1 //===- OutputSections.cpp -------------------------------------------------===//
2 //
3 // The LLVM Linker
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9
10 #include "OutputSections.h"
11 #include "Config.h"
12 #include "LinkerScript.h"
13 #include "SymbolTable.h"
14 #include "SyntheticSections.h"
15 #include "Target.h"
16 #include "lld/Common/Memory.h"
17 #include "lld/Common/Strings.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"
24
25 using namespace llvm;
26 using namespace llvm::dwarf;
27 using namespace llvm::object;
28 using namespace llvm::support::endian;
29 using namespace llvm::ELF;
30
31 using namespace lld;
32 using namespace lld::elf;
33
34 uint8_t Out::First;
35 PhdrEntry *Out::TlsPhdr;
36 OutputSection *Out::ElfHeader;
37 OutputSection *Out::ProgramHeaders;
38 OutputSection *Out::PreinitArray;
39 OutputSection *Out::InitArray;
40 OutputSection *Out::FiniArray;
41
42 std::vector<OutputSection *> elf::OutputSections;
43
getPhdrFlags() const44 uint32_t OutputSection::getPhdrFlags() const {
45 uint32_t Ret = 0;
46 if (Config->EMachine != EM_ARM || !(Flags & SHF_ARM_PURECODE))
47 Ret |= PF_R;
48 if (Flags & SHF_WRITE)
49 Ret |= PF_W;
50 if (Flags & SHF_EXECINSTR)
51 Ret |= PF_X;
52 return Ret;
53 }
54
55 template <class ELFT>
writeHeaderTo(typename ELFT::Shdr * Shdr)56 void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
57 Shdr->sh_entsize = Entsize;
58 Shdr->sh_addralign = Alignment;
59 Shdr->sh_type = Type;
60 Shdr->sh_offset = Offset;
61 Shdr->sh_flags = Flags;
62 Shdr->sh_info = Info;
63 Shdr->sh_link = Link;
64 Shdr->sh_addr = Addr;
65 Shdr->sh_size = Size;
66 Shdr->sh_name = ShName;
67 }
68
OutputSection(StringRef Name,uint32_t Type,uint64_t Flags)69 OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
70 : BaseCommand(OutputSectionKind),
71 SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
72 /*Info*/ 0, /*Link*/ 0) {
73 Live = false;
74 }
75
76 // We allow sections of types listed below to merged into a
77 // single progbits section. This is typically done by linker
78 // scripts. Merging nobits and progbits will force disk space
79 // to be allocated for nobits sections. Other ones don't require
80 // any special treatment on top of progbits, so there doesn't
81 // seem to be a harm in merging them.
canMergeToProgbits(unsigned Type)82 static bool canMergeToProgbits(unsigned Type) {
83 return Type == SHT_NOBITS || Type == SHT_PROGBITS || Type == SHT_INIT_ARRAY ||
84 Type == SHT_PREINIT_ARRAY || Type == SHT_FINI_ARRAY ||
85 Type == SHT_NOTE;
86 }
87
addSection(InputSection * IS)88 void OutputSection::addSection(InputSection *IS) {
89 if (!Live) {
90 // If IS is the first section to be added to this section,
91 // initialize Type, Entsize and flags from IS.
92 Live = true;
93 Type = IS->Type;
94 Entsize = IS->Entsize;
95 Flags = IS->Flags;
96 } else {
97 // Otherwise, check if new type or flags are compatible with existing ones.
98 unsigned Mask = SHF_TLS | SHF_LINK_ORDER;
99 if ((Flags & Mask) != (IS->Flags & Mask))
100 error("incompatible section flags for " + Name + "\n>>> " + toString(IS) +
101 ": 0x" + utohexstr(IS->Flags) + "\n>>> output section " + Name +
102 ": 0x" + utohexstr(Flags));
103
104 if (Type != IS->Type) {
105 if (!canMergeToProgbits(Type) || !canMergeToProgbits(IS->Type))
106 error("section type mismatch for " + IS->Name + "\n>>> " +
107 toString(IS) + ": " +
108 getELFSectionTypeName(Config->EMachine, IS->Type) +
109 "\n>>> output section " + Name + ": " +
110 getELFSectionTypeName(Config->EMachine, Type));
111 Type = SHT_PROGBITS;
112 }
113 }
114
115 IS->Parent = this;
116 uint64_t AndMask =
117 Config->EMachine == EM_ARM ? (uint64_t)SHF_ARM_PURECODE : 0;
118 uint64_t OrMask = ~AndMask;
119 uint64_t AndFlags = (Flags & IS->Flags) & AndMask;
120 uint64_t OrFlags = (Flags | IS->Flags) & OrMask;
121 Flags = AndFlags | OrFlags;
122
123 Alignment = std::max(Alignment, IS->Alignment);
124
125 // If this section contains a table of fixed-size entries, sh_entsize
126 // holds the element size. If it contains elements of different size we
127 // set sh_entsize to 0.
