cause very counter-
476 // intuitive consequences.
478 // So, what is the copy relocation? It's for linking non-position
479 // independent code to DSOs. In an ideal world, all references to data
480 // exported by DSOs should go indirectly through GOT. But if object files
481 // are compiled as non-PIC, all data references are direct. There is no
482 // way for the linker to transform the code to use GOT, as machine
483 // instructions are already set in stone in object files. This is where
484 // the copy relocation takes a role.
486 // A copy relocation instructs the dynamic linker to copy data from a DSO
487 // to a specified address (which is usually in .bss) at load-time. If the
488 // static linker (that's us) finds a direct data reference to a DSO
489 // symbol, it creates a copy relocation, so that the symbol can be
490 // resolved as if it were in .bss rather than in a DSO.
492 // As you can see in this function, we create a copy relocation for the
493 // dynamic linker, and the relocation contains not only symbol name but
494 // various other informtion about the symbol. So, such attributes become a
497 // Note for application developers: I can give you a piece of advice if
498 // you are writing a shared library. You probably should export only
499 // functions from your library. You shouldn't export variables.
501 // As an example what can happen when you export variables without knowing
502 // the semantics of copy relocations, assume that you have an exported
503 // variable of type T. It is an ABI-breaking change to add new members at
504 // end of T even though doing that doesn't change the layout of the
505 // existing members. That's because the space for the new members are not
506 // reserved in .bss unless you recompile the main program. That means they
507 // are likely to overlap with other data that happens to be laid out next
508 // to the variable in .bss. This kind of issue is sometimes very hard to
509 // debug. What's a solution? Instead of exporting a varaible V from a DSO,
510 // define an accessor getV().
511 template <class ELFT> static void addCopyRelSymbol(SharedSymbol *SS) {
512 // Copy relocation against zero-sized symbol doesn't make sense.
513 uint64_t SymSize = SS->template getSize<ELFT>();
515 fatal("cannot create a copy relocation for symbol " + toString(*SS));
517 // See if this symbol is in a read-only segment. If so, preserve the symbol's
518 // memory protection by reserving space in the .bss.rel.ro section.
519 bool IsReadOnly = isReadOnly<ELFT>(SS);
520 BssSection *Sec = IsReadOnly ? In<ELFT>::BssRelRo : In<ELFT>::Bss;
521 uint64_t Off = Sec->reserveSpace(SymSize, SS->getAlignment<ELFT>());
523 // Look through the DSO's dynamic symbol table for aliases and create a
524 // dynamic symbol for each one. This causes the copy relocation to correctly
525 // interpose any aliases.
526 for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS)) {
527 Sym->NeedsCopy = true;
528 Sym->CopyRelSec = Sec;
529 Sym->CopyRelSecOff = Off;
530 Sym->symbol()->IsUsedInRegularObj = true;
533 In<ELFT>::RelaDyn->addReloc({Target->CopyRel, Sec, Off, false, SS, 0});
536 template <class ELFT>
537 static RelExpr adjustExpr(SymbolBody &Body, RelExpr Expr, uint32_t Type,
538 const uint8_t *Data, InputSectionBase &S,
539 typename ELFT::uint RelOff) {
540 if (Body.isGnuIFunc()) {
542 } else if (!isPreemptible(Body, Type)) {
544 Expr = fromPlt(Expr);
545 if (Expr == R_GOT_PC && !isAbsoluteValue(Body))
546 Expr = Target->adjustRelaxExpr(Type, Data, Expr);
549 bool IsWrite = !Config->ZText || (S.Flags & SHF_WRITE);
550 if (IsWrite || isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, S, RelOff))
553 // This relocation would require the dynamic linker to write a value to read
554 // only memory. We can hack around it if we are producing an executable and
555 // the refered symbol can be preemepted to refer to the executable.
556 if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {
557 error("can't create dynamic relocation " + toString(Type) + " against " +
558 (Body.getName().empty() ? "local symbol in readonly segment"
559 : "symbol: " + toString(Body)) +
560 getLocation<ELFT>(S, Body, RelOff));
564 if (Body.getVisibility() != STV_DEFAULT) {
565 error("cannot preempt symbol: " + toString(Body) +
566 getLocation<ELFT>(S, Body, RelOff));
570 if (Body.isObject()) {
571 // Produce a copy relocation.
