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 static void 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   // 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;
190   Hdr->ch_size = Size;
191   Hdr->ch_addralign = Alignment;
192 
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)));
198 
199   // Update section headers.
200   Size = sizeof(Elf_Chdr) + CompressedData.size();
201   Flags |= SHF_COMPRESSED;
202 }
203 
204 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
205   if (Size == 1)
206     *Buf = Data;
207   else if (Size == 2)
208     write16(Buf, Data, Config->Endianness);
209   else if (Size == 4)
210     write32(Buf, Data, Config->Endianness);
211   else if (Size == 8)
212     write64(Buf, Data, Config->Endianness);
213   else
214     llvm_unreachable("unsupported Size argument");
215 }
216 
217 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
218   if (Type == SHT_NOBITS)
219     return;
220 
221   Loc = Buf;
222 
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());
230     return;
231   }
232 
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)
238         if (IS->Live)
239           Sections.push_back(IS);
240   uint32_t Filler = getFiller();
241   if (Filler)
242     fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
243 
244   parallelForEachN(0, Sections.size(), [&](size_t I) {
245     InputSection *IS = Sections[I];
246     IS->writeTo<ELFT>(Buf);
247 
248     // Fill gaps between sections.
249     if (Filler) {
250       uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
251       uint8_t *End;
252       if (I + 1 == Sections.size())
253         End = Buf + Size;
254       else
255         End = Buf + Sections[I + 1]->OutSecOff;
256       fill(Start, End - Start, Filler);
257     }
258   });
259 
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);
265 }
266 
267 template <class ELFT>
268 static void finalizeShtGroup(OutputSection *OS,
269                              InputSection *Section) {
270   assert(Config->Relocatable);
271 
272   // sh_link field for SHT_GROUP sections should contain the section index of
273   // the symbol table.
274   OS->Link = InX::SymTab->getParent()->SectionIndex;
275 
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]);
281 }
282 
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())
288         continue;
289       if (First == nullptr)
290         First = ISD->Sections.front();
291     }
292     if (isa<ByteCommand>(Base) && Type == SHT_NOBITS)
293       Type = SHT_PROGBITS;
294   }
295 
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;
303   }
304 
305   if (Type == SHT_GROUP) {
306     finalizeShtGroup<ELFT>(this, First);
307     return;
308   }
309 
310   if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
311     return;
312 
313   if (isa<SyntheticSection>(First))
314     return;
315 
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;
322 }
323 
324 // Returns true if S matches /Filename.?\.o$/.
325 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
326   if (!S.endswith(".o"))
327     return false;
328   S = S.drop_back(2);
329   if (S.endswith(Filename))
330     return true;
331   return !S.empty() && S.drop_back().endswith(Filename);
332 }
333 
334 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
335 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
336 
337 // .ctors and .dtors are sorted by this priority from highest to lowest.
338 //
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.)
342 //
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.
346 //
347 //  3. The section is just ".ctors" or ".dtors".
348 //
349 //  4. The section was contained in crtend, which contains an end marker.
350 //
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)
359     return BeginA;
360   bool EndA = isCrtend(A->File->getName());
361   bool EndB = isCrtend(B->File->getName());
362   if (EndA != EndB)
363     return EndB;
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"));
368   X = X.substr(6);
369   Y = Y.substr(6);
370   if (X.empty() && Y.empty())
371     return false;
372   return X < Y;
373 }
374 
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);
382 }
383 
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
386 // lowest priority.
387 int elf::getPriority(StringRef S) {
388   size_t Pos = S.rfind('.');
389   if (Pos == StringRef::npos)
390     return 65536;
391   int V;
392   if (!to_integer(S.substr(Pos + 1), V, 10))
393     return 65536;
394   return V;
395 }
396 
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); });
406 }
407 
408 uint32_t OutputSection::getFiller() {
409   if (Filler)
410     return *Filler;
411   if (Flags & SHF_EXECINSTR)
412     return Target->TrapInstr;
413   return 0;
414 }
415 
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);
420 
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);
425 
426 template void OutputSection::maybeCompress<ELF32LE>();
427 template void OutputSection::maybeCompress<ELF32BE>();
428 template void OutputSection::maybeCompress<ELF64LE>();
429 template void OutputSection::maybeCompress<ELF64BE>();
430 
431 template void OutputSection::finalize<ELF32LE>();
432 template void OutputSection::finalize<ELF32BE>();
433 template void OutputSection::finalize<ELF64LE>();
434 template void OutputSection::finalize<ELF64BE>();
435