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"
46 #include "LinkerScript.h"
47 #include "OutputSections.h"
48 #include "SymbolTable.h"
50 #include "SyntheticSections.h"
53 #include "lld/Common/ErrorHandler.h"
54 #include "lld/Common/Memory.h"
55 #include "lld/Common/Strings.h"
56 #include "llvm/ADT/SmallSet.h"
57 #include "llvm/Support/Endian.h"
58 #include "llvm/Support/raw_ostream.h"
62 using namespace llvm::ELF;
63 using namespace llvm::object;
64 using namespace llvm::support::endian;
67 using namespace lld::elf;
69 static Optional<std::string> getLinkerScriptLocation(const Symbol &Sym) {
70 for (BaseCommand *Base : Script->SectionCommands)
71 if (auto *Cmd = dyn_cast<SymbolAssignment>(Base))
77 // Construct a message in the following format.
79 // >>> defined in /home/alice/src/foo.o
80 // >>> referenced by bar.c:12 (/home/alice/src/bar.c:12)
81 // >>> /home/alice/src/bar.o:(.text+0x1)
82 static std::string getLocation(InputSectionBase &S, const Symbol &Sym,
84 std::string Msg = "\n>>> defined in ";
86 Msg += toString(Sym.File);
87 else if (Optional<std::string> Loc = getLinkerScriptLocation(Sym))
90 Msg += "\n>>> referenced by ";
91 std::string Src = S.getSrcMsg(Sym, Off);
93 Msg += Src + "\n>>> ";
94 return Msg + S.getObjMsg(Off);
97 // This function is similar to the `handleTlsRelocation`. MIPS does not
98 // support any relaxations for TLS relocations so by factoring out MIPS
99 // handling in to the separate function we can simplify the code and do not
100 // pollute other `handleTlsRelocation` by MIPS `ifs` statements.
101 // Mips has a custom MipsGotSection that handles the writing of GOT entries
102 // without dynamic relocations.
103 static unsigned handleMipsTlsRelocation(RelType Type, Symbol &Sym,
104 InputSectionBase &C, uint64_t Offset,
105 int64_t Addend, RelExpr Expr) {
106 if (Expr == R_MIPS_TLSLD) {
107 In.MipsGot->addTlsIndex(*C.File);
108 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
111 if (Expr == R_MIPS_TLSGD) {
112 In.MipsGot->addDynTlsEntry(*C.File, Sym);
113 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
119 // This function is similar to the `handleMipsTlsRelocation`. ARM also does not
120 // support any relaxations for TLS relocations. ARM is logically similar to Mips
121 // in how it handles TLS, but Mips uses its own custom GOT which handles some
122 // of the cases that ARM uses GOT relocations for.
124 // We look for TLS global dynamic and local dynamic relocations, these may
125 // require the generation of a pair of GOT entries that have associated
126 // dynamic relocations. When the results of the dynamic relocations can be
127 // resolved at static link time we do so. This is necessary for static linking
128 // as there will be no dynamic loader to resolve them at load-time.
130 // The pair of GOT entries created are of the form
131 // GOT[e0] Module Index (Used to find pointer to TLS block at run-time)
132 // GOT[e1] Offset of symbol in TLS block
133 template <class ELFT>
134 static unsigned handleARMTlsRelocation(RelType Type, Symbol &Sym,
135 InputSectionBase &C, uint64_t Offset,
136 int64_t Addend, RelExpr Expr) {
137 // The Dynamic TLS Module Index Relocation for a symbol defined in an
138 // executable is always 1. If the target Symbol is not preemptible then
139 // we know the offset into the TLS block at static link time.
140 bool NeedDynId = Sym.IsPreemptible || Config->Shared;
141 bool NeedDynOff = Sym.IsPreemptible;
143 auto AddTlsReloc = [&](uint64_t Off, RelType Type, Symbol *Dest, bool Dyn) {
145 In.RelaDyn->addReloc(Type, In.Got, Off, Dest);
147 In.Got->Relocations.push_back({R_ABS, Type, Off, 0, Dest});
150 // Local Dynamic is for access to module local TLS variables, while still
151 // being suitable for being dynamically loaded via dlopen.
152 // GOT[e0] is the module index, with a special value of 0 for the current
153 // module. GOT[e1] is unused. There only needs to be one module index entry.
154 if (Expr == R_TLSLD_PC && In.Got->addTlsIndex()) {
155 AddTlsReloc(In.Got->getTlsIndexOff(), Target->TlsModuleIndexRel,
156 NeedDynId ? nullptr : &Sym, NeedDynId);
157 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
161 // Global Dynamic is the most general purpose access model. When we know
162 // the module index and offset of symbol in TLS block we can fill these in
163 // using static GOT relocations.
164 if (Expr == R_TLSGD_PC) {
165 if (In.Got->addDynTlsEntry(Sym)) {
166 uint64_t Off = In.Got->getGlobalDynOffset(Sym);
167 AddTlsReloc(Off, Target->TlsModuleIndexRel, &Sym, NeedDynId);
168 AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Sym,
171 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
177 // Returns the number of relocations processed.
178 template <class ELFT>
180 handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
181 typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
185 if (Config->EMachine == EM_ARM)
186 return handleARMTlsRelocation<ELFT>(Type, Sym, C, Offset, Addend, Expr);
187 if (Config->EMachine == EM_MIPS)
188 return handleMipsTlsRelocation(Type, Sym, C, Offset, Addend, Expr);
190 if (isRelExprOneOf<R_TLSDESC, R_AARCH64_TLSDESC_PAGE, R_TLSDESC_CALL>(Expr) &&
192 if (In.Got->addDynTlsEntry(Sym)) {
193 uint64_t Off = In.Got->getGlobalDynOffset(Sym);
194 In.RelaDyn->addReloc(
195 {Target->TlsDescRel, In.Got, Off, !Sym.IsPreemptible, &Sym, 0});
197 if (Expr != R_TLSDESC_CALL)
198 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
202 if (isRelExprOneOf<R_TLSLD_GOT, R_TLSLD_GOT_FROM_END, R_TLSLD_PC,
203 R_TLSLD_HINT>(Expr)) {
204 // Local-Dynamic relocs can be relaxed to Local-Exec.
205 if (!Config->Shared) {
206 C.Relocations.push_back(
207 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_LD_TO_LE), Type,
208 Offset, Addend, &Sym});
209 return Target->TlsGdRelaxSkip;
211 if (Expr == R_TLSLD_HINT)
213 if (In.Got->addTlsIndex())
214 In.RelaDyn->addReloc(Target->TlsModuleIndexRel, In.Got,
215 In.Got->getTlsIndexOff(), nullptr);
216 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
220 // Local-Dynamic relocs can be relaxed to Local-Exec.
221 if (Expr == R_ABS && !Config->Shared) {
222 C.Relocations.push_back(
223 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_LD_TO_LE), Type,
224 Offset, Addend, &Sym});
228 // Local-Dynamic sequence where offset of tls variable relative to dynamic
229 // thread pointer is stored in the got.
