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"
49 #include "SymbolTable.h"
51 #include "SyntheticSections.h"
54 #include "lld/Common/Memory.h"
56 #include "llvm/Support/Endian.h"
57 #include "llvm/Support/raw_ostream.h"
61 using namespace llvm::ELF;
62 using namespace llvm::object;
63 using namespace llvm::support::endian;
66 using namespace lld::elf;
68 // Construct a message in the following format.
70 // >>> defined in /home/alice/src/foo.o
71 // >>> referenced by bar.c:12 (/home/alice/src/bar.c:12)
72 // >>> /home/alice/src/bar.o:(.text+0x1)
74 static std::string getLocation(InputSectionBase &S, const Symbol &Sym,
77 "\n>>> defined in " + toString(Sym.File) + "\n>>> referenced by ";
78 std::string Src = S.getSrcMsg<ELFT>(Sym, Off);
80 Msg += Src + "\n>>> ";
81 return Msg + S.getObjMsg(Off);
84 // This is a MIPS-specific rule.
86 // In case of MIPS GP-relative relocations always resolve to a definition
87 // in a regular input file, ignoring the one-definition rule. So we,
88 // for example, should not attempt to create a dynamic relocation even
89 // if the target symbol is preemptible. There are two two MIPS GP-relative
90 // relocations R_MIPS_GPREL16 and R_MIPS_GPREL32. But only R_MIPS_GPREL16
91 // can be against a preemptible symbol.
93 // To get MIPS relocation type we apply 0xff mask. In case of O32 ABI all
94 // relocation types occupy eight bit. In case of N64 ABI we extract first
95 // relocation from 3-in-1 packet because only the first relocation can
96 // be against a real symbol.
97 static bool isMipsGprel(RelType Type) {
98 if (Config->EMachine != EM_MIPS)
101 return Type == R_MIPS_GPREL16 || Type == R_MICROMIPS_GPREL16 ||
102 Type == R_MICROMIPS_GPREL7_S2;
105 // This function is similar to the `handleTlsRelocation`. MIPS does not
106 // support any relaxations for TLS relocations so by factoring out MIPS
107 // handling in to the separate function we can simplify the code and do not
108 // pollute other `handleTlsRelocation` by MIPS `ifs` statements.
109 // Mips has a custom MipsGotSection that handles the writing of GOT entries
110 // without dynamic relocations.
111 template <class ELFT>
112 static unsigned handleMipsTlsRelocation(RelType Type, Symbol &Sym,
113 InputSectionBase &C, uint64_t Offset,
114 int64_t Addend, RelExpr Expr) {
115 if (Expr == R_MIPS_TLSLD) {
116 if (InX::MipsGot->addTlsIndex() && Config->Pic)
117 InX::RelaDyn->addReloc({Target->TlsModuleIndexRel, InX::MipsGot,
118 InX::MipsGot->getTlsIndexOff(), false, nullptr,
120 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
124 if (Expr == R_MIPS_TLSGD) {
125 if (InX::MipsGot->addDynTlsEntry(Sym) && Sym.IsPreemptible) {
126 uint64_t Off = InX::MipsGot->getGlobalDynOffset(Sym);
127 InX::RelaDyn->addReloc(
128 {Target->TlsModuleIndexRel, InX::MipsGot, Off, false, &Sym, 0});
129 if (Sym.IsPreemptible)
130 InX::RelaDyn->addReloc({Target->TlsOffsetRel, InX::MipsGot,
131 Off + Config->Wordsize, false, &Sym, 0});
133 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
139 // This function is similar to the `handleMipsTlsRelocation`. ARM also does not
140 // support any relaxations for TLS relocations. ARM is logically similar to Mips
141 // in how it handles TLS, but Mips uses its own custom GOT which handles some
142 // of the cases that ARM uses GOT relocations for.
144 // We look for TLS global dynamic and local dynamic relocations, these may
145 // require the generation of a pair of GOT entries that have associated
146 // dynamic relocations. When the results of the dynamic relocations can be
147 // resolved at static link time we do so. This is necessary for static linking
148 // as there will be no dynamic loader to resolve them at load-time.
150 // The pair of GOT entries created are of the form
151 // GOT[e0] Module Index (Used to find pointer to TLS block at run-time)
152 // GOT[e1] Offset of symbol in TLS block
153 template <class ELFT>
154 static unsigned handleARMTlsRelocation(RelType Type, Symbol &Sym,
155 InputSectionBase &C, uint64_t Offset,
156 int64_t Addend, RelExpr Expr) {
157 // The Dynamic TLS Module Index Relocation for a symbol defined in an
158 // executable is always 1. If the target Symbol is not preemptible then
159 // we know the offset into the TLS block at static link time.
160 bool NeedDynId = Sym.IsPreemptible || Config->Shared;
161 bool NeedDynOff = Sym.IsPreemptible;
163 auto AddTlsReloc = [&](uint64_t Off, RelType Type, Symbol *Dest, bool Dyn) {
165 InX::RelaDyn->addReloc({Type, InX::Got, Off, false, Dest, 0});
167 InX::Got->Relocations.push_back({R_ABS, Type, Off, 0, Dest});
170 // Local Dynamic is for access to module local TLS variables, while still
171 // being suitable for being dynamically loaded via dlopen.
172 // GOT[e0] is the module index, with a special value of 0 for the current
173 // module. GOT[e1] is unused. There only needs to be one module index entry.
174 if (Expr == R_TLSLD_PC && InX::Got->addTlsIndex()) {
175 AddTlsReloc(InX::Got->getTlsIndexOff(), Target->TlsModuleIndexRel,
176 NeedDynId ? nullptr : &Sym, NeedDynId);
177 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
181 // Global Dynamic is the most general purpose access model. When we know
182 // the module index and offset of symbol in TLS block we can fill these in
183 // using static GOT relocations.
184 if (Expr == R_TLSGD_PC) {
185 if (InX::Got->addDynTlsEntry(Sym)) {
186 uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
187 AddTlsReloc(Off, Target->TlsModuleIndexRel, &Sym, NeedDynId);
188 AddTlsReloc(Off + Config->Wordsize, Target->TlsOffsetRel, &Sym,
191 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
197 // Returns the number of relocations processed.
198 template <class ELFT>
200 handleTlsRelocation(RelType Type, Symbol &Sym, InputSectionBase &C,
201 typename ELFT::uint Offset, int64_t Addend, RelExpr Expr) {
202 if (!(C.Flags & SHF_ALLOC))
208 if (Config->EMachine == EM_ARM)
209 return handleARMTlsRelocation<ELFT>(Type, Sym, C, Offset, Addend, Expr);
210 if (Config->EMachine == EM_MIPS)
211 return handleMipsTlsRelocation<ELFT>(Type, Sym, C, Offset, Addend, Expr);
213 if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL>(Expr) &&
215 if (InX::Got->addDynTlsEntry(Sym)) {
216 uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
217 InX::RelaDyn->addReloc(
218 {Target->TlsDescRel, InX::Got, Off, !Sym.IsPreemptible, &Sym, 0});
220 if (Expr != R_TLSDESC_CALL)
221 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
225 if (isRelExprOneOf<R_TLSLD_PC, R_TLSLD>(Expr)) {
226 // Local-Dynamic relocs can be relaxed to Local-Exec.