572 auto *B = cast<SharedSymbol>(&Body);
574 if (Config->ZNocopyreloc)
575 error("unresolvable relocation " + toString(Type) +
576 " against symbol '" + toString(*B) +
577 "'; recompile with -fPIC or remove '-z nocopyreloc'" +
578 getLocation<ELFT>(S, Body, RelOff));
580 addCopyRelSymbol<ELFT>(B);
586 // This handles a non PIC program call to function in a shared library. In
587 // an ideal world, we could just report an error saying the relocation can
588 // overflow at runtime. In the real world with glibc, crt1.o has a
589 // R_X86_64_PC32 pointing to libc.so.
591 // The general idea on how to handle such cases is to create a PLT entry and
592 // use that as the function value.
594 // For the static linking part, we just return a plt expr and everything
595 // else will use the the PLT entry as the address.
597 // The remaining problem is making sure pointer equality still works. We
598 // need the help of the dynamic linker for that. We let it know that we have
599 // a direct reference to a so symbol by creating an undefined symbol with a
600 // non zero st_value. Seeing that, the dynamic linker resolves the symbol to
601 // the value of the symbol we created. This is true even for got entries, so
602 // pointer equality is maintained. To avoid an infinite loop, the only entry
603 // that points to the real function is a dedicated got entry used by the
604 // plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT,
605 // R_386_JMP_SLOT, etc).
606 Body.NeedsPltAddr = true;
610 error("symbol '" + toString(Body) + "' defined in " + toString(Body.File) +
615 // Returns an addend of a given relocation. If it is RELA, an addend
616 // is in a relocation itself. If it is REL, we need to read it from an
618 template <class ELFT, class RelTy>
619 static int64_t computeAddend(const RelTy &Rel, const uint8_t *Buf) {
620 uint32_t Type = Rel.getType(Config->IsMips64EL);
621 int64_t A = RelTy::IsRela
622 ? getAddend<ELFT>(Rel)
623 : Target->getImplicitAddend(Buf + Rel.r_offset, Type);
625 if (Config->EMachine == EM_PPC64 && Config->Pic && Type == R_PPC64_TOC)
626 A += getPPC64TocBase();
630 // MIPS has an odd notion of "paired" relocations to calculate addends.
631 // For example, if a relocation is of R_MIPS_HI16, there must be a
632 // R_MIPS_LO16 relocation after that, and an addend is calculated using
633 // the two relocations.
634 template <class ELFT, class RelTy>
635 static int64_t computeMipsAddend(const RelTy &Rel, InputSectionBase &Sec,
636 RelExpr Expr, SymbolBody &Body,
638 if (Expr == R_MIPS_GOTREL && Body.isLocal())
639 return Sec.getFile<ELFT>()->MipsGp0;
641 // The ABI says that the paired relocation is used only for REL.
642 // See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
646 uint32_t Type = Rel.getType(Config->IsMips64EL);
647 uint32_t PairTy = getMipsPairType(Type, Body);
648 if (PairTy == R_MIPS_NONE)
651 const uint8_t *Buf = Sec.Data.data();
652 uint32_t SymIndex = Rel.getSymbol(Config->IsMips64EL);
654 // To make things worse, paired relocations might not be contiguous in
655 // the relocation table, so we need to do linear search. *sigh*
656 for (const RelTy *RI = &Rel; RI != End; ++RI) {
657 if (RI->getType(Config->IsMips64EL) != PairTy)
659 if (RI->getSymbol(Config->IsMips64EL) != SymIndex)
662 endianness E = Config->Endianness;
663 int32_t Hi = (read32(Buf + Rel.r_offset, E) & 0xffff) << 16;
664 int32_t Lo = SignExtend32<16>(read32(Buf + RI->r_offset, E));
668 warn("can't find matching " + toString(PairTy) + " relocation for " +
673 template <class ELFT>
674 static void reportUndefined(SymbolBody &Sym, InputSectionBase &S,
676 if (Config->UnresolvedSymbols == UnresolvedPolicy::IgnoreAll)
679 bool CanBeExternal = Sym.symbol()->computeBinding() != STB_LOCAL &&
680 Sym.getVisibility() == STV_DEFAULT;
681 if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
685 "undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
687 std::string Src = S.getSrcMsg<ELFT>(Offset);
689 Msg += Src + "\n>>> ";
690 Msg += S.getObjMsg<ELFT>(Offset);
692 if (Config->UnresolvedSymbols == UnresolvedPolicy::WarnAll ||
693 (Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal)) {
698 if (Config->ArchiveWithoutSymbolsSeen) {
699 message("At least one archive listed no symbols in its index."
