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 "Strings.h"
14 #include "SymbolTable.h"
15 #include "SyntheticSections.h"
16 #include "Target.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"
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 OutputSection *Out::Opd;
36 uint8_t *Out::OpdBuf;
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;
44 
45 std::vector<OutputSection *> elf::OutputSections;
46 
47 uint32_t OutputSection::getPhdrFlags() const {
48   uint32_t Ret = PF_R;
49   if (Flags & SHF_WRITE)
50     Ret |= PF_W;
51   if (Flags & SHF_EXECINSTR)
52     Ret |= PF_X;
53   return Ret;
54 }
55 
56 template <class ELFT>
57 void OutputSection::writeHeaderTo(typename ELFT::Shdr *Shdr) {
58   Shdr->sh_entsize = Entsize;
59   Shdr->sh_addralign = Alignment;
60   Shdr->sh_type = Type;
61   Shdr->sh_offset = Offset;
62   Shdr->sh_flags = Flags;
63   Shdr->sh_info = Info;
64   Shdr->sh_link = Link;
65   Shdr->sh_addr = Addr;
66   Shdr->sh_size = Size;
67   Shdr->sh_name = ShName;
68 }
69 
70 OutputSection::OutputSection(StringRef Name, uint32_t Type, uint64_t Flags)
71     : BaseCommand(OutputSectionKind),
72       SectionBase(Output, Name, Flags, /*Entsize*/ 0, /*Alignment*/ 1, Type,
73                   /*Info*/ 0,
74                   /*Link*/ 0),
75       SectionIndex(INT_MAX) {
76   Live = false;
77 }
78 
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 ||
88          Type == SHT_NOTE;
89 }
90 
91 void OutputSection::addSection(InputSection *IS) {
92   if (!Live) {
93     // If IS is the first section to be added to this section,
94     // initialize Type and Entsize from IS.
95     Live = true;
96     Type = IS->Type;
97     Entsize = IS->Entsize;
98   } else {
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));
104 
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));
112       Type = SHT_PROGBITS;
113     }
114   }
115 
116   IS->Parent = this;
117   Flags |= IS->Flags;
118   Alignment = std::max(Alignment, IS->Alignment);
119   IS->OutSecOff = Size++;
120 
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)
125     Entsize = 0;
126 
127   if (!IS->Assigned) {
128     IS->Assigned = true;
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);
134   }
135 }
136 
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; };
141 
142   std::vector<Pair> V;
143   for (InputSection *S : In)
144     V.push_back({Order(S), S});
145   std::stable_sort(V.begin(), V.end(), Comp);
146 
147   for (size_t I = 0; I < V.size(); ++I)
148     In[I] = V[I].second;
149 }
150 
151 uint64_t elf::getHeaderSize() {
152   if (Config->OFormatBinary)
153     return 0;
154   return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
155 }
156 
157 bool OutputSection::classof(const BaseCommand *C) {
158   return C->Kind == OutputSectionKind;
159 }
160 
161 void OutputSection::sort(std::function<int(InputSectionBase *S)> Order) {
162   assert(Live);
163   assert(SectionCommands.size() == 1);
164   sortByOrder(cast<InputSectionDescription>(SectionCommands[0])->Sections,
165               Order);
166 }
167 
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) {
171   size_t I = 0;
172   for (; I + 4 < Size; I += 4)
173     memcpy(Buf + I, &Filler, 4);
174   memcpy(Buf + I, &Filler, Size - I);
175 }
176 
177 // Compress section contents if this section contains debug info.
178 template <class ELFT> void OutputSection::maybeCompress() {
179   typedef typename ELFT::Chdr Elf_Chdr;
180 
181   // Compress only DWARF debug sections.
182   if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
183       !Name.startswith(".debug_"))
184     return;
185 
186   // Calculate the section offsets and size pre-compression.
187   Size = 0;
188   for (BaseCommand *Cmd : SectionCommands)
189     if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
190       for (InputSection *IS : ISD->Sections) {
191         IS->OutSecOff = alignTo(Size, IS->Alignment);
192         this->Size = IS->OutSecOff + IS->getSize();
193       }
194 
195   // Create a section header.
196   ZDebugHeader.resize(sizeof(Elf_Chdr));
197   auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
198   Hdr->ch_type = ELFCOMPRESS_ZLIB;
199   Hdr->ch_size = Size;
200   Hdr->ch_addralign = Alignment;
201 
202   // Write section contents to a temporary buffer and compress it.