227 if (!Config->Shared) {
228 C.Relocations.push_back(
229 {R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Sym});
232 if (InX::Got->addTlsIndex())
233 InX::RelaDyn->addReloc({Target->TlsModuleIndexRel, InX::Got,
234 InX::Got->getTlsIndexOff(), false, nullptr, 0});
235 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
239 // Local-Dynamic relocs can be relaxed to Local-Exec.
240 if (isRelExprOneOf<R_ABS, R_TLSLD, R_TLSLD_PC>(Expr) && !Config->Shared) {
241 C.Relocations.push_back({R_RELAX_TLS_LD_TO_LE, Type, Offset, Addend, &Sym});
245 if (isRelExprOneOf<R_TLSDESC, R_TLSDESC_PAGE, R_TLSDESC_CALL, R_TLSGD,
247 if (Config->Shared) {
248 if (InX::Got->addDynTlsEntry(Sym)) {
249 uint64_t Off = InX::Got->getGlobalDynOffset(Sym);
250 InX::RelaDyn->addReloc(
251 {Target->TlsModuleIndexRel, InX::Got, Off, false, &Sym, 0});
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 InX::RelaDyn->addReloc(
258 {Target->TlsOffsetRel, InX::Got, OffsetOff, false, &Sym, 0});
260 InX::Got->Relocations.push_back(
261 {R_ABS, Target->TlsOffsetRel, OffsetOff, 0, &Sym});
263 C.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
267 // Global-Dynamic relocs can be relaxed to Initial-Exec or Local-Exec
268 // depending on the symbol being locally defined or not.
269 if (Sym.IsPreemptible) {
270 C.Relocations.push_back(
271 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_IE), Type,
272 Offset, Addend, &Sym});
273 if (!Sym.isInGot()) {
274 InX::Got->addEntry(Sym);
275 InX::RelaDyn->addReloc(
276 {Target->TlsGotRel, InX::Got, Sym.getGotOffset(), false, &Sym, 0});
279 C.Relocations.push_back(
280 {Target->adjustRelaxExpr(Type, nullptr, R_RELAX_TLS_GD_TO_LE), Type,
281 Offset, Addend, &Sym});
283 return Target->TlsGdRelaxSkip;
286 // Initial-Exec relocs can be relaxed to Local-Exec if the symbol is locally
288 if (isRelExprOneOf<R_GOT, R_GOT_FROM_END, R_GOT_PC, R_GOT_PAGE_PC>(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_TLSDESC_CALL)
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_PLT_OPD, R_PLT, R_PLT_PAGE_PC>(Expr);
343 // Returns true if Expr refers a GOT entry. Note that this function
344 // returns false for TLS variables even though they need GOT, because
345 // TLS variables uses GOT differently than the regular variables.
346 static bool needsGot(RelExpr Expr) {
347 return isRelExprOneOf<R_GOT, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOT_OFF,
348 R_MIPS_GOT_OFF32, R_GOT_PAGE_PC, R_GOT_PC,
349 R_GOT_FROM_END>(Expr);
352 // True if this expression is of the form Sym - X, where X is a position in the
353 // file (PC, or GOT for example).
354 static bool isRelExpr(RelExpr Expr) {
355 return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
356 R_PAGE_PC, R_RELAX_GOT_PC>(Expr);
359 // Returns true if a given relocation can be computed at link-time.
361 // For instance, we know the offset from a relocation to its target at
362 // link-time if the relocation is PC-relative and refers a
363 // non-interposable function in the same executable. This function
364 // will return true for such relocation.
366 // If this function returns false, that means we need to emit a
367 // dynamic relocation so that the relocation will be fixed at load-time.
368 template <class ELFT>
369 static bool isStaticLinkTimeConstant(RelExpr E, RelType Type, const Symbol &Sym,
370 InputSectionBase &S, uint64_t RelOff) {
371 // These expressions always compute a constant
372 if (isRelExprOneOf<R_SIZE, R_GOT_FROM_END, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE,
373 R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_MIPS_GOT_GP_PC,
374 R_MIPS_TLSGD, R_GOT_PAGE_PC, R_GOT_PC, R_GOTONLY_PC,
375 R_GOTONLY_PC_FROM_END, R_PLT_PC, R_TLSGD_PC, R_TLSGD,
376 R_PPC_PLT_OPD, R_TLSDESC_CALL, R_TLSDESC_PAGE, R_HINT>(E))
379 // These never do, except if the entire file is position dependent or if
380 // only the low bits are used.
381 if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
382 return Target->usesOnlyLowPageBits(Type) || !Config->Pic;
384 if (Sym.IsPreemptible)
389 // For the target and the relocation, we want to know if they are
390 // absolute or relative.
391 bool AbsVal = isAbsoluteValue(Sym);
392 bool RelE = isRelExpr(E);
397 if (!AbsVal && !RelE)
398 return Target->usesOnlyLowPageBits(Type);
400 // Relative relocation to an absolute value. This is normally unrepresentable,
401 // but if the relocation refers to a weak undefined symbol, we allow it to
402 // resolve to the image base. This is a little strange, but it allows us to
403 // link function calls to such symbols. Normally such a call will be guarded
404 // with a comparison, which will load a zero from the GOT.
405 // Another special case is MIPS _gp_disp symbol which represents offset
406 // between start of a function and '_gp' value and defined as absolute just
407 // to simplify the code.
408 assert(AbsVal && RelE);
409 if (Sym.isUndefWeak())
412 error("relocation " + toString(Type) + " cannot refer to absolute symbol: " +
413 toString(Sym) + getLocation<ELFT>(S, Sym, RelOff));
417 static RelExpr toPlt(RelExpr Expr) {
418 if (Expr == R_PPC_OPD)
419 return R_PPC_PLT_OPD;
422 if (Expr == R_PAGE_PC)
423 return R_PLT_PAGE_PC;
429 static RelExpr fromPlt(RelExpr Expr) {
430 // We decided not to use a plt. Optimize a reference to the plt to a
431 // reference to the symbol itself.
432 if (Expr == R_PLT_PC)
434 if (Expr == R_PPC_PLT_OPD)
441 // Returns true if a given shared symbol is in a read-only segment in a DSO.
