1 //===- OutputSections.cpp -------------------------------------------------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "OutputSections.h"
10 #include "Config.h"
11 #include "LinkerScript.h"
12 #include "SymbolTable.h"
13 #include "SyntheticSections.h"
14 #include "Target.h"
15 #include "lld/Common/Memory.h"
16 #include "lld/Common/Strings.h"
17 #include "lld/Common/Threads.h"
18 #include "llvm/BinaryFormat/Dwarf.h"
19 #include "llvm/Support/Compression.h"
20 #include "llvm/Support/MD5.h"
21 #include "llvm/Support/MathExtras.h"
22 #include "llvm/Support/SHA1.h"
23 
24 using namespace llvm;
25 using namespace llvm::dwarf;
26 using namespace llvm::object;
27 using namespace llvm::support::endian;
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_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,
174                  const std::array<uint8_t, 4> &Filler) {
175   size_t I = 0;
176   for (; I + 4 < Size; I += 4)
177     memcpy(Buf + I, Filler.data(), 4);
178   memcpy(Buf + I, Filler.data(), 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   std::array<uint8_t, 4> Filler = getFiller();
240   bool NonZeroFiller = read32(Filler.data()) != 0;
241   if (NonZeroFiller)
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 (NonZeroFiller) {
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 = In.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 = In.SymTab->getSymbolIndex(Symbols[Section->Info]);
281 }
282 
283 template <class ELFT> void OutputSection::finalize() {
284   if (Type == SHT_NOBITS)
285     for (BaseCommand *Base : SectionCommands)
286       if (isa<ByteCommand>(Base))
287         Type = SHT_PROGBITS;
288 
289   std::vector<InputSection *> V = getInputSections(this);
290   InputSection *First = V.empty() ? nullptr : V[0];
291 
292   if (Flags & SHF_LINK_ORDER) {
293     // We must preserve the link order dependency of sections with the
294     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
295     // need to translate the InputSection sh_link to the OutputSection sh_link,
296     // all InputSections in the OutputSection have the same dependency.
297     if (auto *D = First->getLinkOrderDep())
298       Link = D->getParent()->SectionIndex;
299   }
300 
301   if (Type == SHT_GROUP) {
302     finalizeShtGroup<ELFT>(this, First);
303     return;
304   }
305 
306   if (!Config->CopyRelocs || (Type != SHT_RELA && Type != SHT_REL))
307     return;
308 
309   if (isa<SyntheticSection>(First))
310     return;
311 
312   Link = In.SymTab->getParent()->SectionIndex;
313   // sh_info for SHT_REL[A] sections should contain the section header index of
314   // the section to which the relocation applies.
315   InputSectionBase *S = First->getRelocatedSection();
316   Info = S->getOutputSection()->SectionIndex;
317   Flags |= SHF_INFO_LINK;
318 }
319 
320 // Returns true if S matches /Filename.?\.o$/.
321 static bool isCrtBeginEnd(StringRef S, StringRef Filename) {
322   if (!S.endswith(".o"))
323     return false;
324   S = S.drop_back(2);
325   if (S.endswith(Filename))
326     return true;
327   return !S.empty() && S.drop_back().endswith(Filename);
328 }
329 
330 static bool isCrtbegin(StringRef S) { return isCrtBeginEnd(S, "crtbegin"); }
331 static bool isCrtend(StringRef S) { return isCrtBeginEnd(S, "crtend"); }
332 
333 // .ctors and .dtors are sorted by this priority from highest to lowest.
334 //
335 //  1. The section was contained in crtbegin (crtbegin contains
336 //     some sentinel value in its .ctors and .dtors so that the runtime
337 //     can find the beginning of the sections.)
338 //
339 //  2. The section has an optional priority value in the form of ".ctors.N"
340 //     or ".dtors.N" where N is a number. Unlike .{init,fini}_array,
341 //     they are compared as string rather than number.
342 //
343 //  3. The section is just ".ctors" or ".dtors".
344 //
345 //  4. The section was contained in crtend, which contains an end marker.
346 //
347 // In an ideal world, we don't need this function because .init_array and
348 // .ctors are duplicate features (and .init_array is newer.) However, there
349 // are too many real-world use cases of .ctors, so we had no choice to
350 // support that with this rather ad-hoc semantics.