230 if (Expr == R_TLSLD_GOT_OFF) {
231 // Local-Dynamic relocs can be relaxed to local-exec
232 if (!Config->Shared) {
233 C.Relocations.push_back({R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Sym});
236 if (!Sym.isInGot()) {
237 In.Got->addEntry(Sym);
238 uint64_t Off = Sym.getGotOffset();
239 In.Got->Relocations.push_back(
240 {R_ABS, Target->TlsOffsetRel, Off, 0, &Sym});
242 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
246 if (isRelExprOneOf<R_TLSDESC, R_AARCH64_TLSDESC_PAGE, R_TLSDESC_CALL,
247 R_TLSGD_GOT, R_TLSGD_GOT_FROM_END, R_TLSGD_PC>(Expr)) {
248 if (Config->Shared) {
249 if (In.Got->addDynTlsEntry(Sym)) {
250 uint64_t Off = In.Got->getGlobalDynOffset(Sym);
251 In.RelaDyn->addReloc(Target->TlsModuleIndexRel, In.Got, Off, &Sym);
253 // If the symbol is preemptible we need the dynamic linker to write
255 uint64_t OffsetOff = Off + Config->Wordsize;
256 if (Sym.IsPreemptible)
257 In.RelaDyn->addReloc(Target->TlsOffsetRel, In.Got, OffsetOff, &Sym);
259 In.Got->Relocations.push_back(
260 {R_ABS, Target->TlsOffsetRel, OffsetOff, 0, &Sym});
262 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
266 // Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec
267 // depending on the symbol being locally defined or not.
268 if (Sym.IsPreemptible) {
269 C.Relocations.push_back(
270 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type,
271 Offset, Addend, &Sym});
272 if (!Sym.isInGot()) {
273 In.Got->addEntry(Sym);
274 In.RelaDyn->addReloc(Target->TlsGotRel, In.Got, Sym.getGotOffset(),
278 C.Relocations.push_back(
279 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type,
280 Offset, Addend, &Sym});
282 return Target->TlsGdRelaxSkip;
285 // Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally
287 if (isRelExprOneOf<R_GOT, R_GOT_FROM_END, R_GOT_PC, R_AARCH64_GOT_PAGE_PC,
288 R_GOT_OFF, R_TLSIE_HINT>(Expr) &&
289 !Config->Shared && !Sym.IsPreemptible) {
290 C.Relocations.push_back({R_RELAX_TLS_IE_TO_LE, Type, Offset, Addend, &Sym});
294 if (Expr == R_TLSIE_HINT)
299 static RelType getMipsPairType(RelType Type, bool IsLocal) {
304 // In case of global symbol, the R_MIPS_GOT16 relocation does not
305 // have a pair. Each global symbol has a unique entry in the GOT
306 // and a corresponding instruction with help of the R_MIPS_GOT16
307 // relocation loads an address of the symbol. In case of local
308 // symbol, the R_MIPS_GOT16 relocation creates a GOT entry to hold
309 // the high 16 bits of the symbol's value. A paired R_MIPS_LO16
310 // relocations handle low 16 bits of the address. That allows
311 // to allocate only one GOT entry for every 64 KBytes of local data.
312 return IsLocal ? R_MIPS_LO16 : R_MIPS_NONE;
313 case R_MICROMIPS_GOT16:
314 return IsLocal ? R_MICROMIPS_LO16 : R_MIPS_NONE;
316 return R_MIPS_PCLO16;
317 case R_MICROMIPS_HI16:
318 return R_MICROMIPS_LO16;
324 // True if non-preemptable symbol always has the same value regardless of where
325 // the DSO is loaded.
326 static bool isAbsolute(const Symbol &Sym) {
327 if (Sym.isUndefWeak())
329 if (const auto *DR = dyn_cast<Defined>(&Sym))
330 return DR->Section == nullptr; // Absolute symbol.
334 static bool isAbsoluteValue(const Symbol &Sym) {
335 return isAbsolute(Sym) || Sym.isTls();
338 // Returns true if Expr refers a PLT entry.
339 static bool needsPlt(RelExpr Expr) {
340 return isRelExprOneOf<R_PLT_PC, R_PPC_CALL_PLT, R_PLT, R_AARCH64_PLT_PAGE_PC,
341 R_GOT_PLT, R_AARCH64_GOT_PAGE_PC_PLT>(Expr);
344 // Returns true if Expr refers a GOT entry. Note that this function
345 // returns false for TLS variables even though they need GOT, because
346 // TLS variables uses GOT differently than the regular variables.
347 static bool needsGot(RelExpr Expr) {
348 return isRelExprOneOf<R_GOT, R_GOT_OFF, R_HEXAGON_GOT, R_MIPS_GOT_LOCAL_PAGE,
349 R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_AARCH64_GOT_PAGE_PC,
350 R_AARCH64_GOT_PAGE_PC_PLT, R_GOT_PC, R_GOT_FROM_END,
354 // True if this expression is of the form Sym - X, where X is a position in the
355 // file (PC, or GOT for example).
356 static bool isRelExpr(RelExpr Expr) {
357 return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
358 R_PPC_CALL, R_PPC_CALL_PLT, R_AARCH64_PAGE_PC,
359 R_AARCH64_PLT_PAGE_PC, R_RELAX_GOT_PC>(Expr);
362 // Returns true if a given relocation can be computed at link-time.
364 // For instance, we know the offset from a relocation to its target at
365 // link-time if the relocation is PC-relative and refers a
366 // non-interposable function in the same executable. This function
367 // will return true for such relocation.
369 // If this function returns false, that means we need to emit a
370 // dynamic relocation so that the relocation will be fixed at load-time.
371 static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
372 InputSectionBase &S, uint64_t RelOff) {
373 // These expressions always compute a constant
374 if (isRelExprOneOf<R_GOT_FROM_END, R_GOT_OFF, R_HEXAGON_GOT, R_TLSLD_GOT_OFF,
375 R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOTREL, R_MIPS_GOT_OFF,
376 R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC, R_MIPS_TLSGD,
377 R_AARCH64_GOT_PAGE_PC, R_AARCH64_GOT_PAGE_PC_PLT, R_GOT_PC,
378 R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_PLT_PC, R_TLSGD_GOT,
379 R_TLSGD_GOT_FROM_END, R_TLSGD_PC, R_PPC_CALL_PLT,
380 R_TLSDESC_CALL, R_AARCH64_TLSDESC_PAGE, R_HINT,
381 R_TLSLD_HINT, R_TLSIE_HINT>(E))
384 // The computation involves output from the ifunc resolver.
385 if (Sym.isGnuIFunc() && Config->ZIfuncnoplt)
388 // These never do, except if the entire file is position dependent or if
389 // only the low bits are used.
390 if (E == R_GOT || E == R_GOT_PLT || E == R_PLT || E == R_TLSDESC)
391 return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
393 if (Sym.IsPreemptible)
398 // The size of a non preemptible symbol is a constant.
402 // For the target and the relocation, we want to know if they are
403 // absolute or relative.
404 bool AbsVal = isAbsoluteValue(Sym);
405 bool RelE = isRelExpr(E);
410 if (!AbsVal && !RelE)
411 return Target->usesOnlyLowPageBits(Type);
413 // Relative relocation to an absolute value. This is normally unrepresentable,
414 // but if the relocation refers to a weak undefined symbol, we allow it to
415 // resolve to the image base. This is a little strange, but it allows us to
416 // link function calls to such symbols. Normally such a call will be guarded
417 // with a comparison, which will load a zero from the GOT.
418 // Another special case is MIPS _gp_disp symbol which represents offset
419 // between start of a function and '_gp' value and defined as absolute just
420 // to simplify the code.