442 template <class ELFT> static bool isReadOnly(SharedSymbol *SS) {
443 typedef typename ELFT::Phdr Elf_Phdr;
445 // Determine if the symbol is read-only by scanning the DSO's program headers.
446 const SharedFile<ELFT> *File = SS->getFile<ELFT>();
447 for (const Elf_Phdr &Phdr : check(File->getObj().program_headers()))
448 if ((Phdr.p_type == ELF::PT_LOAD || Phdr.p_type == ELF::PT_GNU_RELRO) &&
449 !(Phdr.p_flags & ELF::PF_W) && SS->Value >= Phdr.p_vaddr &&
450 SS->Value < Phdr.p_vaddr + Phdr.p_memsz)
455 // Returns symbols at the same offset as a given symbol, including SS itself.
457 // If two or more symbols are at the same offset, and at least one of
458 // them are copied by a copy relocation, all of them need to be copied.
459 // Otherwise, they would refer different places at runtime.
460 template <class ELFT>
461 static std::vector<SharedSymbol *> getSymbolsAt(SharedSymbol *SS) {
462 typedef typename ELFT::Sym Elf_Sym;
464 SharedFile<ELFT> *File = SS->getFile<ELFT>();
466 std::vector<SharedSymbol *> Ret;
467 for (const Elf_Sym &S : File->getGlobalELFSyms()) {
468 if (S.st_shndx == SHN_UNDEF || S.st_shndx == SHN_ABS ||
469 S.st_value != SS->Value)
471 StringRef Name = check(S.getName(File->getStringTable()));
472 Symbol *Sym = Symtab->find(Name);
473 if (auto *Alias = dyn_cast_or_null<SharedSymbol>(Sym))
474 Ret.push_back(Alias);
479 // Reserve space in .bss or .bss.rel.ro for copy relocation.
481 // The copy relocation is pretty much a hack. If you use a copy relocation
482 // in your program, not only the symbol name but the symbol's size, RW/RO
483 // bit and alignment become part of the ABI. In addition to that, if the
484 // symbol has aliases, the aliases become part of the ABI. That's subtle,
485 // but if you violate that implicit ABI, that can cause very counter-
486 // intuitive consequences.
488 // So, what is the copy relocation? It's for linking non-position
489 // independent code to DSOs. In an ideal world, all references to data
490 // exported by DSOs should go indirectly through GOT. But if object files
491 // are compiled as non-PIC, all data references are direct. There is no
492 // way for the linker to transform the code to use GOT, as machine
493 // instructions are already set in stone in object files. This is where
494 // the copy relocation takes a role.
496 // A copy relocation instructs the dynamic linker to copy data from a DSO
497 // to a specified address (which is usually in .bss) at load-time. If the
498 // static linker (that's us) finds a direct data reference to a DSO
499 // symbol, it creates a copy relocation, so that the symbol can be
500 // resolved as if it were in .bss rather than in a DSO.
502 // As you can see in this function, we create a copy relocation for the
503 // dynamic linker, and the relocation contains not only symbol name but
504 // various other informtion about the symbol. So, such attributes become a
507 // Note for application developers: I can give you a piece of advice if
508 // you are writing a shared library. You probably should export only
509 // functions from your library. You shouldn't export variables.
511 // As an example what can happen when you export variables without knowing
512 // the semantics of copy relocations, assume that you have an exported
513 // variable of type T. It is an ABI-breaking change to add new members at
514 // end of T even though doing that doesn't change the layout of the
515 // existing members. That's because the space for the new members are not
516 // reserved in .bss unless you recompile the main program. That means they
517 // are likely to overlap with other data that happens to be laid out next
518 // to the variable in .bss. This kind of issue is sometimes very hard to
519 // debug. What's a solution? Instead of exporting a varaible V from a DSO,
520 // define an accessor getV().
521 template <class ELFT> static void addCopyRelSymbol(SharedSymbol *SS) {
522 // Copy relocation against zero-sized symbol doesn't make sense.
523 uint64_t SymSize = SS->getSize();
525 fatal("cannot create a copy relocation for symbol " + toString(*SS));
527 // See if this symbol is in a read-only segment. If so, preserve the symbol's
528 // memory protection by reserving space in the .bss.rel.ro section.
529 bool IsReadOnly = isReadOnly<ELFT>(SS);
530 BssSection *Sec = make<BssSection>(IsReadOnly ? ".bss.rel.ro" : ".bss",
531 SymSize, SS->Alignment);
533 InX::BssRelRo->getParent()->addSection(Sec);
535 InX::Bss->getParent()->addSection(Sec);
537 // Look through the DSO's dynamic symbol table for aliases and create a
538 // dynamic symbol for each one. This causes the copy relocation to correctly
539 // interpose any aliases.
540 for (SharedSymbol *Sym : getSymbolsAt<ELFT>(SS)) {
541 Sym->CopyRelSec = Sec;
542 Sym->IsPreemptible = false;
543 Sym->IsUsedInRegularObj = true;
547 InX::RelaDyn->addReloc({Target->CopyRel, Sec, 0, false, SS, 0});
550 static void errorOrWarn(const Twine &Msg) {
551 if (!Config->NoinhibitExec)
557 template <class ELFT>
558 static RelExpr adjustExpr(Symbol &Sym, RelExpr Expr, RelType Type,
559 InputSectionBase &S, uint64_t RelOff) {
560 // We can create any dynamic relocation if a section is simply writable.
561 if (S.Flags & SHF_WRITE)
564 // Or, if we are allowed to create dynamic relocations against
565 // read-only sections (i.e. unless "-z notext" is given),
566 // we can create a dynamic relocation as we want, too.
570 // If a relocation can be applied at link-time, we don't need to
571 // create a dynamic relocation in the first place.
572 if (isStaticLinkTimeConstant<ELFT>(Expr, Type, Sym, S, RelOff))
575 // If we got here we know that this relocation would require the dynamic
576 // linker to write a value to read only memory.
578 // If the relocation is to a weak undef, give up on it and produce a
579 // non preemptible 0.
580 if (Sym.isUndefWeak()) {
581 Sym.IsPreemptible = false;
585 // We can hack around it if we are producing an executable and
586 // the refered symbol can be preemepted to refer to the executable.
587 if (Config->Shared || (Config->Pic && !isRelExpr(Expr))) {
589 "can't create dynamic relocation " + toString(Type) + " against " +
590 (Sym.getName().empty() ? "local symbol" : "symbol: " + toString(Sym)) +
591 " in readonly segment; recompile object files with -fPIC" +
592 getLocation<ELFT>(S, Sym, RelOff));
596 if (Sym.getVisibility() != STV_DEFAULT) {
597 error("cannot preempt symbol: " + toString(Sym) +
598 getLocation<ELFT>(S, Sym, RelOff));
602 if (Sym.isObject()) {
603 // Produce a copy relocation.