700 " This can happen when creating archives with a version"
701 " of ar that does not understand the object files in"
702 " the archive. For example, if you are using LLVM"
703 " bitcode objects (such as created by -flto), you may"
704 " need to use llvm-ar or GNU ar with a plugin.");
705 // Reset to false so that we print the message only once.
706 Config->ArchiveWithoutSymbolsSeen = false;
711 template <class RelTy>
712 static std::pair<uint32_t, uint32_t>
713 mergeMipsN32RelTypes(uint32_t Type, uint32_t Offset, RelTy *I, RelTy *E) {
714 // MIPS N32 ABI treats series of successive relocations with the same offset
715 // as a single relocation. The similar approach used by N64 ABI, but this ABI
716 // packs all relocations into the single relocation record. Here we emulate
717 // this for the N32 ABI. Iterate over relocation with the same offset and put
718 // theirs types into the single bit-set.
719 uint32_t Processed = 0;
720 for (; I != E && Offset == I->r_offset; ++I) {
722 Type |= I->getType(Config->IsMips64EL) << (8 * Processed);
724 return std::make_pair(Type, Processed);
727 // .eh_frame sections are mergeable input sections, so their input
728 // offsets are not linearly mapped to output section. For each input
729 // offset, we need to find a section piece containing the offset and
730 // add the piece's base address to the input offset to compute the
731 // output offset. That isn't cheap.
733 // This class is to speed up the offset computation. When we process
734 // relocations, we access offsets in the monotonically increasing
735 // order. So we can optimize for that access pattern.
737 // For sections other than .eh_frame, this class doesn't do anything.
741 explicit OffsetGetter(InputSectionBase &Sec) {
742 if (auto *Eh = dyn_cast<EhInputSection>(&Sec)) {
744 Size = Eh->Pieces.size();
748 // Translates offsets in input sections to offsets in output sections.
749 // Given offset must increase monotonically. We assume that P is
750 // sorted by InputOff.
751 uint64_t get(uint64_t Off) {
755 while (I != Size && P[I].InputOff + P[I].size() <= Off)
760 // P must be contiguous, so there must be no holes in between.
761 assert(P[I].InputOff <= Off && "Relocation not in any piece");
763 // Offset -1 means that the piece is dead (i.e. garbage collected).
764 if (P[I].OutputOff == -1)
766 return P[I].OutputOff + Off - P[I].InputOff;
770 ArrayRef<EhSectionPiece> P;
776 template <class ELFT, class GotPltSection>
777 static void addPltEntry(PltSection *Plt, GotPltSection *GotPlt,
778 RelocationSection<ELFT> *Rel, uint32_t Type,
779 SymbolBody &Sym, bool UseSymVA) {
780 Plt->addEntry<ELFT>(Sym);
781 GotPlt->addEntry(Sym);
782 Rel->addReloc({Type, GotPlt, Sym.getGotPltOffset(), UseSymVA, &Sym, 0});
785 template <class ELFT>
786 static void addGotEntry(SymbolBody &Sym, bool Preemptible) {
787 In<ELFT>::Got->addEntry(Sym);
789 uint64_t Off = Sym.getGotOffset();
791 RelExpr Expr = R_ABS;
794 DynType = Target->TlsGotRel;
796 } else if (!Preemptible && Config->Pic && !isAbsolute(Sym)) {
797 DynType = Target->RelativeRel;
799 DynType = Target->GotRel;
802 bool Constant = !Preemptible && !(Config->Pic && !isAbsolute(Sym));
804 In<ELFT>::RelaDyn->addReloc(
805 {DynType, In<ELFT>::Got, Off, !Preemptible, &Sym, 0});
807 if (Constant || (!Config->IsRela && !Preemptible))
808 In<ELFT>::Got->Relocations.push_back({Expr, DynType, Off, 0, &Sym});
811 // The reason we have to do this early scan is as follows
812 // * To mmap the output file, we need to know the size
813 // * For that, we need to know how many dynamic relocs we will have.
