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::ELF;
29 
30 using namespace lld;
31 using namespace lld::elf;
32 
33 uint8_t Out::First;
34 PhdrEntry *Out::TlsPhdr;
35 OutputSection *Out::DebugInfo;
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 
44 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>
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 
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.
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 
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_ALLOC | 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 
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 
155 uint64_t elf::getHeaderSize() {
156   if (Config->OFormatBinary)
157     return 0;
158   return Out::ElfHeader->Size + Out::ProgramHeaders->Size;
159 }
160 
161 bool OutputSection::classof(const BaseCommand *C) {
162   return C->Kind == OutputSectionKind;
163 }
164 
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.
174 static void fill(uint8_t *Buf, size_t Size, uint32_t Filler) {
175   size_t I = 0;
176   for (; I + 4 < Size; I += 4)
177     memcpy(Buf + I, &Filler, 4);
178   memcpy(Buf + I, &Filler, Size - I);
179 }
180 
181 // Compress section contents if this section contains debug info.
182 template <class ELFT> void OutputSection::maybeCompress() {
183   typedef typename ELFT::Chdr Elf_Chdr;
184 
185   // Compress only DWARF debug sections.
186   if (!Config->CompressDebugSections || (Flags & SHF_ALLOC) ||
187       !Name.startswith(".debug_"))
188     return;
189 
190   // Create a section header.
191   ZDebugHeader.resize(sizeof(Elf_Chdr));
192   auto *Hdr = reinterpret_cast<Elf_Chdr *>(ZDebugHeader.data());
193   Hdr->ch_type = ELFCOMPRESS_ZLIB;
194   Hdr->ch_size = Size;
195   Hdr->ch_addralign = Alignment;
196 
197   // Write section contents to a temporary buffer and compress it.
198   std::vector<uint8_t> Buf(Size);
199   writeTo<ELFT>(Buf.data());
200   if (Error E = zlib::compress(toStringRef(Buf), CompressedData))
201     fatal("compress failed: " + llvm::toString(std::move(E)));
202 
203   // Update section headers.
204   Size = sizeof(Elf_Chdr) + CompressedData.size();
205   Flags |= SHF_COMPRESSED;
206 }
207 
208 static void writeInt(uint8_t *Buf, uint64_t Data, uint64_t Size) {
209   if (Size == 1)
210     *Buf = Data;
211   else if (Size == 2)
212     write16(Buf, Data);
213   else if (Size == 4)
214     write32(Buf, Data);
215   else if (Size == 8)
216     write64(Buf, Data);
217   else
218     llvm_unreachable("unsupported Size argument");
219 }
220 
221 template <class ELFT> void OutputSection::writeTo(uint8_t *Buf) {
222   if (Type == SHT_NOBITS)
223     return;
224 
225   Loc = Buf;
226 
227   // If -compress-debug-section is specified and if this is a debug seciton,
228   // we've already compressed section contents. If that's the case,
229   // just write it down.
230   if (!CompressedData.empty()) {
231     memcpy(Buf, ZDebugHeader.data(), ZDebugHeader.size());
232     memcpy(Buf + ZDebugHeader.size(), CompressedData.data(),
233            CompressedData.size());
234     return;
235   }
236 
237   // Write leading padding.
238   std::vector<InputSection *> Sections = getInputSections(this);
239   uint32_t Filler = getFiller();
240   if (Filler)
241     fill(Buf, Sections.empty() ? Size : Sections[0]->OutSecOff, Filler);
242 
243   parallelForEachN(0, Sections.size(), [&](size_t I) {
244     InputSection *IS = Sections[I];
245     IS->writeTo<ELFT>(Buf);
246 
247     // Fill gaps between sections.
248     if (Filler) {
249       uint8_t *Start = Buf + IS->OutSecOff + IS->getSize();
250       uint8_t *End;
251       if (I + 1 == Sections.size())
252         End = Buf + Size;
253       else
254         End = Buf + Sections[I + 1]->OutSecOff;
255       fill(Start, End - Start, Filler);
256     }
257   });
258 
259   // Linker scripts may have BYTE()-family commands with which you
260   // can write arbitrary bytes to the output. Process them if any.
261   for (BaseCommand *Base : SectionCommands)
262     if (auto *Data = dyn_cast<ByteCommand>(Base))
263       writeInt(Buf + Data->Offset, Data->Expression().getValue(), Data->Size);
264 }
265 
266 template <class ELFT>
267 static void finalizeShtGroup(OutputSection *OS,
268                              InputSection *Section) {
269   assert(Config->Relocatable);
270 
271   // sh_link field for SHT_GROUP sections should contain the section index of
272   // the symbol table.
273   OS->Link = InX::SymTab->getParent()->SectionIndex;
274 
275   // sh_info then contain index of an entry in symbol table section which
276   // provides signature of the section group.
277   ObjFile<ELFT> *Obj = Section->getFile<ELFT>();
278   ArrayRef<Symbol *> Symbols = Obj->getSymbols();
279   OS->Info = InX::SymTab->getSymbolIndex(Symbols[Section->Info]);
280 }
281 
282 template <class ELFT> void OutputSection::finalize() {
283   if (Type == SHT_NOBITS)
284     for (BaseCommand *Base : SectionCommands)
285       if (isa<ByteCommand>(Base))
286         Type = SHT_PROGBITS;
287 
288   std::vector<InputSection *> V = getInputSections(this);
289   InputSection *First = V.empty() ? nullptr : V[0];
290 
291   if (Flags & SHF_LINK_ORDER) {
292     // We must preserve the link order dependency of sections with the
293     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
294     // need to translate the InputSection sh_link to the OutputSection sh_link,
295     // all InputSections in the OutputSection have the same dependency.
