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