814 // It might be possible to avoid this by outputting the file with write:
815 // * Write the allocated output sections, computing addresses.
816 // * Apply relocations, recording which ones require a dynamic reloc.
817 // * Write the dynamic relocations.
818 // * Write the rest of the file.
819 // This would have some drawbacks. For example, we would only know if .rela.dyn
820 // is needed after applying relocations. If it is, it will go after rw and rx
821 // sections. Given that it is ro, we will need an extra PT_LOAD. This
822 // complicates things for the dynamic linker and means we would have to reserve
823 // space for the extra PT_LOAD even if we end up not using it.
824 template <class ELFT, class RelTy>
825 static void scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
826 OffsetGetter GetOffset(Sec);
828 for (auto I = Rels.begin(), End = Rels.end(); I != End; ++I) {
829 const RelTy &Rel = *I;
830 SymbolBody &Body = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
831 uint32_t Type = Rel.getType(Config->IsMips64EL);
833 if (Config->MipsN32Abi) {
835 std::tie(Type, Processed) =
836 mergeMipsN32RelTypes(Type, Rel.r_offset, I + 1, End);
840 // Compute the offset of this section in the output section.
841 uint64_t Offset = GetOffset.get(Rel.r_offset);
842 if (Offset == uint64_t(-1))
845 // Report undefined symbols. The fact that we report undefined
846 // symbols here means that we report undefined symbols only when
847 // they have relocations pointing to them. We don't care about
848 // undefined symbols that are in dead-stripped sections.
849 if (!Body.isLocal() && Body.isUndefined() && !Body.symbol()->isWeak())
850 reportUndefined<ELFT>(Body, Sec, Rel.r_offset);
853 Target->getRelExpr(Type, Body, Sec.Data.begin() + Rel.r_offset);
855 // Ignore "hint" relocations because they are only markers for relaxation.
856 if (isRelExprOneOf<R_HINT, R_NONE>(Expr))
859 bool Preemptible = isPreemptible(Body, Type);
860 Expr = adjustExpr<ELFT>(Body, Expr, Type, Sec.Data.data() + Rel.r_offset,
865 // This relocation does not require got entry, but it is relative to got and
866 // needs it to be created. Here we request for that.
867 if (isRelExprOneOf<R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_GOTREL,
868 R_GOTREL_FROM_END, R_PPC_TOC>(Expr))
869 In<ELFT>::Got->HasGotOffRel = true;
872 int64_t Addend = computeAddend<ELFT>(Rel, Sec.Data.data());
873 if (Config->EMachine == EM_MIPS)
874 Addend += computeMipsAddend<ELFT>(Rel, Sec, Expr, Body, End);
876 // Process some TLS relocations, including relaxing TLS relocations.
877 // Note that this function does not handle all TLS relocations.
878 if (unsigned Processed =
879 handleTlsRelocation<ELFT>(Type, Body, Sec, Offset, Addend, Expr)) {
880 I += (Processed - 1);
884 // If a relocation needs PLT, we create PLT and GOTPLT slots for the symbol.
885 if (needsPlt(Expr) && !Body.isInPlt()) {
886 if (Body.isGnuIFunc() && !Preemptible)
887 addPltEntry(InX::Iplt, In<ELFT>::IgotPlt, In<ELFT>::RelaIplt,
888 Target->IRelativeRel, Body, true);
890 addPltEntry(InX::Plt, In<ELFT>::GotPlt, In<ELFT>::RelaPlt,
891 Target->PltRel, Body, !Preemptible);
894 // Create a GOT slot if a relocation needs GOT.
895 if (needsGot(Expr)) {
896 if (Config->EMachine == EM_MIPS) {
897 // MIPS ABI has special rules to process GOT entries and doesn't
898 // require relocation entries for them. A special case is TLS
899 // relocations. In that case dynamic loader applies dynamic
900 // relocations to initialize TLS GOT entries.
