1 //===- OutputSections.cpp -------------------------------------------------===//
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 #include "OutputSections.h"
12 #include "LinkerScript.h"
14 #include "SymbolTable.h"
15 #include "SyntheticSections.h"
17 #include "lld/Common/Memory.h"
18 #include "lld/Common/Threads.h"
19 #include "llvm/BinaryFormat/Dwarf.h"
20 #include "llvm/Support/Compression.h"
21 #include "llvm/Support/MD5.h"
22 #include "llvm/Support/MathExtras.h"
23 #include "llvm/Support/SHA1.h"
26 using namespace llvm::dwarf;
27 using namespace llvm::object;
28 using namespace llvm::support::endian;
29 using namespace llvm::ELF;
32 using namespace lld::elf;
35 OutputSection *Out::Opd;
37 PhdrEntry *Out::TlsPhdr;
38 OutputSection *Out::DebugInfo;
39 OutputSection *Out::ElfHeader;
40 OutputSection *Out::ProgramHeaders;
41 OutputSection *Out::PreinitArray;
42 OutputSection *Out::InitArray;
43 OutputSection *Out::FiniArray;
45 std::vector<OutputSection *> elf::OutputSections;
47 uint32_t OutputSection::getPhdrFlags() const {
49 if (Flags & SHF_WRITE)
51 if (Flags & SHF_EXECINSTR)
57 void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
58 Shdr->sh_entsize = Entsize;
59 Shdr->sh_addralign = Alignment;
61 Shdr->sh_offset = Offset;
62 Shdr->sh_flags = Flags;
67 Shdr->sh_name = ShName;
70 OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
71 : BaseCommand(OutputSectionKind),
72 SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
75 SectionIndex(INT_MAX) {
79 // We allow sections of types listed below to merged into a
80 // single progbits section. This is typically done by linker
81 // scripts. Merging nobits and progbits will force disk space
82 // to be allocated for nobits sections. Other ones don't require
83 // any special treatment on top of progbits, so there doesn't
84 // seem to be a harm in merging them.
85 static bool canMergeToProgbits(unsigned Type) {
86 return Type == SHT_NOBITS || Type == SHT_PROGBITS || Type == SHT_INIT_ARRAY ||
87 Type == SHT_PREINIT_ARRAY || Type == SHT_FINI_ARRAY ||
91 void OutputSection::addSection(InputSection *IS) {
93 // If IS is the first section to be added to this section,
94 // initialize Type and Entsize from IS.
97 Entsize = IS->Entsize;
99 // Otherwise, check if new type or flags are compatible with existing ones.
100 if ((Flags & (SHF_ALLOC | SHF_TLS)) != (IS->Flags & (SHF_ALLOC | SHF_TLS)))
101 error("incompatible section flags for " + Name + "\n>>> " + toString(IS) +
102 ": 0x" + utohexstr(IS->Flags) + "\n>>> output section " + Name +
103 ": 0x" + utohexstr(Flags));
105 if (Type != IS->Type) {
106 if (!canMergeToProgbits(Type) || !canMergeToProgbits(IS->Type))
107 error("section type mismatch for " + IS->Name + "\n>>> " +
108 toString(IS) + ": " +
109 getELFSectionTypeName(Config->EMachine, IS->Type) +
110 "\n>>> output section " + Name + ": " +
111 getELFSectionTypeName(Config->EMachine, Type));
118 Alignment = std::max(Alignment, IS->Alignment);
119 IS->OutSecOff = Size++;
121 // If this section contains a table of fixed-size entries, sh_entsize
122 // holds the element size. If it contains elements of different size we
123 // set sh_entsize to 0.
124 if (Entsize != IS->Entsize)
129 if (SectionCommands.empty() ||
130 !isa<InputSectionDescription>(SectionCommands.back()))
131 SectionCommands.push_back(make<InputSectionDescription>(""));
132 auto *ISD = cast<InputSectionDescription>(SectionCommands.back());
133 ISD->Sections.push_back(IS);
137 void elf::sortByOrder(MutableArrayRef<InputSection *> In,
138 std::function<int(InputSectionBase *S)> Order) {
139 typedef std::pair<int, InputSection *> Pair;
140 auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };
143 for (InputSection *S : In)
144 V.push_back({Order(S), S});
145 std::stable_sort(V.begin(), V.end(), Comp);
147 for (size_t I = 0; I < V.size(); ++I)
151 uint64_t elf::getHeaderSize() {
152 if (Config->OFormatBinary)
154 return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
157 bool OutputSection::classof(const BaseCommand *C) {
158 return C->Kind == OutputSectionKind;
161 void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
163 assert(SectionCommands.size() == 1);
164 sortByOrder(cast<InputSectionDescription>(SectionCommands[0])->Sections,
168 // Fill [Buf, Buf + Size) with Filler.