604 auto *B = dyn_cast<SharedSymbol>(&Sym);
605 if (B && !B->CopyRelSec) {
606 if (Config->ZNocopyreloc)
607 error("unresolvable relocation " + toString(Type) +
608 " against symbol '" + toString(*B) +
609 "'; recompile with -fPIC or remove '-z nocopyreloc'" +
610 getLocation<ELFT>(S, Sym, RelOff));
612 addCopyRelSymbol<ELFT>(B);
618 // This handles a non PIC program call to function in a shared library. In
619 // an ideal world, we could just report an error saying the relocation can
620 // overflow at runtime. In the real world with glibc, crt1.o has a
621 // R_X86_64_PC32 pointing to libc.so.
623 // The general idea on how to handle such cases is to create a PLT entry and
624 // use that as the function value.
626 // For the static linking part, we just return a plt expr and everything
627 // else will use the the PLT entry as the address.
629 // The remaining problem is making sure pointer equality still works. We
630 // need the help of the dynamic linker for that. We let it know that we have
631 // a direct reference to a so symbol by creating an undefined symbol with a
632 // non zero st_value. Seeing that, the dynamic linker resolves the symbol to
633 // the value of the symbol we created. This is true even for got entries, so
634 // pointer equality is maintained. To avoid an infinite loop, the only entry
635 // that points to the real function is a dedicated got entry used by the
636 // plt. That is identified by special relocation types (R_X86_64_JUMP_SLOT,
637 // R_386_JMP_SLOT, etc).
638 Sym.NeedsPltAddr = true;
639 Sym.IsPreemptible = false;
643 errorOrWarn("symbol '" + toString(Sym) + "' defined in " +
644 toString(Sym.File) + " has no type");
648 // MIPS has an odd notion of "paired" relocations to calculate addends.
649 // For example, if a relocation is of R_MIPS_HI16, there must be a
650 // R_MIPS_LO16 relocation after that, and an addend is calculated using
651 // the two relocations.
652 template <class ELFT, class RelTy>
653 static int64_t computeMipsAddend(const RelTy &Rel, const RelTy *End,
654 InputSectionBase &Sec, RelExpr Expr,
656 if (Expr == R_MIPS_GOTREL && IsLocal)
657 return Sec.getFile<ELFT>()->MipsGp0;
659 // The ABI says that the paired relocation is used only for REL.
660 // See p. 4-17 at ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
664 RelType Type = Rel.getType(Config->IsMips64EL);
665 uint32_t PairTy = getMipsPairType(Type, IsLocal);
666 if (PairTy == R_MIPS_NONE)
669 const uint8_t *Buf = Sec.Data.data();
670 uint32_t SymIndex = Rel.getSymbol(Config->IsMips64EL);
672 // To make things worse, paired relocations might not be contiguous in
673 // the relocation table, so we need to do linear search. *sigh*
674 for (const RelTy *RI = &Rel; RI != End; ++RI)
675 if (RI->getType(Config->IsMips64EL) == PairTy &&
676 RI->getSymbol(Config->IsMips64EL) == SymIndex)
677 return Target->getImplicitAddend(Buf + RI->r_offset, PairTy);
679 warn("can't find matching " + toString(PairTy) + " relocation for " +
684 // Returns an addend of a given relocation. If it is RELA, an addend
685 // is in a relocation itself. If it is REL, we need to read it from an
687 template <class ELFT, class RelTy>
688 static int64_t computeAddend(const RelTy &Rel, const RelTy *End,
689 InputSectionBase &Sec, RelExpr Expr,
692 RelType Type = Rel.getType(Config->IsMips64EL);
695 Addend = getAddend<ELFT>(Rel);
697 const uint8_t *Buf = Sec.Data.data();
698 Addend = Target->getImplicitAddend(Buf + Rel.r_offset, Type);
701 if (Config->EMachine == EM_PPC64 && Config->Pic && Type == R_PPC64_TOC)
702 Addend += getPPC64TocBase();
703 if (Config->EMachine == EM_MIPS)
704 Addend += computeMipsAddend<ELFT>(Rel, End, Sec, Expr, IsLocal);
709 // Report an undefined symbol if necessary.
710 // Returns true if this function printed out an error message.
711 template <class ELFT>
712 static bool maybeReportUndefined(Symbol &Sym, InputSectionBase &Sec,
714 if (Config->UnresolvedSymbols == UnresolvedPolicy::IgnoreAll)
717 if (Sym.isLocal() || !Sym.isUndefined() || Sym.isWeak())
721 Sym.computeBinding() != STB_LOCAL && Sym.getVisibility() == STV_DEFAULT;
722 if (Config->UnresolvedSymbols == UnresolvedPolicy::Ignore && CanBeExternal)
726 "undefined symbol: " + toString(Sym) + "\n>>> referenced by ";
728 std::string Src = Sec.getSrcMsg<ELFT>(Sym, Offset);
730 Msg += Src + "\n>>> ";
731 Msg += Sec.getObjMsg(Offset);
733 if ((Config->UnresolvedSymbols == UnresolvedPolicy::Warn && CanBeExternal) ||
734 Config->NoinhibitExec) {
743 // MIPS N32 ABI treats series of successive relocations with the same offset
744 // as a single relocation. The similar approach used by N64 ABI, but this ABI
745 // packs all relocations into the single relocation record. Here we emulate
746 // this for the N32 ABI. Iterate over relocation with the same offset and put
747 // theirs types into the single bit-set.
748 template <class RelTy> static RelType getMipsN32RelType(RelTy *&Rel, RelTy *End) {
749 RelType Type = Rel->getType(Config->IsMips64EL);
750 uint64_t Offset = Rel->r_offset;
753 while (Rel + 1 != End && (Rel + 1)->r_offset == Offset)
754 Type |= (++Rel)->getType(Config->IsMips64EL) << (8 * ++N);
758 // .eh_frame sections are mergeable input sections, so their input
759 // offsets are not linearly mapped to output section. For each input
760 // offset, we need to find a section piece containing the offset and
761 // add the piece's base address to the input offset to compute the
762 // output offset. That isn't cheap.
764 // This class is to speed up the offset computation. When we process
765 // relocations, we access offsets in the monotonically increasing
766 // order. So we can optimize for that access pattern.
768 // For sections other than .eh_frame, this class doesn't do anything.
772 explicit OffsetGetter(InputSectionBase &Sec) {
773 if (auto *Eh = dyn_cast<EhInputSection>(&Sec))
777 // Translates offsets in input sections to offsets in output sections.
778 // Given offset must increase monotonically. We assume that Piece is
779 // sorted by InputOff.