901 // See "Global Offset Table" in Chapter 5 in the following document
902 // for detailed description:
903 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
904 In<ELFT>::MipsGot->addEntry(Body, Addend, Expr);
905 if (Body.isTls() && Body.isPreemptible())
906 In<ELFT>::RelaDyn->addReloc({Target->TlsGotRel, In<ELFT>::MipsGot,
907 Body.getGotOffset(), false, &Body, 0});
908 } else if (!Body.isInGot()) {
909 addGotEntry<ELFT>(Body, Preemptible);
913 if (!needsPlt(Expr) && !needsGot(Expr) && isPreemptible(Body, Type)) {
914 // We don't know anything about the finaly symbol. Just ask the dynamic
915 // linker to handle the relocation for us.
916 if (!Target->isPicRel(Type))
917 error("relocation " + toString(Type) +
918 " cannot be used against shared object; recompile with -fPIC" +
919 getLocation<ELFT>(Sec, Body, Offset));
921 In<ELFT>::RelaDyn->addReloc(
922 {Target->getDynRel(Type), &Sec, Offset, false, &Body, Addend});
924 // MIPS ABI turns using of GOT and dynamic relocations inside out.
925 // While regular ABI uses dynamic relocations to fill up GOT entries
926 // MIPS ABI requires dynamic linker to fills up GOT entries using
927 // specially sorted dynamic symbol table. This affects even dynamic
928 // relocations against symbols which do not require GOT entries
929 // creation explicitly, i.e. do not have any GOT-relocations. So if
930 // a preemptible symbol has a dynamic relocation we anyway have
931 // to create a GOT entry for it.
932 // If a non-preemptible symbol has a dynamic relocation against it,
933 // dynamic linker takes it st_value, adds offset and writes down
934 // result of the dynamic relocation. In case of preemptible symbol
935 // dynamic linker performs symbol resolution, writes the symbol value
936 // to the GOT entry and reads the GOT entry when it needs to perform
937 // a dynamic relocation.
938 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19
939 if (Config->EMachine == EM_MIPS)
940 In<ELFT>::MipsGot->addEntry(Body, Addend, Expr);
944 // If the relocation points to something in the file, we can process it.
946 isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, Sec, Rel.r_offset);
948 // If the output being produced is position independent, the final value
949 // is still not known. In that case we still need some help from the
950 // dynamic linker. We can however do better than just copying the incoming
951 // relocation. We can process some of it and and just ask the dynamic
952 // linker to add the load address.
954 In<ELFT>::RelaDyn->addReloc(
955 {Target->RelativeRel, &Sec, Offset, true, &Body, Addend});
957 // If the produced value is a constant, we just remember to write it
958 // when outputting this section. We also have to do it if the format
959 // uses Elf_Rel, since in that case the written value is the addend.
960 if (IsConstant || !RelTy::IsRela)
961 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Body});
965 template <class ELFT> void elf::scanRelocations(InputSectionBase &S) {
967 scanRelocs<ELFT>(S, S.relas<ELFT>());
969 scanRelocs<ELFT>(S, S.rels<ELFT>());
972 // Insert the Thunks for OutputSection OS into their designated place
973 // in the Sections vector, and recalculate the InputSection output section
975 // This may invalidate any output section offsets stored outside of InputSection
976 template <class ELFT>
977 void ThunkCreator<ELFT>::mergeThunks(OutputSection *OS,
978 std::vector<ThunkSection *> &Thunks) {
979 // Order Thunks in ascending OutSecOff
980 auto ThunkCmp = [](const ThunkSection *A, const ThunkSection *B) {
981 return A->OutSecOff < B->OutSecOff;
983 std::stable_sort(Thunks.begin(), Thunks.end(), ThunkCmp);
985 // Merge sorted vectors of Thunks and InputSections by OutSecOff
986 std::vector<InputSection *> Tmp;
987 Tmp.reserve(OS->Sections.size() + Thunks.size());
988 auto MergeCmp = [](const InputSection *A, const InputSection *B) {
989 // std::merge requires a strict weak ordering.
990 if (A->OutSecOff < B->OutSecOff)
992 if (A->OutSecOff == B->OutSecOff)
993 // Check if Thunk is immediately before any specific Target InputSection
994 // for example Mips LA25 Thunks.
