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"
15 #include "SymbolTable.h"
16 #include "SyntheticSections.h"
19 #include "llvm/BinaryFormat/Dwarf.h"
20 #include "llvm/Support/MD5.h"
21 #include "llvm/Support/MathExtras.h"
22 #include "llvm/Support/SHA1.h"
25 using namespace llvm::dwarf;
26 using namespace llvm::object;
27 using namespace llvm::support::endian;
28 using namespace llvm::ELF;
31 using namespace lld::elf;
34 OutputSection *Out::Opd;
36 PhdrEntry *Out::TlsPhdr;
37 OutputSection *Out::DebugInfo;
38 OutputSection *Out::ElfHeader;
39 OutputSection *Out::ProgramHeaders;
40 OutputSection *Out::PreinitArray;
41 OutputSection *Out::InitArray;
42 OutputSection *Out::FiniArray;
44 std::vector<OutputSection *> elf::OutputSections;
45 std::vector<OutputSectionCommand *> elf::OutputSectionCommands;
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 : SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
74 SectionIndex(INT_MAX) {}
76 static uint64_t updateOffset(uint64_t Off, InputSection *S) {
77 Off = alignTo(Off, S->Alignment);
79 return Off + S->getSize();
82 void OutputSection::addSection(InputSection *S) {
84 Sections.push_back(S);
86 this->updateAlignment(S->Alignment);
88 // The actual offsets will be computed by assignAddresses. For now, use
89 // crude approximation so that it is at least easy for other code to know the
90 // section order. It is also used to calculate the output section size early
91 // for compressed debug sections.
92 this->Size = updateOffset(Size, S);
94 // If this section contains a table of fixed-size entries, sh_entsize
95 // holds the element size. Consequently, if this contains two or more
96 // input sections, all of them must have the same sh_entsize. However,
97 // you can put different types of input sections into one output
98 // sectin by using linker scripts. I don't know what to do here.
99 // Probably we sholuld handle that as an error. But for now we just
100 // pick the largest sh_entsize.
101 this->Entsize = std::max(this->Entsize, S->Entsize);
104 // This function is called after we sort input sections
105 // and scan relocations to setup sections' offsets.
106 void OutputSection::assignOffsets() {
107 OutputSectionCommand *Cmd = Script->getCmd(this);
109 for (BaseCommand *Base : Cmd->Commands)
110 if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
111 for (InputSection *S : ISD->Sections)
112 Off = updateOffset(Off, S);
116 void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
117 typedef std::pair<unsigned, InputSection *> Pair;
118 auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };
121 for (InputSection *S : Sections)
122 V.push_back({Order(S), S});
123 std::stable_sort(V.begin(), V.end(), Comp);
126 Sections.push_back(P.second);
129 // Sorts input sections by section name suffixes, so that .foo.N comes
130 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
131 // We want to keep the original order if the priorities are the same
132 // because the compiler keeps the original initialization order in a
133 // translation unit and we need to respect that.
134 // For more detail, read the section of the GCC's manual about init_priority.
135 void OutputSection::sortInitFini() {
136 // Sort sections by priority.
137 sort([](InputSectionBase *S) { return getPriority(S->Name); });
140 // Returns true if S matches /Filename.?\.o$/.
141 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
142 if (!S.endswith(".o"))
145 if (S.endswith(Filename))
147 return !S.empty() && S.drop_back().endswith(Filename);
150 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
151 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
153 // .ctors and .dtors are sorted by this priority from highest to lowest.
155 // 1. The section was contained in crtbegin (crtbegin contains
156 // some sentinel value in its .ctors and .dtors so that the runtime
157 // can find the beginning of the sections.)
159 // 2. The section has an optional priority value in the form of ".ctors.N"
160 // or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
161 // they are compared as string rather than number.
163 // 3. The section is just ".ctors" or ".dtors".
165 // 4. The section was contained in crtend, which contains an end marker.