421 assert(AbsVal && RelE);
422 if (Sym.isUndefWeak())
425 error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
426 toString(Sym) + getLocation(S, Sym, RelOff));
430 static RelExpr toPlt(RelExpr Expr) {
433 return R_PPC_CALL_PLT;
436 case R_AARCH64_PAGE_PC:
437 return R_AARCH64_PLT_PAGE_PC;
438 case R_AARCH64_GOT_PAGE_PC:
439 return R_AARCH64_GOT_PAGE_PC_PLT;
449 static RelExpr fromPlt(RelExpr Expr) {
450 // We decided not to use a plt. Optimize a reference to the plt to a
451 // reference to the symbol itself.
464 // Returns true if a given shared symbol is in a read-only segment in a DSO.
465 template <class ELFT> static bool isReadOnly(SharedSymbol &SS) {
466 typedef typename ELFT::Phdr Elf_Phdr;
468 // Determine if the symbol is read-only by scanning the DSO's program headers.
469 const SharedFile<ELFT> &File = SS.getFile<ELFT>();
470 for (const Elf_Phdr &Phdr : check(File.getObj().program_headers()))
471 if ((Phdr.p_type == ELF::PT_LOAD || Phdr.p_type == ELF::PT_GNU_RELRO) &&
472 !(Phdr.p_flags & ELF::PF_W) && SS.Value >= Phdr.p_vaddr &&
473 SS.Value < Phdr.p_vaddr + Phdr.p_memsz)
478 // Returns symbols at the same offset as a given symbol, including SS itself.
480 // If two or more symbols are at the same offset, and at least one of
481 // them are copied by a copy relocation, all of them need to be copied.
482 // Otherwise, they would refer to different places at runtime.
483 template <class ELFT>
484 static SmallSet<SharedSymbol *, 4> getSymbolsAt(SharedSymbol &SS) {
485 typedef typename ELFT::Sym Elf_Sym;
487 SharedFile<ELFT> &File = SS.getFile<ELFT>();
489 SmallSet<SharedSymbol *, 4> Ret;
490 for (const Elf_Sym &S : File.getGlobalELFSyms()) {
491 if (S.st_shndx == SHN_UNDEF || S.st_shndx == SHN_ABS ||
492 S.getType() == STT_TLS || S.st_value != SS.Value)
494 StringRef Name = check(S.getName(File.getStringTable()));
495 Symbol *Sym = Symtab->find(Name);
496 if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
502 // When a symbol is copy relocated or we create a canonical plt entry, it is
503 // effectively a defined symbol. In the case of copy relocation the symbol is
504 // in .bss and in the case of a canonical plt entry it is in .plt. This function
505 // replaces the existing symbol with a Defined pointing to the appropriate
507 static void replaceWithDefined(Symbol &Sym, SectionBase *Sec, uint64_t Value,
510 replaceSymbol<Defined>(&Sym, Sym.File, Sym.getName(), Sym.Binding,
511 Sym.StOther, Sym.Type, Value, Size, Sec);
512 Sym.PltIndex = Old.PltIndex;
513 Sym.GotIndex = Old.GotIndex;
514 Sym.VerdefIndex = Old.VerdefIndex;
515 Sym.PPC64BranchltIndex = Old.PPC64BranchltIndex;
516 Sym.IsPreemptible = true;
517 Sym.ExportDynamic = true;
518 Sym.IsUsedInRegularObj = true;
522 // Reserve space in .bss or .bss.rel.ro for copy relocation.
524 // The copy relocation is pretty much a hack. If you use a copy relocation
525 // in your program, not only the symbol name but the symbol's size, RW/RO
526 // bit and alignment become part of the ABI. In addition to that, if the
527 // symbol has aliases, the aliases become part of the ABI. That's subtle,
528 // but if you violate that implicit ABI, that can cause very counter-
529 // intuitive consequences.
531 // So, what is the copy relocation? It's for linking non-position
532 // independent code to DSOs. In an ideal world, all references to data
533 // exported by DSOs should go indirectly through GOT. But if object files
534 // are compiled as non-PIC, all data references are direct. There is no
535 // way for the linker to transform the code to use GOT, as machine
536 // instructions are already set in stone in object files. This is where
537 // the copy relocation takes a role.
539 // A copy relocation instructs the dynamic linker to copy data from a DSO
540 // to a specified address (which is usually in .bss) at load-time. If the
541 // static linker (that's us) finds a direct data reference to a DSO
542 // symbol, it creates a copy relocation, so that the symbol can be
543 // resolved as if it were in .bss rather than in a DSO.
545 // As you can see in this function, we create a copy relocation for the
546 // dynamic linker, and the relocation contains not only symbol name but
547 // various other informtion about the symbol. So, such attributes become a
550 // Note for application developers: I can give you a piece of advice if
551 // you are writing a shared library. You probably should export only
552 // functions from your library. You shouldn't export variables.
554 // As an example what can happen when you export variables without knowing
555 // the semantics of copy relocations, assume that you have an exported
556 // variable of type T. It is an ABI-breaking change to add new members at
557 // end of T even though doing that doesn't change the layout of the
558 // existing members. That's because the space for the new members are not
559 // reserved in .bss unless you recompile the main program. That means they
560 // are likely to overlap with other data that happens to be laid out next
561 // to the variable in .bss. This kind of issue is sometimes very hard to
562 // debug. What's a solution? Instead of exporting a varaible V from a DSO,
563 // define an accessor getV().
564 template <class ELFT> static void addCopyRelSymbol(SharedSymbol &SS) {
565 // Copy relocation against zero-sized symbol doesn't make sense.
566 uint64_t SymSize = SS.getSize();
567 if (SymSize == 0 || SS.Alignment == 0)
568 fatal("cannot create a copy relocation for symbol " + toString(SS));
570 // See if this symbol is in a read-only segment. If so, preserve the symbol's
571 // memory protection by reserving space in the .bss.rel.ro section.
572 bool IsReadOnly = isReadOnly<ELFT>(SS);
573 BssSection *Sec = make<BssSection>(IsReadOnly ? ".bss.rel.ro" : ".bss",
574 SymSize, SS.Alignment);
576 In.BssRelRo->getParent()->addSection(Sec);
578 In.Bss->getParent()->addSection(Sec);
580 // Look through the DSO's dynamic symbol table for aliases and create a
581 // dynamic symbol for each one. This causes the copy relocation to correctly
582 // interpose any aliases.
583 for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS))
584 replaceWithDefined(*Sym, Sec, 0, Sym->Size);
586 In.RelaDyn->addReloc(Target->CopyRel, Sec, 0, &SS);
589 // MIPS has an odd notion of "paired" relocations to calculate addends.
590 // For example, if a relocation is of R_MIPS_HI16, there must be a
591 // R_MIPS_LO16 relocation after that, and an addend is calculated using
592 // the two relocations.
593 template <class ELFT, class RelTy>
594 static int64_t computeMipsAddend(const RelTy &Rel, const RelTy *End,
595 InputSectionBase &Sec, RelExpr Expr,
597 if (Expr == R_MIPS_GOTREL && IsLocal)
598 return Sec.getFile<ELFT>()->MipsGp0;
600 // The ABI says that the paired relocation is used only for REL.