296     if (auto *D = First->getLinkOrderDep())
297       Link = D->getParent()->SectionIndex;
298   }
299 
300   if (Type == SHT_GROUP) {
301     finalizeShtGroup<ELFT>(this, First);
302     return;
303   }
304 
305   if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
306     return;
307 
308   if (isa<SyntheticSection>(First))
309     return;
310 
311   Link = InX::SymTab->getParent()->SectionIndex;
312   // sh_info for SHT_REL[A] sections should contain the section header index of
313   // the section to which the relocation applies.
314   InputSectionBase *S = First->getRelocatedSection();
315   Info = S->getOutputSection()->SectionIndex;
316   Flags |= SHF_INFO_LINK;
317 }
318 
319 // Returns true if S matches /Filename.?\.o$/.
320 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
321   if (!S.endswith(".o"))
322     return false;
323   S = S.drop_back(2);
324   if (S.endswith(Filename))
325     return true;
326   return !S.empty() && S.drop_back().endswith(Filename);
327 }
328 
329 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
330 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
331 
332 // .ctors and .dtors are sorted by this priority from highest to lowest.
333 //
334 //  1. The section was contained in crtbegin (crtbegin contains
335 //     some sentinel value in its .ctors and .dtors so that the runtime
336 //     can find the beginning of the sections.)
337 //
338 //  2. The section has an optional priority value in the form of ".ctors.N"
339 //     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
340 //     they are compared as string rather than number.
341 //
342 //  3. The section is just ".ctors" or ".dtors".
343 //
344 //  4. The section was contained in crtend, which contains an end marker.
345 //
346 // In an ideal world, we don't need this function because .init_array and
347 // .ctors are duplicate features (and .init_array is newer.) However, there
348 // are too many real-world use cases of .ctors, so we had no choice to
349 // support that with this rather ad-hoc semantics.
350 static bool compCtors(const InputSection *A, const InputSection *B) {
351   bool BeginA = isCrtbegin(A->File->getName());
352   bool BeginB = isCrtbegin(B->File->getName());
353   if (BeginA != BeginB)
354     return BeginA;
355   bool EndA = isCrtend(A->File->getName());
356   bool EndB = isCrtend(B->File->getName());
357   if (EndA != EndB)
358     return EndB;
359   StringRef X = A->Name;
360   StringRef Y = B->Name;
361   assert(X.startswith(".ctors") || X.startswith(".dtors"));
362   assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
363   X = X.substr(6);
364   Y = Y.substr(6);
365   return X < Y;
366 }
367 
368 // Sorts input sections by the special rules for .ctors and .dtors.
369 // Unfortunately, the rules are different from the one for .{init,fini}_array.
370 // Read the comment above.
371 void OutputSection::sortCtorsDtors() {
372   assert(SectionCommands.size() == 1);
373   auto *ISD = cast<InputSectionDescription>(SectionCommands[0]);
374   std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
375 }
376 
377 // If an input string is in the form of "foo.N" where N is a number,
378 // return N. Otherwise, returns 65536, which is one greater than the
379 // lowest priority.
380 int elf::getPriority(StringRef S) {
381   size_t Pos = S.rfind('.');
382   if (Pos == StringRef::npos)
383     return 65536;
384   int V;
385   if (!to_integer(S.substr(Pos + 1), V, 10))
386     return 65536;
387   return V;
388 }
389 
390 std::vector<InputSection *> elf::getInputSections(OutputSection *OS) {
391   std::vector<InputSection *> Ret;
392   for (BaseCommand *Base : OS->SectionCommands)
393     if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
394       Ret.insert(Ret.end(), ISD->Sections.begin(), ISD->Sections.end());
395   return Ret;
396 }
397 
398 // Sorts input sections by section name suffixes, so that .foo.N comes
399 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
400 // We want to keep the original order if the priorities are the same
401 // because the compiler keeps the original initialization order in a
402 // translation unit and we need to respect that.
403 // For more detail, read the section of the GCC's manual about init_priority.
404 void OutputSection::sortInitFini() {
405   // Sort sections by priority.
406   sort([](InputSectionBase *S) { return getPriority(S->Name); });
407 }
408 
409 uint32_t OutputSection::getFiller() {
410   if (Filler)
411     return *Filler;
412   if (Flags & SHF_EXECINSTR)
413     return Target->TrapInstr;
414   return 0;
415 }
416 
417 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
418 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
419 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
420 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
421 
422 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
423 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
424 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
425 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
426 
427 template void OutputSection::maybeCompress<ELF32LE>();
428 template void OutputSection::maybeCompress<ELF32BE>();
429 template void OutputSection::maybeCompress<ELF64LE>();
430 template void OutputSection::maybeCompress<ELF64BE>();
431 
432 template void OutputSection::finalize<ELF32LE>();
433 template void OutputSection::finalize<ELF32BE>();
434 template void OutputSection::finalize<ELF64LE>();
435 template void OutputSection::finalize<ELF64BE>();
436