169 // This is used for linker script "=fillexp" command.
170 static void fill(uint8_t *Buf, size_t Size, uint32_t Filler) {
172 for (; I + 4 < Size; I += 4)
173 memcpy(Buf + I, &Filler, 4);
174 memcpy(Buf + I, &Filler, Size - I);
177 // Compress section contents if this section contains debug info.
178 template <class ELFT> void OutputSection::maybeCompress() {
179 typedef typename ELFT::Chdr Elf_Chdr;
181 // Compress only DWARF debug sections.
182 if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
183 !Name.startswith(".debug_"))
186 // Create a section header.
187 ZDebugHeader.resize(sizeof(Elf_Chdr));
188 auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
189 Hdr->ch_type = ELFCOMPRESS_ZLIB;
191 Hdr->ch_addralign = Alignment;
193 // Write section contents to a temporary buffer and compress it.
194 std::vector<uint8_t> Buf(Size);
195 writeTo<ELFT>(Buf.data());
196 if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
197 fatal("compress failed: " + llvm::toString(std::move(E)));
199 // Update section headers.
200 Size = sizeof(Elf_Chdr) + CompressedData.size();
201 Flags |= SHF_COMPRESSED;
204 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
208 write16(Buf, Data, Config->Endianness);
210 write32(Buf, Data, Config->Endianness);
212 write64(Buf, Data, Config->Endianness);
214 llvm_unreachable("unsupported Size argument");
217 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
218 if (Type == SHT_NOBITS)
223 // If -compress-debug-section is specified and if this is a debug seciton,
224 // we've already compressed section contents. If that's the case,
225 // just write it down.
226 if (!CompressedData.empty()) {
227 memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
228 memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
229 CompressedData.size());
233 // Write leading padding.
234 std::vector<InputSection *> Sections;
235 for (BaseCommand *Cmd : SectionCommands)
236 if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
237 for (InputSection *IS : ISD->Sections)
239 Sections.push_back(IS);
240 uint32_t Filler = getFiller();
242 fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
244 parallelForEachN(0, Sections.size(), [&](size_t I) {
245 InputSection *IS = Sections[I];
246 IS->writeTo<ELFT>(Buf);
248 // Fill gaps between sections.
250 uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
252 if (I + 1 == Sections.size())
255 End = Buf + Sections[I + 1]->OutSecOff;
256 fill(Start, End - Start, Filler);
260 // Linker scripts may have BYTE()-family commands with which you
261 // can write arbitrary bytes to the output. Process them if any.
262 for (BaseCommand *Base : SectionCommands)
263 if (auto *Data = dyn_cast<ByteCommand>(Base))
264 writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
267 template <class ELFT>
268 static void finalizeShtGroup(OutputSection *OS,
269 InputSection *Section) {
270 assert(Config->Relocatable);
272 // sh_link field for SHT_GROUP sections should contain the section index of
274 OS->Link = InX::SymTab->getParent()->SectionIndex;
276 // sh_info then contain index of an entry in symbol table section which
277 // provides signature of the section group.
278 ObjFile<ELFT> *Obj = Section->getFile<ELFT>();
279 ArrayRef<Symbol *> Symbols = Obj->getSymbols();
280 OS->Info = InX::SymTab->getSymbolIndex(Symbols[Section->Info]);
283 template <class ELFT> void OutputSection::finalize() {
284 InputSection *First = nullptr;
285 for (BaseCommand *Base : SectionCommands) {
286 if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
287 if (ISD->Sections.empty())
289 if (First == nullptr)
290 First = ISD->Sections.front();
292 if (isa<ByteCommand>(Base) && Type == SHT_NOBITS)
296 if (Flags & SHF_LINK_ORDER) {
297 // We must preserve the link order dependency of sections with the
298 // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
299 // need to translate the InputSection sh_link to the OutputSection sh_link,
300 // all InputSections in the OutputSection have the same dependency.