780 uint64_t get(uint64_t Off) {
784 while (I != Pieces.size() && Pieces[I].InputOff + Pieces[I].Size <= Off)
786 if (I == Pieces.size())
789 // Pieces must be contiguous, so there must be no holes in between.
790 assert(Pieces[I].InputOff <= Off && "Relocation not in any piece");
792 // Offset -1 means that the piece is dead (i.e. garbage collected).
793 if (Pieces[I].OutputOff == -1)
795 return Pieces[I].OutputOff + Off - Pieces[I].InputOff;
799 ArrayRef<EhSectionPiece> Pieces;
804 template <class ELFT, class GotPltSection>
805 static void addPltEntry(PltSection *Plt, GotPltSection *GotPlt,
806 RelocationBaseSection *Rel, RelType Type, Symbol &Sym,
808 Plt->addEntry<ELFT>(Sym);
809 GotPlt->addEntry(Sym);
810 Rel->addReloc({Type, GotPlt, Sym.getGotPltOffset(), UseSymVA, &Sym, 0});
813 template <class ELFT> static void addGotEntry(Symbol &Sym, bool Preemptible) {
814 InX::Got->addEntry(Sym);
816 RelExpr Expr = Sym.isTls() ? R_TLS : R_ABS;
817 uint64_t Off = Sym.getGotOffset();
819 // If a GOT slot value can be calculated at link-time, which is now,
820 // we can just fill that out.
822 // (We don't actually write a value to a GOT slot right now, but we
823 // add a static relocation to a Relocations vector so that
824 // InputSection::relocate will do the work for us. We may be able
825 // to just write a value now, but it is a TODO.)
826 bool IsLinkTimeConstant = !Preemptible && (!Config->Pic || isAbsolute(Sym));
827 if (IsLinkTimeConstant) {
828 InX::Got->Relocations.push_back({Expr, Target->GotRel, Off, 0, &Sym});
832 // Otherwise, we emit a dynamic relocation to .rel[a].dyn so that
833 // the GOT slot will be fixed at load-time.
836 Type = Target->TlsGotRel;
837 else if (!Preemptible && Config->Pic && !isAbsolute(Sym))
838 Type = Target->RelativeRel;
840 Type = Target->GotRel;
841 InX::RelaDyn->addReloc({Type, InX::Got, Off, !Preemptible, &Sym, 0});
843 // REL type relocations don't have addend fields unlike RELAs, and
844 // their addends are stored to the section to which they are applied.
845 // So, store addends if we need to.
847 // This is ugly -- the difference between REL and RELA should be
848 // handled in a better way. It's a TODO.
849 if (!Config->IsRela && !Preemptible)
850 InX::Got->Relocations.push_back({R_ABS, Target->GotRel, Off, 0, &Sym});
853 // The reason we have to do this early scan is as follows
854 // * To mmap the output file, we need to know the size
855 // * For that, we need to know how many dynamic relocs we will have.
856 // It might be possible to avoid this by outputting the file with write:
857 // * Write the allocated output sections, computing addresses.
858 // * Apply relocations, recording which ones require a dynamic reloc.
859 // * Write the dynamic relocations.
860 // * Write the rest of the file.
861 // This would have some drawbacks. For example, we would only know if .rela.dyn
862 // is needed after applying relocations. If it is, it will go after rw and rx
863 // sections. Given that it is ro, we will need an extra PT_LOAD. This
864 // complicates things for the dynamic linker and means we would have to reserve
865 // space for the extra PT_LOAD even if we end up not using it.
866 template <class ELFT, class RelTy>
867 static void scanRelocs(InputSectionBase &Sec, ArrayRef<RelTy> Rels) {
868 OffsetGetter GetOffset(Sec);
870 // Not all relocations end up in Sec.Relocations, but a lot do.
871 Sec.Relocations.reserve(Rels.size());
873 for (auto I = Rels.begin(), End = Rels.end(); I != End; ++I) {
874 const RelTy &Rel = *I;
875 Symbol &Sym = Sec.getFile<ELFT>()->getRelocTargetSym(Rel);
876 RelType Type = Rel.getType(Config->IsMips64EL);
878 // Deal with MIPS oddity.
879 if (Config->MipsN32Abi)
880 Type = getMipsN32RelType(I, End);
882 // Get an offset in an output section this relocation is applied to.
883 uint64_t Offset = GetOffset.get(Rel.r_offset);
884 if (Offset == uint64_t(-1))
887 // Skip if the target symbol is an erroneous undefined symbol.
888 if (maybeReportUndefined<ELFT>(Sym, Sec, Rel.r_offset))
892 Target->getRelExpr(Type, Sym, Sec.Data.begin() + Rel.r_offset);
894 // Ignore "hint" relocations because they are only markers for relaxation.
895 if (isRelExprOneOf<R_HINT, R_NONE>(Expr))
898 // Handle yet another MIPS-ness.
899 if (isMipsGprel(Type)) {
900 int64_t Addend = computeAddend<ELFT>(Rel, End, Sec, Expr, Sym.isLocal());
901 Sec.Relocations.push_back({R_MIPS_GOTREL, Type, Offset, Addend, &Sym});
905 bool Preemptible = Sym.IsPreemptible;
907 // Strenghten or relax a PLT access.
909 // GNU ifunc symbols must be accessed via PLT because their addresses
910 // are determined by runtime.
912 // On the other hand, if we know that a PLT entry will be resolved within
913 // the same ELF module, we can skip PLT access and directly jump to the
914 // destination function. For example, if we are linking a main exectuable,
915 // all dynamic symbols that can be resolved within the executable will
916 // actually be resolved that way at runtime, because the main exectuable
917 // is always at the beginning of a search list. We can leverage that fact.
918 if (Sym.isGnuIFunc())
920 else if (!Preemptible && Expr == R_GOT_PC && !isAbsoluteValue(Sym))
922 Target->adjustRelaxExpr(Type, Sec.Data.data() + Rel.r_offset, Expr);
923 else if (!Preemptible)
924 Expr = fromPlt(Expr);
926 Expr = adjustExpr<ELFT>(Sym, Expr, Type, Sec, Rel.r_offset);
930 // This relocation does not require got entry, but it is relative to got and
931 // needs it to be created. Here we request for that.
932 if (isRelExprOneOf<R_GOTONLY_PC, R_GOTONLY_PC_FROM_END, R_GOTREL,
933 R_GOTREL_FROM_END, R_PPC_TOC>(Expr))
934 InX::Got->HasGotOffRel = true;
937 int64_t Addend = computeAddend<ELFT>(Rel, End, Sec, Expr, Sym.isLocal());
939 // Process some TLS relocations, including relaxing TLS relocations.
940 // Note that this function does not handle all TLS relocations.
941 if (unsigned Processed =
942 handleTlsRelocation<ELFT>(Type, Sym, Sec, Offset, Addend, Expr)) {
943 I += (Processed - 1);
947 // If a relocation needs PLT, we create PLT and GOTPLT slots for the symbol.