601 // See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
605 RelType Type = Rel.getType(Config->IsMips64EL);
606 uint32_t PairTy = getMipsPairType(Type, IsLocal);
607 if (PairTy == R_MIPS_NONE)
610 const uint8_t *Buf = Sec.data().data();
611 uint32_t SymIndex = Rel.getSymbol(Config->IsMips64EL);
613 // To make things worse, paired relocations might not be contiguous in
614 // the relocation table, so we need to do linear search. *sigh*
615 for (const RelTy *RI = &Rel; RI != End; ++RI)
616 if (RI->getType(Config->IsMips64EL) == PairTy &&
617 RI->getSymbol(Config->IsMips64EL) == SymIndex)
618 return Target->getImplicitAddend(Buf + RI->r_offset, PairTy);
620 warn("can't find matching " + toString(PairTy) + " relocation for " +
625 // Returns an addend of a given relocation. If it is RELA, an addend
626 // is in a relocation itself. If it is REL, we need to read it from an
628 template <class ELFT, class RelTy>
629 static int64_t computeAddend(const RelTy &Rel, const RelTy *End,
630 InputSectionBase &Sec, RelExpr Expr,
633 RelType Type = Rel.getType(Config->IsMips64EL);
636 Addend = getAddend<ELFT>(Rel);
638 const uint8_t *Buf = Sec.data().data();
639 Addend = Target->getImplicitAddend(Buf + Rel.r_offset, Type);
642 if (Config->EMachine == EM_PPC64 && Config->Pic && Type == R_PPC64_TOC)
643 Addend += getPPC64TocBase();
644 if (Config->EMachine == EM_MIPS)
645 Addend += computeMipsAddend<ELFT>(Rel, End, Sec, Expr, IsLocal);
650 // Report an undefined symbol if necessary.
651 // Returns true if this function printed out an error message.
652 static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
654 if (Sym.isLocal() || !Sym.isUndefined() || Sym.isWeak())
658 Sym.computeBinding() != STB_LOCAL && Sym.Visibility == STV_DEFAULT;
659 if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
663 "undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
665 std::string Src = Sec.getSrcMsg(Sym, Offset);
667 Msg += Src + "\n>>> ";
668 Msg += Sec.getObjMsg(Offset);
670 if (Sym.getName().startswith("_ZTV"))
671 Msg += "\nthe vtable symbol may be undefined because the class is missing "
672 "its key function (see https://lld.llvm.org/missingkeyfunction)";
674 if ((Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal) ||
675 Config->NoinhibitExec) {
684 // MIPS N32 ABI treats series of successive relocations with the same offset
685 // as a single relocation. The similar approach used by N64 ABI, but this ABI
686 // packs all relocations into the single relocation record. Here we emulate
687 // this for the N32 ABI. Iterate over relocation with the same offset and put
688 // theirs types into the single bit-set.
689 template <class RelTy> static RelType getMipsN32RelType(RelTy *&Rel, RelTy *End) {
691 uint64_t Offset = Rel->r_offset;
694 while (Rel != End && Rel->r_offset == Offset)
695 Type |= (Rel++)->getType(Config->IsMips64EL) << (8 * N++);
699 // .eh_frame sections are mergeable input sections, so their input
700 // offsets are not linearly mapped to output section. For each input
701 // offset, we need to find a section piece containing the offset and
702 // add the piece's base address to the input offset to compute the
703 // output offset. That isn't cheap.
705 // This class is to speed up the offset computation. When we process
706 // relocations, we access offsets in the monotonically increasing
707 // order. So we can optimize for that access pattern.
709 // For sections other than .eh_frame, this class doesn't do anything.
713 explicit OffsetGetter(InputSectionBase &Sec) {
714 if (auto *Eh = dyn_cast<EhInputSection>(&Sec))
718 // Translates offsets in input sections to offsets in output sections.
719 // Given offset must increase monotonically. We assume that Piece is
720 // sorted by InputOff.
721 uint64_t get(uint64_t Off) {
725 while (I != Pieces.size() && Pieces[I].InputOff + Pieces[I].Size <= Off)
727 if (I == Pieces.size())
728 fatal(".eh_frame: relocation is not in any piece");
730 // Pieces must be contiguous, so there must be no holes in between.
731 assert(Pieces[I].InputOff <= Off && "Relocation not in any piece");
733 // Offset -1 means that the piece is dead (i.e. garbage collected).
734 if (Pieces[I].OutputOff == -1)
736 return Pieces[I].OutputOff + Off - Pieces[I].InputOff;
740 ArrayRef<EhSectionPiece> Pieces;
745 static void addRelativeReloc(InputSectionBase *IS, uint64_t OffsetInSec,
746 Symbol *Sym, int64_t Addend, RelExpr Expr,
748 // Add a relative relocation. If RelrDyn section is enabled, and the
749 // relocation offset is guaranteed to be even, add the relocation to
750 // the RelrDyn section, otherwise add it to the RelaDyn section.
751 // RelrDyn sections don't support odd offsets. Also, RelrDyn sections
752 // don't store the addend values, so we must write it to the relocated
754 if (In.RelrDyn && IS->Alignment >= 2 && OffsetInSec % 2 == 0) {
755 IS->Relocations.push_back({Expr, Type, OffsetInSec, Addend, Sym});
756 In.RelrDyn->Relocs.push_back({IS, OffsetInSec});
759 In.RelaDyn->addReloc(Target->RelativeRel, IS, OffsetInSec, Sym, Addend, Expr,
763 template <class ELFT, class GotPltSection>
764 static void addPltEntry(PltSection *Plt, GotPltSection *GotPlt,
765 RelocationBaseSection *Rel, RelType Type, Symbol &Sym) {
766 Plt->addEntry<ELFT>(Sym);
767 GotPlt->addEntry(Sym);
769 {Type, GotPlt, Sym.getGotPltOffset(), !Sym.IsPreemptible, &Sym, 0});
772 template <class ELFT> static void addGotEntry(Symbol &Sym) {
773 In.Got->addEntry(Sym);
778 else if (Sym.isGnuIFunc())
783 uint64_t Off = Sym.getGotOffset();
785 // If a GOT slot value can be calculated at link-time, which is now,
786 // we can just fill that out.
788 // (We don't actually write a value to a GOT slot right now, but we
789 // add a static relocation to a Relocations vector so that
790 // InputSection::relocate will do the work for us. We may be able
791 // to just write a value now, but it is a TODO.)
792 bool IsLinkTimeConstant =
793 !Sym.IsPreemptible && (!Config->Pic || isAbsolute(Sym));
794 if (IsLinkTimeConstant) {
795 In.Got->Relocations.push_back({Expr, Target->GotRel, Off, 0, &Sym});
799 // Otherwise, we emit a dynamic relocation to .rel[a].dyn so that
800 // the GOT slot will be fixed at load-time.
801 if (!Sym.isTls() && !Sym.IsPreemptible && Config->Pic && !isAbsolute(Sym)) {
802 addRelativeReloc(In.Got, Off, &Sym, 0, R_ABS, Target->GotRel);
805 In.RelaDyn->addReloc(Sym.isTls() ? Target->TlsGotRel : Target->GotRel, In.Got,
806 Off, &Sym, 0, Sym.IsPreemptible ? R_ADDEND : R_ABS,
810 // Return true if we can define a symbol in the executable that
811 // contains the value/function of a symbol defined in a shared
813 static bool canDefineSymbolInExecutable(Symbol &Sym) {
814 // If the symbol has default visibility the symbol defined in the
815 // executable will preempt it.