128 if (Entsize != IS->Entsize)
129 Entsize = 0;
130
131 if (!IS->Assigned) {
132 IS->Assigned = true;
133 if (SectionCommands.empty() ||
134 !isa<InputSectionDescription>(SectionCommands.back()))
135 SectionCommands.push_back(make<InputSectionDescription>(""));
136 auto *ISD = cast<InputSectionDescription>(SectionCommands.back());
137 ISD->Sections.push_back(IS);
138 }
139 }
140
sortByOrder(MutableArrayRef<InputSection * > In,llvm::function_ref<int (InputSectionBase * S)> Order)141 static void sortByOrder(MutableArrayRef<InputSection *> In,
142 llvm::function_ref<int(InputSectionBase *S)> Order) {
143 typedef std::pair<int, InputSection *> Pair;
144 auto Comp = [](const Pair &A, const Pair &B) { return A.first < B.first; };
145
146 std::vector<Pair> V;
147 for (InputSection *S : In)
148 V.push_back({Order(S), S});
149 std::stable_sort(V.begin(), V.end(), Comp);
150
151 for (size_t I = 0; I < V.size(); ++I)
152 In[I] = V[I].second;
153 }
154
getHeaderSize()155 uint64_t elf::getHeaderSize() {
156 if (Config->OFormatBinary)
157 return 0;
158 return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
159 }
160
classof(const BaseCommand * C)161 bool OutputSection::classof(const BaseCommand *C) {
162 return C->Kind == OutputSectionKind;
163 }
164
sort(llvm::function_ref<int (InputSectionBase * S)> Order)165 void OutputSection::sort(llvm::function_ref<int(InputSectionBase *S)> Order) {
166 assert(Live);
167 for (BaseCommand *B : SectionCommands)
168 if (auto *ISD = dyn_cast<InputSectionDescription>(B))
169 sortByOrder(ISD->Sections, Order);
170 }
171
172 // Fill [Buf, Buf + Size) with Filler.
173 // This is used for linker script "=fillexp" command.
fill(uint8_t * Buf,size_t Size,const std::array<uint8_t,4> & Filler)174 static void fill(uint8_t *Buf, size_t Size,
175 const std::array<uint8_t, 4> &Filler) {
176 size_t I = 0;
177 for (; I + 4 < Size; I += 4)
178 memcpy(Buf + I, Filler.data(), 4);
179 memcpy(Buf + I, Filler.data(), Size - I);
180 }
181
182 // Compress section contents if this section contains debug info.
maybeCompress()183 template <class ELFT> void OutputSection::maybeCompress() {
184 typedef typename ELFT::Chdr Elf_Chdr;
185
186 // Compress only DWARF debug sections.
187 if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
188 !Name.startswith(".debug_"))
189 return;
190
191 // Create a section header.
192 ZDebugHeader.resize(sizeof(Elf_Chdr));
193 auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
194 Hdr->ch_type = ELFCOMPRESS_ZLIB;
195 Hdr->ch_size = Size;
196 Hdr->ch_addralign = Alignment;
197
198 // Write section contents to a temporary buffer and compress it.
199 std::vector<uint8_t> Buf(Size);
200 writeTo<ELFT>(Buf.data());
201 if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
202 fatal("compress failed: " + llvm::toString(std::move(E)));
203
204 // Update section headers.