301 if (auto *D = First->getLinkOrderDep())
302 Link = D->getParent()->SectionIndex;
305 if (Type == SHT_GROUP) {
306 finalizeShtGroup<ELFT>(this, First);
310 if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
313 if (isa<SyntheticSection>(First))
316 Link = InX::SymTab->getParent()->SectionIndex;
317 // sh_info for SHT_REL[A] sections should contain the section header index of
318 // the section to which the relocation applies.
319 InputSectionBase *S = First->getRelocatedSection();
320 Info = S->getOutputSection()->SectionIndex;
321 Flags |= SHF_INFO_LINK;
324 // Returns true if S matches /Filename.?\.o$/.
325 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
326 if (!S.endswith(".o"))
329 if (S.endswith(Filename))
331 return !S.empty() && S.drop_back().endswith(Filename);
334 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
335 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
337 // .ctors and .dtors are sorted by this priority from highest to lowest.
339 // 1. The section was contained in crtbegin (crtbegin contains
340 // some sentinel value in its .ctors and .dtors so that the runtime
341 // can find the beginning of the sections.)
343 // 2. The section has an optional priority value in the form of ".ctors.N"
344 // or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
345 // they are compared as string rather than number.
347 // 3. The section is just ".ctors" or ".dtors".
349 // 4. The section was contained in crtend, which contains an end marker.
351 // In an ideal world, we don't need this function because .init_array and
352 // .ctors are duplicate features (and .init_array is newer.) However, there
353 // are too many real-world use cases of .ctors, so we had no choice to
354 // support that with this rather ad-hoc semantics.
355 static bool compCtors(const InputSection *A, const InputSection *B) {
356 bool BeginA = isCrtbegin(A->File->getName());
357 bool BeginB = isCrtbegin(B->File->getName());
358 if (BeginA != BeginB)
360 bool EndA = isCrtend(A->File->getName());
361 bool EndB = isCrtend(B->File->getName());
364 StringRef X = A->Name;
365 StringRef Y = B->Name;
366 assert(X.startswith(".ctors") || X.startswith(".dtors"));
367 assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
370 if (X.empty() && Y.empty())
375 // Sorts input sections by the special rules for .ctors and .dtors.
376 // Unfortunately, the rules are different from the one for .{init,fini}_array.
377 // Read the comment above.
378 void OutputSection::sortCtorsDtors() {
379 assert(SectionCommands.size() == 1);
380 auto *ISD = cast<InputSectionDescription>(SectionCommands[0]);
381 std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
384 // If an input string is in the form of "foo.N" where N is a number,
385 // return N. Otherwise, returns 65536, which is one greater than the
387 int elf::getPriority(StringRef S) {
388 size_t Pos = S.rfind('.');
389 if (Pos == StringRef::npos)
392 if (!to_integer(S.substr(Pos + 1), V, 10))
397 // Sorts input sections by section name suffixes, so that .foo.N comes
398 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
399 // We want to keep the original order if the priorities are the same
400 // because the compiler keeps the original initialization order in a
401 // translation unit and we need to respect that.
402 // For more detail, read the section of the GCC's manual about init_priority.
403 void OutputSection::sortInitFini() {
404 // Sort sections by priority.
405 sort([](InputSectionBase *S) { return getPriority(S->Name); });
408 uint32_t OutputSection::getFiller() {
411 if (Flags & SHF_EXECINSTR)
412 return Target->TrapInstr;
416 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
417 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
418 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
419 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
421 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
422 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
423 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
424 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
426 template void OutputSection::maybeCompress<ELF32LE>();
427 template void OutputSection::maybeCompress<ELF32BE>();
428 template void OutputSection::maybeCompress<ELF64LE>();
429 template void OutputSection::maybeCompress<ELF64BE>();
431 template void OutputSection::finalize<ELF32LE>();
432 template void OutputSection::finalize<ELF32BE>();
433 template void OutputSection::finalize<ELF64LE>();
434 template void OutputSection::finalize<ELF64BE>();