167 // In an ideal world, we don't need this function because .init_array and
168 // .ctors are duplicate features (and .init_array is newer.) However, there
169 // are too many real-world use cases of .ctors, so we had no choice to
170 // support that with this rather ad-hoc semantics.
171 static bool compCtors(const InputSection *A, const InputSection *B) {
172 bool BeginA = isCrtbegin(A->File->getName());
173 bool BeginB = isCrtbegin(B->File->getName());
174 if (BeginA != BeginB)
176 bool EndA = isCrtend(A->File->getName());
177 bool EndB = isCrtend(B->File->getName());
180 StringRef X = A->Name;
181 StringRef Y = B->Name;
182 assert(X.startswith(".ctors") || X.startswith(".dtors"));
183 assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
186 if (X.empty() && Y.empty())
191 // Sorts input sections by the special rules for .ctors and .dtors.
192 // Unfortunately, the rules are different from the one for .{init,fini}_array.
193 // Read the comment above.
194 void OutputSection::sortCtorsDtors() {
195 std::stable_sort(Sections.begin(), Sections.end(), compCtors);
198 static SectionKey createKey(InputSectionBase *C, StringRef OutsecName) {
199 // The ELF spec just says
200 // ----------------------------------------------------------------
201 // In the first phase, input sections that match in name, type and
202 // attribute flags should be concatenated into single sections.
203 // ----------------------------------------------------------------
205 // However, it is clear that at least some flags have to be ignored for
206 // section merging. At the very least SHF_GROUP and SHF_COMPRESSED have to be
207 // ignored. We should not have two output .text sections just because one was
208 // in a group and another was not for example.
210 // It also seems that that wording was a late addition and didn't get the
211 // necessary scrutiny.
213 // Merging sections with different flags is expected by some users. One
214 // reason is that if one file has
216 // int *const bar __attribute__((section(".foo"))) = (int *)0;
218 // gcc with -fPIC will produce a read only .foo section. But if another
222 // int *const bar __attribute__((section(".foo"))) = (int *)&zed;
224 // gcc with -fPIC will produce a read write section.
226 // Last but not least, when using linker script the merge rules are forced by
227 // the script. Unfortunately, linker scripts are name based. This means that
228 // expressions like *(.foo*) can refer to multiple input sections with
229 // different flags. We cannot put them in different output sections or we
230 // would produce wrong results for
232 // start = .; *(.foo.*) end = .; *(.bar)
234 // and a mapping of .foo1 and .bar1 to one section and .foo2 and .bar2 to
235 // another. The problem is that there is no way to layout those output
236 // sections such that the .foo sections are the only thing between the start
239 // Given the above issues, we instead merge sections by name and error on
240 // incompatible types and flags.
242 uint32_t Alignment = 0;
244 if (Config->Relocatable && (C->Flags & SHF_MERGE)) {
245 Alignment = std::max<uint64_t>(C->Alignment, C->Entsize);
246 Flags = C->Flags & (SHF_MERGE | SHF_STRINGS);
249 return SectionKey{OutsecName, Flags, Alignment};
252 OutputSectionFactory::OutputSectionFactory(
253 std::vector<OutputSection *> &OutputSections)
254 : OutputSections(OutputSections) {}
256 static uint64_t getIncompatibleFlags(uint64_t Flags) {
257 return Flags & (SHF_ALLOC | SHF_TLS);
260 // We allow sections of types listed below to merged into a
261 // single progbits section. This is typically done by linker
262 // scripts. Merging nobits and progbits will force disk space
263 // to be allocated for nobits sections. Other ones don't require
264 // any special treatment on top of progbits, so there doesn't
265 // seem to be a harm in merging them.