816 // Note that we want the visibility of the shared symbol itself, not
817 // the visibility of the symbol in the output file we are producing. That is
818 // why we use Sym.StOther.
819 if ((Sym.StOther & 0x3) == STV_DEFAULT)
822 // If we are allowed to break address equality of functions, defining
823 // a plt entry will allow the program to call the function in the
824 // .so, but the .so and the executable will no agree on the address
825 // of the function. Similar logic for objects.
826 return ((Sym.isFunc() && Config->IgnoreFunctionAddressEquality) ||
827 (Sym.isObject() && Config->IgnoreDataAddressEquality));
830 // The reason we have to do this early scan is as follows
831 // * To mmap the output file, we need to know the size
832 // * For that, we need to know how many dynamic relocs we will have.
833 // It might be possible to avoid this by outputting the file with write:
834 // * Write the allocated output sections, computing addresses.
835 // * Apply relocations, recording which ones require a dynamic reloc.
836 // * Write the dynamic relocations.
837 // * Write the rest of the file.
838 // This would have some drawbacks. For example, we would only know if .rela.dyn
839 // is needed after applying relocations. If it is, it will go after rw and rx
840 // sections. Given that it is ro, we will need an extra PT_LOAD. This
841 // complicates things for the dynamic linker and means we would have to reserve
842 // space for the extra PT_LOAD even if we end up not using it.
843 template <class ELFT, class RelTy>
844 static void processRelocAux(InputSectionBase &Sec, RelExpr Expr, RelType Type,
845 uint64_t Offset, Symbol &Sym, const RelTy &Rel,
847 if (isStaticLinkTimeConstant(Expr, Type, Sym, Sec, Offset)) {
848 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
851 if (Sym.isGnuIFunc() && Config->ZIfuncnoplt) {
852 In.RelaDyn->addReloc(Type, &Sec, Offset, &Sym, Addend, R_ADDEND, Type);
855 bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
857 // R_GOT refers to a position in the got, even if the symbol is preemptible.
858 bool IsPreemptibleValue = Sym.IsPreemptible && Expr != R_GOT;
860 if (!IsPreemptibleValue) {
861 addRelativeReloc(&Sec, Offset, &Sym, Addend, Expr, Type);
863 } else if (RelType Rel = Target->getDynRel(Type)) {
864 In.RelaDyn->addReloc(Rel, &Sec, Offset, &Sym, Addend, R_ADDEND, Type);
866 // MIPS ABI turns using of GOT and dynamic relocations inside out.
867 // While regular ABI uses dynamic relocations to fill up GOT entries
868 // MIPS ABI requires dynamic linker to fills up GOT entries using
869 // specially sorted dynamic symbol table. This affects even dynamic
870 // relocations against symbols which do not require GOT entries
871 // creation explicitly, i.e. do not have any GOT-relocations. So if
872 // a preemptible symbol has a dynamic relocation we anyway have
873 // to create a GOT entry for it.
874 // If a non-preemptible symbol has a dynamic relocation against it,
875 // dynamic linker takes it st_value, adds offset and writes down
876 // result of the dynamic relocation. In case of preemptible symbol
877 // dynamic linker performs symbol resolution, writes the symbol value
878 // to the GOT entry and reads the GOT entry when it needs to perform
879 // a dynamic relocation.
880 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19
881 if (Config->EMachine == EM_MIPS)
882 In.MipsGot->addEntry(*Sec.File, Sym, Addend, Expr);
887 // If the relocation is to a weak undef, and we are producing
888 // executable, give up on it and produce a non preemptible 0.
889 if (!Config->Shared && Sym.isUndefWeak()) {
890 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
894 if (!CanWrite && (Config->Pic && !isRelExpr(Expr))) {
896 "can't create dynamic relocation " + toString(Type) + " against " +
897 (Sym.getName().empty() ? "local symbol" : "symbol: " + toString(Sym)) +
898 " in readonly segment; recompile object files with -fPIC "
899 "or pass '-Wl,-z,notext' to allow text relocations in the output" +
900 getLocation(Sec, Sym, Offset));
904 // Copy relocations are only possible if we are creating an executable.
905 if (Config->Shared) {
906 errorOrWarn("relocation " + toString(Type) +
907 " cannot be used against symbol " + toString(Sym) +
908 "; recompile with -fPIC" + getLocation(Sec, Sym, Offset));
912 // If the symbol is undefined we already reported any relevant errors.
913 if (Sym.isUndefined())
916 if (!canDefineSymbolInExecutable(Sym)) {
917 error("cannot preempt symbol: " + toString(Sym) +
918 getLocation(Sec, Sym, Offset));
922 if (Sym.isObject()) {
923 // Produce a copy relocation.
924 if (auto *SS = dyn_cast<SharedSymbol>(&Sym)) {
925 if (!Config->ZCopyreloc)
926 error("unresolvable relocation " + toString(Type) +
927 " against symbol '" + toString(*SS) +
928 "'; recompile with -fPIC or remove '-z nocopyreloc'" +
929 getLocation(Sec, Sym, Offset));
930 addCopyRelSymbol<ELFT>(*SS);
932 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
937 // This handles a non PIC program call to function in a shared library. In
938 // an ideal world, we could just report an error saying the relocation can
939 // overflow at runtime. In the real world with glibc, crt1.o has a
940 // R_X86_64_PC32 pointing to libc.so.
942 // The general idea on how to handle such cases is to create a PLT entry and
943 // use that as the function value.
945 // For the static linking part, we just return a plt expr and everything
946 // else will use the PLT entry as the address.
948 // The remaining problem is making sure pointer equality still works. We
949 // need the help of the dynamic linker for that. We let it know that we have
950 // a direct reference to a so symbol by creating an undefined symbol with a
951 // non zero st_value. Seeing that, the dynamic linker resolves the symbol to
952 // the value of the symbol we created. This is true even for got entries, so
953 // pointer equality is maintained. To avoid an infinite loop, the only entry
954 // that points to the real function is a dedicated got entry used by the
955 // plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT,
956 // R_386_JMP_SLOT, etc).
958 // For position independent executable on i386, the plt entry requires ebx
959 // to be set. This causes two problems:
960 // * If some code has a direct reference to a function, it was probably
961 // compiled without -fPIE/-fPIC and doesn't maintain ebx.
962 // * If a library definition gets preempted to the executable, it will have
963 // the wrong ebx value.
964 if (Config->Pie && Config->EMachine == EM_386)
965 errorOrWarn("symbol '" + toString(Sym) +
966 "' cannot be preempted; recompile with -fPIE" +
967 getLocation(Sec, Sym, Offset));
969 addPltEntry<ELFT>(In.Plt, In.GotPlt, In.RelaPlt, Target->PltRel, Sym);
970 if (!Sym.isDefined())
971 replaceWithDefined(Sym, In.Plt, getPltEntryOffset(Sym.PltIndex), 0);
972 Sym.NeedsPltAddr = true;
973 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
977 errorOrWarn("symbol '" + toString(Sym) + "' has no type" +
978 getLocation(Sec, Sym, Offset));
981 template <class ELFT, class RelTy>
982 static void scanReloc(InputSectionBase &Sec, OffsetGetter &GetOffset, RelTy *&I,
984 const RelTy &Rel = *I;
985 Symbol &Sym = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
988 // Deal with MIPS oddity.
989 if (Config->MipsN32Abi) {
990 Type = getMipsN32RelType(I, End);
992 Type = Rel.getType(Config->IsMips64EL);
996 // Get an offset in an output section this relocation is applied to.