948 if (needsPlt(Expr) && !Sym.isInPlt()) {
949 if (Sym.isGnuIFunc() && !Preemptible)
950 addPltEntry<ELFT>(InX::Iplt, InX::IgotPlt, InX::RelaIplt,
951 Target->IRelativeRel, Sym, true);
953 addPltEntry<ELFT>(InX::Plt, InX::GotPlt, InX::RelaPlt, Target->PltRel,
957 // Create a GOT slot if a relocation needs GOT.
958 if (needsGot(Expr)) {
959 if (Config->EMachine == EM_MIPS) {
960 // MIPS ABI has special rules to process GOT entries and doesn't
961 // require relocation entries for them. A special case is TLS
962 // relocations. In that case dynamic loader applies dynamic
963 // relocations to initialize TLS GOT entries.
964 // See "Global Offset Table" in Chapter 5 in the following document
965 // for detailed description:
966 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
967 InX::MipsGot->addEntry(Sym, Addend, Expr);
968 if (Sym.isTls() && Sym.IsPreemptible)
969 InX::RelaDyn->addReloc({Target->TlsGotRel, InX::MipsGot,
970 Sym.getGotOffset(), false, &Sym, 0});
971 } else if (!Sym.isInGot()) {
972 addGotEntry<ELFT>(Sym, Preemptible);
976 if (!needsPlt(Expr) && !needsGot(Expr) && Sym.IsPreemptible) {
977 // We don't know anything about the finaly symbol. Just ask the dynamic
978 // linker to handle the relocation for us.
979 if (!Target->isPicRel(Type))
981 "relocation " + toString(Type) +
982 " cannot be used against shared object; recompile with -fPIC" +
983 getLocation<ELFT>(Sec, Sym, Offset));
985 InX::RelaDyn->addReloc(
986 {Target->getDynRel(Type), &Sec, Offset, false, &Sym, Addend});
988 // MIPS ABI turns using of GOT and dynamic relocations inside out.
989 // While regular ABI uses dynamic relocations to fill up GOT entries
990 // MIPS ABI requires dynamic linker to fills up GOT entries using
991 // specially sorted dynamic symbol table. This affects even dynamic
992 // relocations against symbols which do not require GOT entries
993 // creation explicitly, i.e. do not have any GOT-relocations. So if
994 // a preemptible symbol has a dynamic relocation we anyway have
995 // to create a GOT entry for it.
996 // If a non-preemptible symbol has a dynamic relocation against it,
997 // dynamic linker takes it st_value, adds offset and writes down
998 // result of the dynamic relocation. In case of preemptible symbol
999 // dynamic linker performs symbol resolution, writes the symbol value
1000 // to the GOT entry and reads the GOT entry when it needs to perform
1001 // a dynamic relocation.
1002 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf p.4-19
1003 if (Config->EMachine == EM_MIPS)
1004 InX::MipsGot->addEntry(Sym, Addend, Expr);
1008 // If the relocation points to something in the file, we can process it.
1010 isStaticLinkTimeConstant<ELFT>(Expr, Type, Sym, Sec, Rel.r_offset);
1012 // The size is not going to change, so we fold it in here.
1014 Addend += Sym.getSize();
1016 // If the produced value is a constant, we just remember to write it
1017 // when outputting this section. We also have to do it if the format
1018 // uses Elf_Rel, since in that case the written value is the addend.
1020 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
1024 // If the output being produced is position independent, the final value
1025 // is still not known. In that case we still need some help from the
1026 // dynamic linker. We can however do better than just copying the incoming
1027 // relocation. We can process some of it and and just ask the dynamic
1028 // linker to add the load address.
1029 if (Config->IsRela) {
1030 InX::RelaDyn->addReloc(
1031 {Target->RelativeRel, &Sec, Offset, true, &Sym, Addend});
1033 // In REL, addends are stored to the target section.
1034 InX::RelaDyn->addReloc(
1035 {Target->RelativeRel, &Sec, Offset, true, &Sym, 0});
1036 Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
1041 template <class ELFT> void elf::scanRelocations(InputSectionBase &S) {
1042 if (S.AreRelocsRela)
1043 scanRelocs<ELFT>(S, S.relas<ELFT>());
1045 scanRelocs<ELFT>(S, S.rels<ELFT>());
1048 // Thunk Implementation
1050 // Thunks (sometimes called stubs, veneers or branch islands) are small pieces
1051 // of code that the linker inserts inbetween a caller and a callee. The thunks
1052 // are added at link time rather than compile time as the decision on whether
1053 // a thunk is needed, such as the caller and callee being out of range, can only
1054 // be made at link time.
1056 // It is straightforward to tell given the current state of the program when a
1057 // thunk is needed for a particular call. The more difficult part is that
1058 // the thunk needs to be placed in the program such that the caller can reach
1059 // the thunk and the thunk can reach the callee; furthermore, adding thunks to
1060 // the program alters addresses, which can mean more thunks etc.
1062 // In lld we have a synthetic ThunkSection that can hold many Thunks.
1063 // The decision to have a ThunkSection act as a container means that we can
1064 // more easily handle the most common case of a single block of contiguous
1065 // Thunks by inserting just a single ThunkSection.
1067 // The implementation of Thunks in lld is split across these areas
1068 // Relocations.cpp : Framework for creating and placing thunks
1069 // Thunks.cpp : The code generated for each supported thunk
1070 // Target.cpp : Target specific hooks that the framework uses to decide when
1072 // Synthetic.cpp : Implementation of ThunkSection
1073 // Writer.cpp : Iteratively call framework until no more Thunks added
1075 // Thunk placement requirements:
1076 // Mips LA25 thunks. These must be placed immediately before the callee section
1077 // We can assume that the caller is in range of the Thunk. These are modelled
1078 // by Thunks that return the section they must precede with
1079 // getTargetInputSection().
1081 // ARM interworking and range extension thunks. These thunks must be placed
1082 // within range of the caller. All implemented ARM thunks can always reach the
1083 // callee as they use an indirect jump via a register that has no range
1086 // Thunk placement algorithm:
1087 // For Mips LA25 ThunkSections; the placement is explicit, it has to be before
1088 // getTargetInputSection().