203   std::vector<uint8_t> Buf(Size);
204   writeTo<ELFT>(Buf.data());
205   if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
206     fatal("compress failed: " + llvm::toString(std::move(E)));
207 
208   // Update section headers.
209   Size = sizeof(Elf_Chdr) + CompressedData.size();
210   Flags |= SHF_COMPRESSED;
211 }
212 
213 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
214   if (Size == 1)
215     *Buf = Data;
216   else if (Size == 2)
217     write16(Buf, Data, Config->Endianness);
218   else if (Size == 4)
219     write32(Buf, Data, Config->Endianness);
220   else if (Size == 8)
221     write64(Buf, Data, Config->Endianness);
222   else
223     llvm_unreachable("unsupported Size argument");
224 }
225 
226 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
227   if (Type == SHT_NOBITS)
228     return;
229 
230   Loc = Buf;
231 
232   // If -compress-debug-section is specified and if this is a debug seciton,
233   // we've already compressed section contents. If that's the case,
234   // just write it down.
235   if (!CompressedData.empty()) {
236     memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
237     memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
238            CompressedData.size());
239     return;
240   }
241 
242   // Write leading padding.
243   std::vector<InputSection *> Sections;
244   for (BaseCommand *Cmd : SectionCommands)
245     if (auto *ISD = dyn_cast<InputSectionDescription>(Cmd))
246       for (InputSection *IS : ISD->Sections)
247         if (IS->Live)
248           Sections.push_back(IS);
249   uint32_t Filler = getFiller();
250   if (Filler)
251     fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
252 
253   parallelForEachN(0, Sections.size(), [&](size_t I) {
254     InputSection *IS = Sections[I];
255     IS->writeTo<ELFT>(Buf);
256 
257     // Fill gaps between sections.
258     if (Filler) {
259       uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
260       uint8_t *End;
261       if (I + 1 == Sections.size())
262         End = Buf + Size;
263       else
264         End = Buf + Sections[I + 1]->OutSecOff;
265       fill(Start, End - Start, Filler);
266     }
267   });
268 
269   // Linker scripts may have BYTE()-family commands with which you
270   // can write arbitrary bytes to the output. Process them if any.
271   for (BaseCommand *Base : SectionCommands)
272     if (auto *Data = dyn_cast<ByteCommand>(Base))
273       writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
274 }
275 
276 template <class ELFT>
277 static void finalizeShtGroup(OutputSection *OS,
278                              InputSection *Section) {
279   assert(Config->Relocatable);
280 
281   // sh_link field for SHT_GROUP sections should contain the section index of
282   // the symbol table.
283   OS->Link = InX::SymTab->getParent()->SectionIndex;
284 
285   // sh_info then contain index of an entry in symbol table section which
286   // provides signature of the section group.
287   ObjFile<ELFT> *Obj = Section->getFile<ELFT>();
288   ArrayRef<Symbol *> Symbols = Obj->getSymbols();
289   OS->Info = InX::SymTab->getSymbolIndex(Symbols[Section->Info]);
290 }
291 
292 template <class ELFT> void OutputSection::finalize() {
293   InputSection *First = nullptr;
294   for (BaseCommand *Base : SectionCommands) {
295     if (auto *ISD = dyn_cast<InputSectionDescription>(Base)) {
296       if (ISD->Sections.empty())
297         continue;
298       if (First == nullptr)
299         First = ISD->Sections.front();
300     }
301     if (isa<ByteCommand>(Base) && Type == SHT_NOBITS)
302       Type = SHT_PROGBITS;
303   }
304 
305   if (Flags & SHF_LINK_ORDER) {
306     // We must preserve the link order dependency of sections with the
307     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
308     // need to translate the InputSection sh_link to the OutputSection sh_link,
309     // all InputSections in the OutputSection have the same dependency.
310     if (auto *D = First->getLinkOrderDep())
311       Link = D->getParent()->SectionIndex;
312   }
313 
314   if (Type == SHT_GROUP) {
315     finalizeShtGroup<ELFT>(this, First);
316     return;
317   }
318 
319   if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
320     return;
321 
322   if (isa<SyntheticSection>(First))
323     return;
324 
325   Link = InX::SymTab->getParent()->SectionIndex;
326   // sh_info for SHT_REL[A] sections should contain the section header index of
327   // the section to which the relocation applies.