997 uint64_t Offset = GetOffset.get(Rel.r_offset);
998 if (Offset == uint64_t(-1))
1001 // Skip if the target symbol is an erroneous undefined symbol.
1002 if (maybeReportUndefined(Sym, Sec, Rel.r_offset))
1005 const uint8_t *RelocatedAddr = Sec.data().begin() + Rel.r_offset;
1006 RelExpr Expr = Target->getRelExpr(Type, Sym, RelocatedAddr);
1008 // Ignore "hint" relocations because they are only markers for relaxation.
1009 if (isRelExprOneOf<R_HINT, R_NONE>(Expr))
1012 // Strenghten or relax relocations.
1014 // GNU ifunc symbols must be accessed via PLT because their addresses
1015 // are determined by runtime.
1017 // On the other hand, if we know that a PLT entry will be resolved within
1018 // the same ELF module, we can skip PLT access and directly jump to the
1019 // destination function. For example, if we are linking a main exectuable,
1020 // all dynamic symbols that can be resolved within the executable will
1021 // actually be resolved that way at runtime, because the main exectuable
1022 // is always at the beginning of a search list. We can leverage that fact.
1023 if (Sym.isGnuIFunc() && !Config->ZIfuncnoplt) {
1024 if (!Config->ZText && Config->WarnIfuncTextrel) {
1025 warn("using ifunc symbols when text relocations are allowed may produce "
1026 "a binary that will segfault, if the object file is linked with "
1027 "old version of glibc (glibc 2.28 and earlier). If this applies to "
1028 "you, consider recompiling the object files without -fPIC and "
1029 "without -Wl,-z,notext option. Use -no-warn-ifunc-textrel to "
1030 "turn off this warning." +
1031 getLocation(Sec, Sym, Offset));
1034 } else if (!Sym.IsPreemptible && Expr == R_GOT_PC && !isAbsoluteValue(Sym)) {
1035 Expr = Target->adjustRelaxExpr(Type, RelocatedAddr, Expr);
1036 } else if (!Sym.IsPreemptible) {
1037 Expr = fromPlt(Expr);
1040 // This relocation does not require got entry, but it is relative to got and
1041 // needs it to be created. Here we request for that.
1042 if (isRelExprOneOf<R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_GOTREL,
1043 R_GOTREL_FROM_END, R_PPC_TOC>(Expr))
1044 In.Got->HasGotOffRel = true;
1047 int64_t Addend = computeAddend<ELFT>(Rel, End, Sec, Expr, Sym.isLocal());
1049 // Process some TLS relocations, including relaxing TLS relocations.
1050 // Note that this function does not handle all TLS relocations.
1051 if (unsigned Processed =
1052 handleTlsRelocation<ELFT>(Type, Sym, Sec, Offset, Addend, Expr)) {
1053 I += (Processed - 1);
1057 // If a relocation needs PLT, we create PLT and GOTPLT slots for the symbol.
1058 if (needsPlt(Expr) && !Sym.isInPlt()) {
1059 if (Sym.isGnuIFunc() && !Sym.IsPreemptible)
1060 addPltEntry<ELFT>(In.Iplt, In.IgotPlt, In.RelaIplt, Target->IRelativeRel,
1063 addPltEntry<ELFT>(In.Plt, In.GotPlt, In.RelaPlt, Target->PltRel, Sym);
1066 // Create a GOT slot if a relocation needs GOT.
1067 if (needsGot(Expr)) {
1068 if (Config->EMachine == EM_MIPS) {
1069 // MIPS ABI has special rules to process GOT entries and doesn't
1070 // require relocation entries for them. A special case is TLS
1071 // relocations. In that case dynamic loader applies dynamic
1072 // relocations to initialize TLS GOT entries.
1073 // See "Global Offset Table" in Chapter 5 in the following document
1074 // for detailed description:
1075 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
1076 In.MipsGot->addEntry(*Sec.File, Sym, Addend, Expr);
1077 } else if (!Sym.isInGot()) {
1078 addGotEntry<ELFT>(Sym);
1082 processRelocAux<ELFT>(Sec, Expr, Type, Offset, Sym, Rel, Addend);
1085 template <class ELFT, class RelTy>
1086 static void scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
1087 OffsetGetter GetOffset(Sec);
1089 // Not all relocations end up in Sec.Relocations, but a lot do.
1090 Sec.Relocations.reserve(Rels.size());
1092 for (auto I = Rels.begin(), End = Rels.end(); I != End;)
1093 scanReloc<ELFT>(Sec, GetOffset, I, End);
1095 // Sort relocations by offset to binary search for R_RISCV_PCREL_HI20
1096 if (Config->EMachine == EM_RISCV)
1097 std::stable_sort(Sec.Relocations.begin(), Sec.Relocations.end(),
1098 RelocationOffsetComparator{});
1101 template <class ELFT> void elf::scanRelocations(InputSectionBase &S) {
1102 if (S.AreRelocsRela)
1103 scanRelocs<ELFT>(S, S.relas<ELFT>());
1105 scanRelocs<ELFT>(S, S.rels<ELFT>());
1108 static bool mergeCmp(const InputSection *A, const InputSection *B) {
1109 // std::merge requires a strict weak ordering.
1110 if (A->OutSecOff < B->OutSecOff)
1113 if (A->OutSecOff == B->OutSecOff) {
1114 auto *TA = dyn_cast<ThunkSection>(A);
1115 auto *TB = dyn_cast<ThunkSection>(B);
1117 // Check if Thunk is immediately before any specific Target
1118 // InputSection for example Mips LA25 Thunks.
1119 if (TA && TA->getTargetInputSection() == B)
1122 // Place Thunk Sections without specific targets before
1123 // non-Thunk Sections.
1124 if (TA && !TB && !TA->getTargetInputSection())
1131 // Call Fn on every executable InputSection accessed via the linker script
1132 // InputSectionDescription::Sections.
1133 static void forEachInputSectionDescription(
1134 ArrayRef<OutputSection *> OutputSections,
1135 llvm::function_ref<void(OutputSection *, InputSectionDescription *)> Fn) {
1136 for (OutputSection *OS : OutputSections) {
1137 if (!(OS->Flags & SHF_ALLOC) || !(OS->Flags & SHF_EXECINSTR))
1139 for (BaseCommand *BC : OS->SectionCommands)
1140 if (auto *ISD = dyn_cast<InputSectionDescription>(BC))
1145 // Thunk Implementation
1147 // Thunks (sometimes called stubs, veneers or branch islands) are small pieces
1148 // of code that the linker inserts inbetween a caller and a callee. The thunks
1149 // are added at link time rather than compile time as the decision on whether
1150 // a thunk is needed, such as the caller and callee being out of range, can only
1151 // be made at link time.
1153 // It is straightforward to tell given the current state of the program when a
1154 // thunk is needed for a particular call. The more difficult part is that
1155 // the thunk needs to be placed in the program such that the caller can reach
1156 // the thunk and the thunk can reach the callee; furthermore, adding thunks to
1157 // the program alters addresses, which can mean more thunks etc.
1159 // In lld we have a synthetic ThunkSection that can hold many Thunks.
1160 // The decision to have a ThunkSection act as a container means that we can
1161 // more easily handle the most common case of a single block of contiguous
1162 // Thunks by inserting just a single ThunkSection.