1090 // For thunks that must be placed within range of the caller there are many
1091 // possible choices given that the maximum range from the caller is usually
1092 // much larger than the average InputSection size. Desirable properties include:
1093 // - Maximize reuse of thunks by multiple callers
1094 // - Minimize number of ThunkSections to simplify insertion
1095 // - Handle impact of already added Thunks on addresses
1096 // - Simple to understand and implement
1098 // In lld for the first pass, we pre-create one or more ThunkSections per
1099 // InputSectionDescription at Target specific intervals. A ThunkSection is
1100 // placed so that the estimated end of the ThunkSection is within range of the
1101 // start of the InputSectionDescription or the previous ThunkSection. For
1103 // InputSectionDescription
1113 // The intention is that we can add a Thunk to a ThunkSection that is well
1114 // spaced enough to service a number of callers without having to do a lot
1115 // of work. An important principle is that it is not an error if a Thunk cannot
1116 // be placed in a pre-created ThunkSection; when this happens we create a new
1117 // ThunkSection placed next to the caller. This allows us to handle the vast
1118 // majority of thunks simply, but also handle rare cases where the branch range
1119 // is smaller than the target specific spacing.
1121 // The algorithm is expected to create all the thunks that are needed in a
1122 // single pass, with a small number of programs needing a second pass due to
1123 // the insertion of thunks in the first pass increasing the offset between
1124 // callers and callees that were only just in range.
1126 // A consequence of allowing new ThunkSections to be created outside of the
1127 // pre-created ThunkSections is that in rare cases calls to Thunks that were in
1128 // range in pass K, are out of range in some pass > K due to the insertion of
1129 // more Thunks in between the caller and callee. When this happens we retarget
1130 // the relocation back to the original target and create another Thunk.
1132 // Remove ThunkSections that are empty, this should only be the initial set
1133 // precreated on pass 0.
1135 // Insert the Thunks for OutputSection OS into their designated place
1136 // in the Sections vector, and recalculate the InputSection output section
1138 // This may invalidate any output section offsets stored outside of InputSection
1139 void ThunkCreator::mergeThunks(ArrayRef<OutputSection *> OutputSections) {
1140 forEachInputSectionDescription(
1141 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1142 if (ISD->ThunkSections.empty())
1145 // Remove any zero sized precreated Thunks.
1146 llvm::erase_if(ISD->ThunkSections,
1147 [](const std::pair<ThunkSection *, uint32_t> &TS) {
1148 return TS.first->getSize() == 0;
1150 // ISD->ThunkSections contains all created ThunkSections, including
1151 // those inserted in previous passes. Extract the Thunks created this
1152 // pass and order them in ascending OutSecOff.
1153 std::vector<ThunkSection *> NewThunks;
1154 for (const std::pair<ThunkSection *, uint32_t> TS : ISD->ThunkSections)
1155 if (TS.second == Pass)
1156 NewThunks.push_back(TS.first);
1157 std::stable_sort(NewThunks.begin(), NewThunks.end(),
1158 [](const ThunkSection *A, const ThunkSection *B) {
1159 return A->OutSecOff < B->OutSecOff;
1162 // Merge sorted vectors of Thunks and InputSections by OutSecOff
1163 std::vector<InputSection *> Tmp;
1164 Tmp.reserve(ISD->Sections.size() + NewThunks.size());
1165 auto MergeCmp = [](const InputSection *A, const InputSection *B) {
1166 // std::merge requires a strict weak ordering.
1167 if (A->OutSecOff < B->OutSecOff)
1169 if (A->OutSecOff == B->OutSecOff) {
1170 auto *TA = dyn_cast<ThunkSection>(A);
1171 auto *TB = dyn_cast<ThunkSection>(B);
1172 // Check if Thunk is immediately before any specific Target
1173 // InputSection for example Mips LA25 Thunks.
1174 if (TA && TA->getTargetInputSection() == B)
1176 if (TA && !TB && !TA->getTargetInputSection())
1177 // Place Thunk Sections without specific targets before
1178 // non-Thunk Sections.
1183 std::merge(ISD->Sections.begin(), ISD->Sections.end(),
1184 NewThunks.begin(), NewThunks.end(), std::back_inserter(Tmp),
1186 ISD->Sections = std::move(Tmp);
1190 // Find or create a ThunkSection within the InputSectionDescription (ISD) that
1191 // is in range of Src. An ISD maps to a range of InputSections described by a
1192 // linker script section pattern such as { .text .text.* }.
1193 ThunkSection *ThunkCreator::getISDThunkSec(OutputSection *OS, InputSection *IS,
1194 InputSectionDescription *ISD,
1195 uint32_t Type, uint64_t Src) {
1196 for (std::pair<ThunkSection *, uint32_t> TP : ISD->ThunkSections) {
1197 ThunkSection *TS = TP.first;
1198 uint64_t TSBase = OS->Addr + TS->OutSecOff;
1199 uint64_t TSLimit = TSBase + TS->getSize();
1200 if (Target->inBranchRange(Type, Src, (Src > TSLimit) ? TSBase : TSLimit))
1204 // No suitable ThunkSection exists. This can happen when there is a branch
1205 // with lower range than the ThunkSection spacing or when there are too
1206 // many Thunks. Create a new ThunkSection as close to the InputSection as
1207 // possible. Error if InputSection is so large we cannot place ThunkSection
1208 // anywhere in Range.
1209 uint64_t ThunkSecOff = IS->OutSecOff;
1210 if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff)) {
1211 ThunkSecOff = IS->OutSecOff + IS->getSize();
1212 if (!Target->inBranchRange(Type, Src, OS->Addr + ThunkSecOff))
1213 fatal("InputSection too large for range extension thunk " +
1214 IS->getObjMsg(Src - (OS->Addr + IS->OutSecOff)));
1216 return addThunkSection(OS, ISD, ThunkSecOff);
1219 // Add a Thunk that needs to be placed in a ThunkSection that immediately
1220 // precedes its Target.
1221 ThunkSection *ThunkCreator::getISThunkSec(InputSection *IS) {
1222 ThunkSection *TS = ThunkedSections.lookup(IS);
1226 // Find InputSectionRange within Target Output Section (TOS) that the
1227 // InputSection (IS) that we need to precede is in.
1228 OutputSection *TOS = IS->getParent();
1229 for (BaseCommand *BC : TOS->SectionCommands)
1230 if (auto *ISD = dyn_cast<InputSectionDescription>(BC)) {
1231 if (ISD->Sections.empty())
1233 InputSection *first = ISD->Sections.front();
1234 InputSection *last = ISD->Sections.back();
1235 if (IS->OutSecOff >= first->OutSecOff &&
1236 IS->OutSecOff <= last->OutSecOff) {
1237 TS = addThunkSection(TOS, ISD, IS->OutSecOff);
1238 ThunkedSections[IS] = TS;
1245 // Create one or more ThunkSections per OS that can be used to place Thunks.
1246 // We attempt to place the ThunkSections using the following desirable
1248 // - Within range of the maximum number of callers
1249 // - Minimise the number of ThunkSections
1251 // We follow a simple but conservative heuristic to place ThunkSections at
1252 // offsets that are multiples of a Target specific branch range.