328   InputSectionBase *S = First->getRelocatedSection();
329   Info = S->getOutputSection()->SectionIndex;
330   Flags |= SHF_INFO_LINK;
331 }
332 
333 // Returns true if S matches /Filename.?\.o$/.
334 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
335   if (!S.endswith(".o"))
336     return false;
337   S = S.drop_back(2);
338   if (S.endswith(Filename))
339     return true;
340   return !S.empty() && S.drop_back().endswith(Filename);
341 }
342 
343 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
344 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
345 
346 // .ctors and .dtors are sorted by this priority from highest to lowest.
347 //
348 //  1. The section was contained in crtbegin (crtbegin contains
349 //     some sentinel value in its .ctors and .dtors so that the runtime
350 //     can find the beginning of the sections.)
351 //
352 //  2. The section has an optional priority value in the form of ".ctors.N"
353 //     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
354 //     they are compared as string rather than number.
355 //
356 //  3. The section is just ".ctors" or ".dtors".
357 //
358 //  4. The section was contained in crtend, which contains an end marker.
359 //
360 // In an ideal world, we don't need this function because .init_array and
361 // .ctors are duplicate features (and .init_array is newer.) However, there
362 // are too many real-world use cases of .ctors, so we had no choice to
363 // support that with this rather ad-hoc semantics.
364 static bool compCtors(const InputSection *A, const InputSection *B) {
365   bool BeginA = isCrtbegin(A->File->getName());
366   bool BeginB = isCrtbegin(B->File->getName());
367   if (BeginA != BeginB)
368     return BeginA;
369   bool EndA = isCrtend(A->File->getName());
370   bool EndB = isCrtend(B->File->getName());
371   if (EndA != EndB)
372     return EndB;
373   StringRef X = A->Name;
374   StringRef Y = B->Name;
375   assert(X.startswith(".ctors") || X.startswith(".dtors"));
376   assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
377   X = X.substr(6);
378   Y = Y.substr(6);
379   if (X.empty() && Y.empty())
380     return false;
381   return X < Y;
382 }
383 
384 // Sorts input sections by the special rules for .ctors and .dtors.
385 // Unfortunately, the rules are different from the one for .{init,fini}_array.
386 // Read the comment above.
387 void OutputSection::sortCtorsDtors() {
388   assert(SectionCommands.size() == 1);
389   auto *ISD = cast<InputSectionDescription>(SectionCommands[0]);
390   std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
391 }
392 
393 // If an input string is in the form of "foo.N" where N is a number,
394 // return N. Otherwise, returns 65536, which is one greater than the
395 // lowest priority.
396 int elf::getPriority(StringRef S) {
397   size_t Pos = S.rfind('.');
398   if (Pos == StringRef::npos)
399     return 65536;
400   int V;
401   if (!to_integer(S.substr(Pos + 1), V, 10))
402     return 65536;
403   return V;
404 }
405 
406 // Sorts input sections by section name suffixes, so that .foo.N comes
407 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
408 // We want to keep the original order if the priorities are the same
409 // because the compiler keeps the original initialization order in a
410 // translation unit and we need to respect that.
411 // For more detail, read the section of the GCC's manual about init_priority.
412 void OutputSection::sortInitFini() {
413   // Sort sections by priority.
414   sort([](InputSectionBase *S) { return getPriority(S->Name); });
415 }
416 
417 uint32_t OutputSection::getFiller() {
418   if (Filler)
419     return *Filler;
420   if (Flags & SHF_EXECINSTR)
421     return Target->TrapInstr;
422   return 0;
423 }
424 
425 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
426 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
427 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
428 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
429 
430 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
431 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
432 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
433 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
434 
435 template void OutputSection::maybeCompress<ELF32LE>();
436 template void OutputSection::maybeCompress<ELF32BE>();
437 template void OutputSection::maybeCompress<ELF64LE>();
438 template void OutputSection::maybeCompress<ELF64BE>();
439 
440 template void OutputSection::finalize<ELF32LE>();
441 template void OutputSection::finalize<ELF32BE>();
442 template void OutputSection::finalize<ELF64LE>();
443 template void OutputSection::finalize<ELF64BE>();
444