266 static bool canMergeToProgbits(unsigned Type) {
267 return Type == SHT_NOBITS || Type == SHT_PROGBITS || Type == SHT_INIT_ARRAY ||
268 Type == SHT_PREINIT_ARRAY || Type == SHT_FINI_ARRAY ||
272 void elf::reportDiscarded(InputSectionBase *IS) {
273 if (!Config->PrintGcSections)
275 message("removing unused section from '" + IS->Name + "' in file '" +
276 IS->File->getName() + "'");
279 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
280 StringRef OutsecName) {
281 // Sections with the SHT_GROUP attribute reach here only when the - r option
282 // is given. Such sections define "section groups", and InputFiles.cpp has
283 // dedup'ed section groups by their signatures. For the -r, we want to pass
284 // through all SHT_GROUP sections without merging them because merging them
285 // creates broken section contents.
286 if (IS->Type == SHT_GROUP) {
287 OutputSection *Out = nullptr;
288 addInputSec(IS, OutsecName, Out);
292 // Imagine .zed : { *(.foo) *(.bar) } script. Both foo and bar may have
293 // relocation sections .rela.foo and .rela.bar for example. Most tools do
294 // not allow multiple REL[A] sections for output section. Hence we
295 // should combine these relocation sections into single output.
296 // We skip synthetic sections because it can be .rela.dyn/.rela.plt or any
297 // other REL[A] sections created by linker itself.
298 if (!isa<SyntheticSection>(IS) &&
299 (IS->Type == SHT_REL || IS->Type == SHT_RELA)) {
300 auto *Sec = cast<InputSection>(IS);
301 OutputSection *Out = Sec->getRelocatedSection()->getOutputSection();
302 addInputSec(IS, OutsecName, Out->RelocationSection);
306 SectionKey Key = createKey(IS, OutsecName);
307 OutputSection *&Sec = Map[Key];
308 addInputSec(IS, OutsecName, Sec);
311 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
312 StringRef OutsecName,
313 OutputSection *&Sec) {
320 if (getIncompatibleFlags(Sec->Flags) != getIncompatibleFlags(IS->Flags))
321 error("incompatible section flags for " + Sec->Name +
322 "\n>>> " + toString(IS) + ": 0x" + utohexstr(IS->Flags) +
323 "\n>>> output section " + Sec->Name + ": 0x" +
324 utohexstr(Sec->Flags));
325 if (Sec->Type != IS->Type) {
326 if (canMergeToProgbits(Sec->Type) && canMergeToProgbits(IS->Type))
327 Sec->Type = SHT_PROGBITS;
329 error("section type mismatch for " + IS->Name +
330 "\n>>> " + toString(IS) + ": " +
331 getELFSectionTypeName(Config->EMachine, IS->Type) +
332 "\n>>> output section " + Sec->Name + ": " +
333 getELFSectionTypeName(Config->EMachine, Sec->Type));
335 Sec->Flags |= IS->Flags;
337 Sec = make<OutputSection>(OutsecName, IS->Type, IS->Flags);
338 OutputSections.push_back(Sec);
341 Sec->addSection(cast<InputSection>(IS));
344 OutputSectionFactory::~OutputSectionFactory() {}
346 SectionKey DenseMapInfo<SectionKey>::getEmptyKey() {
347 return SectionKey{DenseMapInfo<StringRef>::getEmptyKey(), 0, 0};
350 SectionKey DenseMapInfo<SectionKey>::getTombstoneKey() {
351 return SectionKey{DenseMapInfo<StringRef>::getTombstoneKey(), 0, 0};
354 unsigned DenseMapInfo<SectionKey>::getHashValue(const SectionKey &Val) {
355 return hash_combine(Val.Name, Val.Flags, Val.Alignment);
358 bool DenseMapInfo<SectionKey>::isEqual(const SectionKey &LHS,
359 const SectionKey &RHS) {
360 return DenseMapInfo<StringRef>::isEqual(LHS.Name, RHS.Name) &&
361 LHS.Flags == RHS.Flags && LHS.Alignment == RHS.Alignment;
364 uint64_t elf::getHeaderSize() {
365 if (Config->OFormatBinary)
367 return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
370 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
371 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
372 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
373 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);