205 Size = sizeof(Elf_Chdr) + CompressedData.size();
206 Flags |= SHF_COMPRESSED;
207 }
208
writeInt(uint8_t * Buf,uint64_t Data,uint64_t Size)209 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
210 if (Size == 1)
211 *Buf = Data;
212 else if (Size == 2)
213 write16(Buf, Data);
214 else if (Size == 4)
215 write32(Buf, Data);
216 else if (Size == 8)
217 write64(Buf, Data);
218 else
219 llvm_unreachable("unsupported Size argument");
220 }
221
writeTo(uint8_t * Buf)222 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
223 if (Type == SHT_NOBITS)
224 return;
225
226 Loc = Buf;
227
228 // If -compress-debug-section is specified and if this is a debug seciton,
229 // we've already compressed section contents. If that's the case,
230 // just write it down.
231 if (!CompressedData.empty()) {
232 memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
233 memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
234 CompressedData.size());
235 return;
236 }
237
238 // Write leading padding.
239 std::vector<InputSection *> Sections = getInputSections(this);
240 std::array<uint8_t, 4> Filler = getFiller();
241 bool NonZeroFiller = read32(Filler.data()) != 0;
242 if (NonZeroFiller)
243 fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
244
245 parallelForEachN(0, Sections.size(), [&](size_t I) {
246 InputSection *IS = Sections[I];
247 IS->writeTo<ELFT>(Buf);
248
249 // Fill gaps between sections.
250 if (NonZeroFiller) {
251 uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
252 uint8_t *End;
253 if (I + 1 == Sections.size())
254 End = Buf + Size;
255 else
256 End = Buf + Sections[I + 1]->OutSecOff;
257 fill(Start, End - Start, Filler);
258 }
259 });
260
261 // Linker scripts may have BYTE()-family commands with which you
262 // can write arbitrary bytes to the output. Process them if any.
263 for (BaseCommand *Base : SectionCommands)
264 if (auto *Data = dyn_cast<ByteCommand>(Base))
265 writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
266 }
267
268 template <class ELFT>
finalizeShtGroup(OutputSection * OS,InputSection * Section)269 static void finalizeShtGroup(OutputSection *OS,
270 InputSection *Section) {
271 assert(Config->Relocatable);
272
273 // sh_link field for SHT_GROUP sections should contain the section index of
274 // the symbol table.
275 OS->Link = In.SymTab->getParent()->SectionIndex;
276
277 // sh_info then contain index of an entry in symbol table section which
278 // provides signature of the section group.
279 ObjFile<ELFT> *Obj = Section->getFile<ELFT>();
280 ArrayRef<Symbol *> Symbols = Obj->getSymbols();
281 OS->Info = In.SymTab->getSymbolIndex(Symbols[Section->Info]);
282 }
283
finalize()284 template <class ELFT> void OutputSection::finalize() {
285 if (Type == SHT_NOBITS)
286 for (BaseCommand *Base : SectionCommands)
287 if (isa<ByteCommand>(Base))
288 Type = SHT_PROGBITS;
289
290 std::vector<InputSection *> V = getInputSections(this);
291 InputSection *First = V.empty() ? nullptr : V[0];
292
293 if (Flags & SHF_LINK_ORDER) {
294 // We must preserve the link order dependency of sections with the
295 // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
296 // need to translate the InputSection sh_link to the OutputSection sh_link,
297 // all InputSections in the OutputSection have the same dependency.
298 if (auto *D = First->getLinkOrderDep())
299 Link = D->getParent()->SectionIndex;
300 }
301
302 if (Type == SHT_GROUP) {
303 finalizeShtGroup<ELFT>(this, First);
304 return;
305 }
306
307 if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
308 return;
309
310 if (isa<SyntheticSection>(First))
311 return;
312
313 Link = In.SymTab->getParent()->SectionIndex;
314 // sh_info for SHT_REL[A] sections should contain the section header index of
315 // the section to which the relocation applies.
316 InputSectionBase *S = First->getRelocatedSection();
317 Info = S->getOutputSection()->SectionIndex;
318 Flags |= SHF_INFO_LINK;
319 }
320
321 // Returns true if S matches /Filename.?\.o$/.
isCrtBeginEnd(StringRef S,StringRef Filename)322 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
323 if (!S.endswith(".o"))
324 return false;
325 S = S.drop_back(2);
326 if (S.endswith(Filename))
327 return true;
328 return !S.empty() && S.drop_back().endswith(Filename);
329 }
330
isCrtbegin(StringRef S)331 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
isCrtend(StringRef S)332 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
333
334 // .ctors and .dtors are sorted by this priority from highest to lowest.