1253 // For an InputSectionRange that is smaller than the range, a single
1254 // ThunkSection at the end of the range will do.
1255 void ThunkCreator::createInitialThunkSections(
1256 ArrayRef<OutputSection *> OutputSections) {
1257 forEachInputSectionDescription(
1258 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1259 if (ISD->Sections.empty())
1262 uint32_t PrevISLimit = ISD->Sections.front()->OutSecOff;
1263 uint32_t ThunkUpperBound = PrevISLimit + Target->ThunkSectionSpacing;
1265 for (const InputSection *IS : ISD->Sections) {
1266 ISLimit = IS->OutSecOff + IS->getSize();
1267 if (ISLimit > ThunkUpperBound) {
1268 addThunkSection(OS, ISD, PrevISLimit);
1269 ThunkUpperBound = PrevISLimit + Target->ThunkSectionSpacing;
1271 PrevISLimit = ISLimit;
1273 addThunkSection(OS, ISD, ISLimit);
1277 ThunkSection *ThunkCreator::addThunkSection(OutputSection *OS,
1278 InputSectionDescription *ISD,
1280 auto *TS = make<ThunkSection>(OS, Off);
1281 ISD->ThunkSections.push_back(std::make_pair(TS, Pass));
1285 std::pair<Thunk *, bool> ThunkCreator::getThunk(Symbol &Sym, RelType Type,
1287 auto Res = ThunkedSymbols.insert({&Sym, std::vector<Thunk *>()});
1289 // Check existing Thunks for Sym to see if they can be reused
1290 for (Thunk *ET : Res.first->second)
1291 if (ET->isCompatibleWith(Type) &&
1292 Target->inBranchRange(Type, Src, ET->ThunkSym->getVA()))
1293 return std::make_pair(ET, false);
1295 // No existing compatible Thunk in range, create a new one
1296 Thunk *T = addThunk(Type, Sym);
1297 Res.first->second.push_back(T);
1298 return std::make_pair(T, true);
1301 // Call Fn on every executable InputSection accessed via the linker script
1302 // InputSectionDescription::Sections.
1303 void ThunkCreator::forEachInputSectionDescription(
1304 ArrayRef<OutputSection *> OutputSections,
1305 std::function<void(OutputSection *, InputSectionDescription *)> Fn) {
1306 for (OutputSection *OS : OutputSections) {
1307 if (!(OS->Flags & SHF_ALLOC) || !(OS->Flags & SHF_EXECINSTR))
1309 for (BaseCommand *BC : OS->SectionCommands)
1310 if (auto *ISD = dyn_cast<InputSectionDescription>(BC))
1315 // Return true if the relocation target is an in range Thunk.
1316 // Return false if the relocation is not to a Thunk. If the relocation target
1317 // was originally to a Thunk, but is no longer in range we revert the
1318 // relocation back to its original non-Thunk target.
1319 bool ThunkCreator::normalizeExistingThunk(Relocation &Rel, uint64_t Src) {
1320 if (Thunk *ET = Thunks.lookup(Rel.Sym)) {
1321 if (Target->inBranchRange(Rel.Type, Src, Rel.Sym->getVA()))
1323 Rel.Sym = &ET->Destination;
1324 if (Rel.Sym->isInPlt())
1325 Rel.Expr = toPlt(Rel.Expr);
1330 // Process all relocations from the InputSections that have been assigned
1331 // to InputSectionDescriptions and redirect through Thunks if needed. The
1332 // function should be called iteratively until it returns false.
1335 // All InputSections that may need a Thunk are reachable from
1336 // OutputSectionCommands.
1338 // All OutputSections have an address and all InputSections have an offset
1339 // within the OutputSection.
1341 // The offsets between caller (relocation place) and callee
1342 // (relocation target) will not be modified outside of createThunks().
1345 // If return value is true then ThunkSections have been inserted into
1346 // OutputSections. All relocations that needed a Thunk based on the information
1347 // available to createThunks() on entry have been redirected to a Thunk. Note
1348 // that adding Thunks changes offsets between caller and callee so more Thunks
1351 // If return value is false then no more Thunks are needed, and createThunks has
1352 // made no changes. If the target requires range extension thunks, currently
1353 // ARM, then any future change in offset between caller and callee risks a
1354 // relocation out of range error.
1355 bool ThunkCreator::createThunks(ArrayRef<OutputSection *> OutputSections) {
1356 bool AddressesChanged = false;
1357 if (Pass == 0 && Target->ThunkSectionSpacing)
1358 createInitialThunkSections(OutputSections);
1359 else if (Pass == 10)
1360 // With Thunk Size much smaller than branch range we expect to
1361 // converge quickly; if we get to 10 something has gone wrong.
1362 fatal("thunk creation not converged");
1364 // Create all the Thunks and insert them into synthetic ThunkSections. The
1365 // ThunkSections are later inserted back into InputSectionDescriptions.
1366 // We separate the creation of ThunkSections from the insertion of the
1367 // ThunkSections as ThunkSections are not always inserted into the same
1368 // InputSectionDescription as the caller.
1369 forEachInputSectionDescription(
1370 OutputSections, [&](OutputSection *OS, InputSectionDescription *ISD) {
1371 for (InputSection *IS : ISD->Sections)
1372 for (Relocation &Rel : IS->Relocations) {
1373 uint64_t Src = OS->Addr + IS->OutSecOff + Rel.Offset;
1375 // If we are a relocation to an existing Thunk, check if it is
1376 // still in range. If not then Rel will be altered to point to its
1377 // original target so another Thunk can be generated.
1378 if (Pass > 0 && normalizeExistingThunk(Rel, Src))
1381 if (!Target->needsThunk(Rel.Expr, Rel.Type, IS->File, Src,
1386 std::tie(T, IsNew) = getThunk(*Rel.Sym, Rel.Type, Src);
1388 AddressesChanged = true;
1389 // Find or create a ThunkSection for the new Thunk
1391 if (auto *TIS = T->getTargetInputSection())
1392 TS = getISThunkSec(TIS);
1394 TS = getISDThunkSec(OS, IS, ISD, Rel.Type, Src);
1396 Thunks[T->ThunkSym] = T;
1398 // Redirect relocation to Thunk, we never go via the PLT to a Thunk
1399 Rel.Sym = T->ThunkSym;
1400 Rel.Expr = fromPlt(Rel.Expr);
1403 // Merge all created synthetic ThunkSections back into OutputSection
1404 mergeThunks(OutputSections);
1406 return AddressesChanged;
1409 template void elf::scanRelocations<ELF32LE>(InputSectionBase &);
1410 template void elf::scanRelocations<ELF32BE>(InputSectionBase &);
1411 template void elf::scanRelocations<ELF64LE>(InputSectionBase &);
1412 template void elf::scanRelocations<ELF64BE>(InputSectionBase &);