1164 // The implementation of Thunks in lld is split across these areas
1165 // Relocations.cpp : Framework for creating and placing thunks
1166 // Thunks.cpp : The code generated for each supported thunk
1167 // Target.cpp : Target specific hooks that the framework uses to decide when
1169 // Synthetic.cpp : Implementation of ThunkSection
1170 // Writer.cpp : Iteratively call framework until no more Thunks added
1172 // Thunk placement requirements:
1173 // Mips LA25 thunks. These must be placed immediately before the callee section
1174 // We can assume that the caller is in range of the Thunk. These are modelled
1175 // by Thunks that return the section they must precede with
1176 // getTargetInputSection().
1178 // ARM interworking and range extension thunks. These thunks must be placed
1179 // within range of the caller. All implemented ARM thunks can always reach the
1180 // callee as they use an indirect jump via a register that has no range
1183 // Thunk placement algorithm:
1184 // For Mips LA25 ThunkSections; the placement is explicit, it has to be before
1185 // getTargetInputSection().
1187 // For thunks that must be placed within range of the caller there are many
1188 // possible choices given that the maximum range from the caller is usually
1189 // much larger than the average InputSection size. Desirable properties include:
1190 // - Maximize reuse of thunks by multiple callers
1191 // - Minimize number of ThunkSections to simplify insertion
1192 // - Handle impact of already added Thunks on addresses
1193 // - Simple to understand and implement
1195 // In lld for the first pass, we pre-create one or more ThunkSections per
1196 // InputSectionDescription at Target specific intervals. A ThunkSection is
1197 // placed so that the estimated end of the ThunkSection is within range of the
1198 // start of the InputSectionDescription or the previous ThunkSection. For
1200 // InputSectionDescription
1210 // The intention is that we can add a Thunk to a ThunkSection that is well
1211 // spaced enough to service a number of callers without having to do a lot
1212 // of work. An important principle is that it is not an error if a Thunk cannot
1213 // be placed in a pre-created ThunkSection; when this happens we create a new
1214 // ThunkSection placed next to the caller. This allows us to handle the vast
1215 // majority of thunks simply, but also handle rare cases where the branch range
1216 // is smaller than the target specific spacing.
1218 // The algorithm is expected to create all the thunks that are needed in a
1219 // single pass, with a small number of programs needing a second pass due to
1220 // the insertion of thunks in the first pass increasing the offset between
1221 // callers and callees that were only just in range.
1223 // A consequence of allowing new ThunkSections to be created outside of the
1224 // pre-created ThunkSections is that in rare cases calls to Thunks that were in
1225 // range in pass K, are out of range in some pass > K due to the insertion of
1226 // more Thunks in between the caller and callee. When this happens we retarget
1227 // the relocation back to the original target and create another Thunk.
1229 // Remove ThunkSections that are empty, this should only be the initial set
1230 // precreated on pass 0.
1232 // Insert the Thunks for OutputSection OS into their designated place
1233 // in the Sections vector, and recalculate the InputSection output section
1235 // This may invalidate any output section offsets stored outside of InputSection
1236 void ThunkCreator::mergeThunks(ArrayRef<OutputSection *> OutputSections) {
1237 forEachInputSectionDescription(
1238 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1239 if (ISD->ThunkSections.empty())
1242 // Remove any zero sized precreated Thunks.
1243 llvm::erase_if(ISD->ThunkSections,
1244 [](const std::pair<ThunkSection *, uint32_t> &TS) {
1245 return TS.first->getSize() == 0;
1248 // ISD->ThunkSections contains all created ThunkSections, including
1249 // those inserted in previous passes. Extract the Thunks created this
1250 // pass and order them in ascending OutSecOff.
1251 std::vector<ThunkSection *> NewThunks;
1252 for (const std::pair<ThunkSection *, uint32_t> TS : ISD->ThunkSections)
1253 if (TS.second == Pass)
1254 NewThunks.push_back(TS.first);
1255 std::stable_sort(NewThunks.begin(), NewThunks.end(),
1256 [](const ThunkSection *A, const ThunkSection *B) {
1257 return A->OutSecOff < B->OutSecOff;
1260 // Merge sorted vectors of Thunks and InputSections by OutSecOff
1261 std::vector<InputSection *> Tmp;
1262 Tmp.reserve(ISD->Sections.size() + NewThunks.size());
1264 std::merge(ISD->Sections.begin(), ISD->Sections.end(),
1265 NewThunks.begin(), NewThunks.end(), std::back_inserter(Tmp),
1268 ISD->Sections = std::move(Tmp);
1272 // Find or create a ThunkSection within the InputSectionDescription (ISD) that
1273 // is in range of Src. An ISD maps to a range of InputSections described by a
1274 // linker script section pattern such as { .text .text.* }.
1275 ThunkSection *ThunkCreator::getISDThunkSec(OutputSection *OS, InputSection *IS,
1276 InputSectionDescription *ISD,
1277 uint32_t Type, uint64_t Src) {
1278 for (std::pair<ThunkSection *, uint32_t> TP : ISD->ThunkSections) {
1279 ThunkSection *TS = TP.first;
1280 uint64_t TSBase = OS->Addr + TS->OutSecOff;
1281 uint64_t TSLimit = TSBase + TS->getSize();
1282 if (Target->inBranchRange(Type, Src, (Src > TSLimit) ? TSBase : TSLimit))
1286 // No suitable ThunkSection exists. This can happen when there is a branch
1287 // with lower range than the ThunkSection spacing or when there are too
1288 // many Thunks. Create a new ThunkSection as close to the InputSection as
1289 // possible. Error if InputSection is so large we cannot place ThunkSection
1290 // anywhere in Range.
1291 uint64_t ThunkSecOff = IS->OutSecOff;
1292 if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff)) {
1293 ThunkSecOff = IS->OutSecOff + IS->getSize();
1294 if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff))
1295 fatal("InputSection too large for range extension thunk " +
1296 IS->getObjMsg(Src - (OS->Addr + IS->OutSecOff)));
1298 return addThunkSection(OS, ISD, ThunkSecOff);
1301 // Add a Thunk that needs to be placed in a ThunkSection that immediately
1302 // precedes its Target.
1303 ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS) {
1304 ThunkSection *TS = ThunkedSections.lookup(IS);
1308 // Find InputSectionRange within Target Output Section (TOS) that the
1309 // InputSection (IS) that we need to precede is in.
1310 OutputSection *TOS = IS->getParent();
1311 for (BaseCommand *BC : TOS->SectionCommands) {
1312 auto *ISD = dyn_cast<InputSectionDescription>(BC);
1313 if (!ISD || ISD->Sections.empty())
1316 InputSection *First = ISD->Sections.front();
1317 InputSection *Last = ISD->Sections.back();
1319 if (IS->OutSecOff < First->OutSecOff || Last->OutSecOff < IS->OutSecOff)
1322 TS = addThunkSection(TOS, ISD, IS->OutSecOff);
1323 ThunkedSections[IS] = TS;
1330 // Create one or more ThunkSections per OS that can be used to place Thunks.
1331 // We attempt to place the ThunkSections using the following desirable
1333 // - Within range of the maximum number of callers
1334 // - Minimise the number of ThunkSections
1336 // We follow a simple but conservative heuristic to place ThunkSections at
1337 // offsets that are multiples of a Target specific branch range.