335 //
336 // 1. The section was contained in crtbegin (crtbegin contains
337 // some sentinel value in its .ctors and .dtors so that the runtime
338 // can find the beginning of the sections.)
339 //
340 // 2. The section has an optional priority value in the form of ".ctors.N"
341 // or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
342 // they are compared as string rather than number.
343 //
344 // 3. The section is just ".ctors" or ".dtors".
345 //
346 // 4. The section was contained in crtend, which contains an end marker.
347 //
348 // In an ideal world, we don't need this function because .init_array and
349 // .ctors are duplicate features (and .init_array is newer.) However, there
350 // are too many real-world use cases of .ctors, so we had no choice to
351 // support that with this rather ad-hoc semantics.
compCtors(const InputSection * A,const InputSection * B)352 static bool compCtors(const InputSection *A, const InputSection *B) {
353 bool BeginA = isCrtbegin(A->File->getName());
354 bool BeginB = isCrtbegin(B->File->getName());
355 if (BeginA != BeginB)
356 return BeginA;
357 bool EndA = isCrtend(A->File->getName());
358 bool EndB = isCrtend(B->File->getName());
359 if (EndA != EndB)
360 return EndB;
361 StringRef X = A->Name;
362 StringRef Y = B->Name;
363 assert(X.startswith(".ctors") || X.startswith(".dtors"));
364 assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
365 X = X.substr(6);
366 Y = Y.substr(6);
367 return X < Y;
368 }
369
370 // Sorts input sections by the special rules for .ctors and .dtors.
371 // Unfortunately, the rules are different from the one for .{init,fini}_array.
372 // Read the comment above.
sortCtorsDtors()373 void OutputSection::sortCtorsDtors() {
374 assert(SectionCommands.size() == 1);
375 auto *ISD = cast<InputSectionDescription>(SectionCommands[0]);
376 std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
377 }
378
379 // If an input string is in the form of "foo.N" where N is a number,
380 // return N. Otherwise, returns 65536, which is one greater than the
381 // lowest priority.
getPriority(StringRef S)382 int elf::getPriority(StringRef S) {
383 size_t Pos = S.rfind('.');
384 if (Pos == StringRef::npos)
385 return 65536;
386 int V;
387 if (!to_integer(S.substr(Pos + 1), V, 10))
388 return 65536;
389 return V;
390 }
391
getInputSections(OutputSection * OS)392 std::vector<InputSection *> elf::getInputSections(OutputSection *OS) {
393 std::vector<InputSection *> Ret;
394 for (BaseCommand *Base : OS->SectionCommands)
395 if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
396 Ret.insert(Ret.end(), ISD->Sections.begin(), ISD->Sections.end());
397 return Ret;
398 }
399
400 // Sorts input sections by section name suffixes, so that .foo.N comes
401 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
402 // We want to keep the original order if the priorities are the same
403 // because the compiler keeps the original initialization order in a
404 // translation unit and we need to respect that.
405 // For more detail, read the section of the GCC's manual about init_priority.
sortInitFini()406 void OutputSection::sortInitFini() {
407 // Sort sections by priority.
408 sort([](InputSectionBase *S) { return getPriority(S->Name); });
409 }
410
getFiller()411 std::array<uint8_t, 4> OutputSection::getFiller() {
412 if (Filler)
413 return *Filler;
414 if (Flags & SHF_EXECINSTR)
415 return Target->TrapInstr;
416 return {0, 0, 0, 0};
417 }
418
419 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
420 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
421 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
422 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
423
424 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
425 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
426 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
427 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
428
429 template void OutputSection::maybeCompress<ELF32LE>();
430 template void OutputSection::maybeCompress<ELF32BE>();
431 template void OutputSection::maybeCompress<ELF64LE>();
432 template void OutputSection::maybeCompress<ELF64BE>();
433
434 template void OutputSection::finalize<ELF32LE>();
435 template void OutputSection::finalize<ELF32BE>();
436 template void OutputSection::finalize<ELF64LE>();
437 template void OutputSection::finalize<ELF64BE>();
438