351 static bool compCtors(const InputSection *A, const InputSection *B) {
352   bool BeginA = isCrtbegin(A->File->getName());
353   bool BeginB = isCrtbegin(B->File->getName());
354   if (BeginA != BeginB)
355     return BeginA;
356   bool EndA = isCrtend(A->File->getName());
357   bool EndB = isCrtend(B->File->getName());
358   if (EndA != EndB)
359     return EndB;
360   StringRef X = A->Name;
361   StringRef Y = B->Name;
362   assert(X.startswith(".ctors") || X.startswith(".dtors"));
363   assert(Y.startswith(".ctors") || Y.startswith(".dtors"));
364   X = X.substr(6);
365   Y = Y.substr(6);
366   return X < Y;
367 }
368 
369 // Sorts input sections by the special rules for .ctors and .dtors.
370 // Unfortunately, the rules are different from the one for .{init,fini}_array.
371 // Read the comment above.
372 void OutputSection::sortCtorsDtors() {
373   assert(SectionCommands.size() == 1);
374   auto *ISD = cast<InputSectionDescription>(SectionCommands[0]);
375   std::stable_sort(ISD->Sections.begin(), ISD->Sections.end(), compCtors);
376 }
377 
378 // If an input string is in the form of "foo.N" where N is a number,
379 // return N. Otherwise, returns 65536, which is one greater than the
380 // lowest priority.
381 int elf::getPriority(StringRef S) {
382   size_t Pos = S.rfind('.');
383   if (Pos == StringRef::npos)
384     return 65536;
385   int V;
386   if (!to_integer(S.substr(Pos + 1), V, 10))
387     return 65536;
388   return V;
389 }
390 
391 std::vector<InputSection *> elf::getInputSections(OutputSection *OS) {
392   std::vector<InputSection *> Ret;
393   for (BaseCommand *Base : OS->SectionCommands)
394     if (auto *ISD = dyn_cast<InputSectionDescription>(Base))
395       Ret.insert(Ret.end(), ISD->Sections.begin(), ISD->Sections.end());
396   return Ret;
397 }
398 
399 // Sorts input sections by section name suffixes, so that .foo.N comes
400 // before .foo.M if N < M. Used to sort .{init,fini}_array.N sections.
401 // We want to keep the original order if the priorities are the same
402 // because the compiler keeps the original initialization order in a
403 // translation unit and we need to respect that.
404 // For more detail, read the section of the GCC's manual about init_priority.
405 void OutputSection::sortInitFini() {
406   // Sort sections by priority.
407   sort([](InputSectionBase *S) { return getPriority(S->Name); });
408 }
409 
410 std::array<uint8_t, 4> OutputSection::getFiller() {
411   if (Filler)
412     return *Filler;
413   if (Flags & SHF_EXECINSTR)
414     return Target->TrapInstr;
415   return {0, 0, 0, 0};
416 }
417 
418 template void OutputSection::writeHeaderTo<ELF32LE>(ELF32LE::Shdr *Shdr);
419 template void OutputSection::writeHeaderTo<ELF32BE>(ELF32BE::Shdr *Shdr);
420 template void OutputSection::writeHeaderTo<ELF64LE>(ELF64LE::Shdr *Shdr);
421 template void OutputSection::writeHeaderTo<ELF64BE>(ELF64BE::Shdr *Shdr);
422 
423 template void OutputSection::writeTo<ELF32LE>(uint8_t *Buf);
424 template void OutputSection::writeTo<ELF32BE>(uint8_t *Buf);
425 template void OutputSection::writeTo<ELF64LE>(uint8_t *Buf);
426 template void OutputSection::writeTo<ELF64BE>(uint8_t *Buf);
427 
428 template void OutputSection::maybeCompress<ELF32LE>();
429 template void OutputSection::maybeCompress<ELF32BE>();
430 template void OutputSection::maybeCompress<ELF64LE>();
431 template void OutputSection::maybeCompress<ELF64BE>();
432 
433 template void OutputSection::finalize<ELF32LE>();
434 template void OutputSection::finalize<ELF32BE>();
435 template void OutputSection::finalize<ELF64LE>();
436 template void OutputSection::finalize<ELF64BE>();
437