995 if (auto *TA = dyn_cast<ThunkSection>(A))
996 if (TA && TA->getTargetInputSection() == B)
1000 std::merge(OS->Sections.begin(), OS->Sections.end(), Thunks.begin(),
1001 Thunks.end(), std::back_inserter(Tmp), MergeCmp);
1002 OS->Sections = std::move(Tmp);
1003 OS->assignOffsets();
1006 template <class ELFT>
1007 ThunkSection *ThunkCreator<ELFT>::getOSThunkSec(ThunkSection *&TS,
1008 OutputSection *OS) {
1009 if (TS == nullptr) {
1011 for (auto *IS : OS->Sections) {
1012 Off = IS->OutSecOff + IS->getSize();
1013 if ((IS->Flags & SHF_EXECINSTR) == 0)
1016 TS = make<ThunkSection>(OS, Off);
1017 ThunkSections[OS].push_back(TS);
1022 template <class ELFT>
1023 ThunkSection *ThunkCreator<ELFT>::getISThunkSec(InputSection *IS,
1024 OutputSection *OS) {
1025 ThunkSection *TS = ThunkedSections.lookup(IS);
1028 auto *TOS = cast<OutputSection>(IS->OutSec);
1029 TS = make<ThunkSection>(TOS, IS->OutSecOff);
1030 ThunkSections[TOS].push_back(TS);
1031 ThunkedSections[IS] = TS;
1035 template <class ELFT>
1036 std::pair<Thunk *, bool> ThunkCreator<ELFT>::getThunk(SymbolBody &Body,
1038 auto res = ThunkedSymbols.insert({&Body, nullptr});
1040 res.first->second = addThunk<ELFT>(Type, Body);
1041 return std::make_pair(res.first->second, res.second);
1044 // Process all relocations from the InputSections that have been assigned
1045 // to OutputSections and redirect through Thunks if needed.
1047 // createThunks must be called after scanRelocs has created the Relocations for
1048 // each InputSection. It must be called before the static symbol table is
1049 // finalized. If any Thunks are added to an OutputSection the output section
1050 // offsets of the InputSections will change.
1052 // FIXME: All Thunks are assumed to be in range of the relocation. Range
1053 // extension Thunks are not yet supported.
1054 template <class ELFT>
1055 bool ThunkCreator<ELFT>::createThunks(
1056 ArrayRef<OutputSection *> OutputSections) {
1057 // Create all the Thunks and insert them into synthetic ThunkSections. The
1058 // ThunkSections are later inserted back into the OutputSection.
1060 // We separate the creation of ThunkSections from the insertion of the
1061 // ThunkSections back into the OutputSection as ThunkSections are not always
1062 // inserted into the same OutputSection as the caller.
1063 for (OutputSection *OS : OutputSections) {
1064 ThunkSection *OSTS = nullptr;
1065 for (InputSection *IS : OS->Sections) {
1066 for (Relocation &Rel : IS->Relocations) {
1067 SymbolBody &Body = *Rel.Sym;
1068 if (!Target->needsThunk(Rel.Expr, Rel.Type, IS->File, Body))
1072 std::tie(T, IsNew) = getThunk(Body, Rel.Type);
1074 // Find or create a ThunkSection for the new Thunk
1076 if (auto *TIS = T->getTargetInputSection())
1077 TS = getISThunkSec(TIS, OS);
1079 TS = getOSThunkSec(OSTS, OS);
1082 // Redirect relocation to Thunk, we never go via the PLT to a Thunk
1083 Rel.Sym = T->ThunkSym;
1084 Rel.Expr = fromPlt(Rel.Expr);
1089 // Merge all created synthetic ThunkSections back into OutputSection
1090 for (auto &KV : ThunkSections)
1091 mergeThunks(KV.first, KV.second);
1092 return !ThunkSections.empty();
1095 template void elf::scanRelocations<ELF32LE>(InputSectionBase &);
1096 template void elf::scanRelocations<ELF32BE>(InputSectionBase &);
1097 template void elf::scanRelocations<ELF64LE>(InputSectionBase &);
1098 template void elf::scanRelocations<ELF64BE>(InputSectionBase &);
1100 template class elf::ThunkCreator<ELF32LE>;
1101 template class elf::ThunkCreator<ELF32BE>;
1102 template class elf::ThunkCreator<ELF64LE>;
1103 template class elf::ThunkCreator<ELF64BE>;