1338 // For an InputSectionDescription that is smaller than the range, a single
1339 // ThunkSection at the end of the range will do.
1341 // For an InputSectionDescription that is more than twice the size of the range,
1342 // we place the last ThunkSection at range bytes from the end of the
1343 // InputSectionDescription in order to increase the likelihood that the
1344 // distance from a thunk to its target will be sufficiently small to
1345 // allow for the creation of a short thunk.
1346 void ThunkCreator::createInitialThunkSections(
1347 ArrayRef<OutputSection *> OutputSections) {
1348 uint32_t ThunkSectionSpacing = Target->getThunkSectionSpacing();
1350 forEachInputSectionDescription(
1351 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1352 if (ISD->Sections.empty())
1355 uint32_t ISDBegin = ISD->Sections.front()->OutSecOff;
1357 ISD->Sections.back()->OutSecOff + ISD->Sections.back()->getSize();
1358 uint32_t LastThunkLowerBound = -1;
1359 if (ISDEnd - ISDBegin > ThunkSectionSpacing * 2)
1360 LastThunkLowerBound = ISDEnd - ThunkSectionSpacing;
1363 uint32_t PrevISLimit = ISDBegin;
1364 uint32_t ThunkUpperBound = ISDBegin + ThunkSectionSpacing;
1366 for (const InputSection *IS : ISD->Sections) {
1367 ISLimit = IS->OutSecOff + IS->getSize();
1368 if (ISLimit > ThunkUpperBound) {
1369 addThunkSection(OS, ISD, PrevISLimit);
1370 ThunkUpperBound = PrevISLimit + ThunkSectionSpacing;
1372 if (ISLimit > LastThunkLowerBound)
1374 PrevISLimit = ISLimit;
1376 addThunkSection(OS, ISD, ISLimit);
1380 ThunkSection *ThunkCreator::addThunkSection(OutputSection *OS,
1381 InputSectionDescription *ISD,
1383 auto *TS = make<ThunkSection>(OS, Off);
1384 ISD->ThunkSections.push_back({TS, Pass});
1388 std::pair<Thunk *, bool> ThunkCreator::getThunk(Symbol &Sym, RelType Type,
1390 std::vector<Thunk *> *ThunkVec = nullptr;
1392 // We use (section, offset) pair to find the thunk position if possible so
1393 // that we create only one thunk for aliased symbols or ICFed sections.
1394 if (auto *D = dyn_cast<Defined>(&Sym))
1395 if (!D->isInPlt() && D->Section)
1396 ThunkVec = &ThunkedSymbolsBySection[{D->Section->Repl, D->Value}];
1398 ThunkVec = &ThunkedSymbols[&Sym];
1400 // Check existing Thunks for Sym to see if they can be reused
1401 for (Thunk *T : *ThunkVec)
1402 if (T->isCompatibleWith(Type) &&
1403 Target->inBranchRange(Type, Src, T->getThunkTargetSym()->getVA()))
1404 return std::make_pair(T, false);
1406 // No existing compatible Thunk in range, create a new one
1407 Thunk *T = addThunk(Type, Sym);
1408 ThunkVec->push_back(T);
1409 return std::make_pair(T, true);
1412 // Return true if the relocation target is an in range Thunk.
1413 // Return false if the relocation is not to a Thunk. If the relocation target
1414 // was originally to a Thunk, but is no longer in range we revert the
1415 // relocation back to its original non-Thunk target.
1416 bool ThunkCreator::normalizeExistingThunk(Relocation &Rel, uint64_t Src) {
1417 if (Thunk *T = Thunks.lookup(Rel.Sym)) {
1418 if (Target->inBranchRange(Rel.Type, Src, Rel.Sym->getVA()))
1420 Rel.Sym = &T->Destination;
1421 if (Rel.Sym->isInPlt())
1422 Rel.Expr = toPlt(Rel.Expr);
1427 // Process all relocations from the InputSections that have been assigned
1428 // to InputSectionDescriptions and redirect through Thunks if needed. The
1429 // function should be called iteratively until it returns false.
1432 // All InputSections that may need a Thunk are reachable from
1433 // OutputSectionCommands.
1435 // All OutputSections have an address and all InputSections have an offset
1436 // within the OutputSection.
1438 // The offsets between caller (relocation place) and callee
1439 // (relocation target) will not be modified outside of createThunks().
1442 // If return value is true then ThunkSections have been inserted into
1443 // OutputSections. All relocations that needed a Thunk based on the information
1444 // available to createThunks() on entry have been redirected to a Thunk. Note
1445 // that adding Thunks changes offsets between caller and callee so more Thunks
1448 // If return value is false then no more Thunks are needed, and createThunks has
1449 // made no changes. If the target requires range extension thunks, currently
1450 // ARM, then any future change in offset between caller and callee risks a
1451 // relocation out of range error.
1452 bool ThunkCreator::createThunks(ArrayRef<OutputSection *> OutputSections) {
1453 bool AddressesChanged = false;
1455 if (Pass == 0 && Target->getThunkSectionSpacing())
1456 createInitialThunkSections(OutputSections);
1458 // With Thunk Size much smaller than branch range we expect to
1459 // converge quickly; if we get to 10 something has gone wrong.
1461 fatal("thunk creation not converged");
1463 // Create all the Thunks and insert them into synthetic ThunkSections. The
1464 // ThunkSections are later inserted back into InputSectionDescriptions.
1465 // We separate the creation of ThunkSections from the insertion of the
1466 // ThunkSections as ThunkSections are not always inserted into the same
1467 // InputSectionDescription as the caller.
1468 forEachInputSectionDescription(
1469 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1470 for (InputSection *IS : ISD->Sections)
1471 for (Relocation &Rel : IS->Relocations) {
1472 uint64_t Src = IS->getVA(Rel.Offset);
1474 // If we are a relocation to an existing Thunk, check if it is
1475 // still in range. If not then Rel will be altered to point to its
1476 // original target so another Thunk can be generated.
1477 if (Pass > 0 && normalizeExistingThunk(Rel, Src))
1480 if (!Target->needsThunk(Rel.Expr, Rel.Type, IS->File, Src,
1486 std::tie(T, IsNew) = getThunk(*Rel.Sym, Rel.Type, Src);
1489 // Find or create a ThunkSection for the new Thunk
1491 if (auto *TIS = T->getTargetInputSection())
1492 TS = getISThunkSec(TIS);
1494 TS = getISDThunkSec(OS, IS, ISD, Rel.Type, Src);
1496 Thunks[T->getThunkTargetSym()] = T;
1499 // Redirect relocation to Thunk, we never go via the PLT to a Thunk
1500 Rel.Sym = T->getThunkTargetSym();
1501 Rel.Expr = fromPlt(Rel.Expr);
1504 for (auto &P : ISD->ThunkSections)
1505 AddressesChanged |= P.first->assignOffsets();
1508 for (auto &P : ThunkedSections)
1509 AddressesChanged |= P.second->assignOffsets();
1511 // Merge all created synthetic ThunkSections back into OutputSection
1512 mergeThunks(OutputSections);
1514 return AddressesChanged;
1517 template void elf::scanRelocations<ELF32LE>(InputSectionBase &);
1518 template void elf::scanRelocations<ELF32BE>(InputSectionBase &);
1519 template void elf::scanRelocations<ELF64LE>(InputSectionBase &);
1520 template void elf::scanRelocations<ELF64BE>(InputSectionBase &);