[ELF] - Do not handle ELF and program header as dummy sections

[grimar]

Summary

During review of D18499 was requested to improve way how
we deal with dummies sections.
This patch does that. Now they are not part of OutputSections list,
but still keeping them as sections helps to simplify the code.

[ruiu]

Generally looks good. A few nits.

ELF/Writer.cpp

103 ↗

(On Diff #52502)

Why +1?

1346 ↗

(On Diff #52502)

This function needs a comment.

1347–1349 ↗

(On Diff #52502)

"Off" is always zero, so please do this.

Out<ELFT>::ElfHeader->setVA(Target->getVAStart());
Out<ELFT>::ElfHeader->setFileOffset(0);

[ruiu]

Please change the title of this patch to something like "Do not handle ELF and program header as dummy sections", since what you are doing here is not improving the dummy sections but instead removing the concept of that.

[grimar]

• Addressed Rui's review comments.

ELF/Writer.cpp

103 ↗

(On Diff #52502)

Because we have one "dummy-zero" section:

Section Headers:
  [Nr] Name              Type             Address           Offset
       Size              EntSize          Flags  Link  Info  Align
  [ 0]                   NULL             0000000000000000  00000000
       0000000000000000  0000000000000000           0     0     0

1346 ↗

(On Diff #52502)

Done.

1347–1349 ↗

(On Diff #52502)

Done.

[grimar]

• Fixed incompletely addressed place.

[ruiu]

ELF/Writer.cpp

103 ↗

(On Diff #52511)

It's a bit weird to count the first null entry of the section table as a section. You can just remove this function, no?

[grimar]

• Addressed review comments.

ELF/Writer.cpp

103 ↗

(On Diff #52511)

Removed, but there was 2 places where it was called, I put (OutputSections.size() + 1) there instead:

EHdr->e_shnum = OutputSections.size() + 1;

FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);

[ruiu]

LGTM

lld/trunk/ELF/Writer.cpp

 
 using namespace llvm;
 using namespace llvm::ELF;
 using namespace llvm::object;
 
 using namespace lld;
 using namespace lld::elf;
 
-// Usually there are 2 dummies sections: ELF header and program header.
-// Relocatable output does not require program headers to be created.
-static unsigned dummySectionsNum() { return Config->Relocatable ? 1 : 2; }
-
 namespace {
 // The writer writes a SymbolTable result to a file.
 template <class ELFT> class Writer {
 public:
   typedef typename ELFT::uint uintX_t;
   typedef typename ELFT::Shdr Elf_Shdr;
   typedef typename ELFT::Ehdr Elf_Ehdr;
   typedef typename ELFT::Phdr Elf_Phdr;
private:
   void scanRelocs(InputSectionBase<ELFT> &C, ArrayRef<RelTy> Rels);
 
   void scanRelocs(InputSection<ELFT> &C);
   void scanRelocs(InputSectionBase<ELFT> &S, const Elf_Shdr &RelSec);
   void createPhdrs();
   void assignAddresses();
   void assignFileOffsets();
   void setPhdrs();
+  void fixHeaders();
   void fixSectionAlignments();
   void fixAbsoluteSymbols();
   void openFile();
   void writeHeader();
   void writeSections();
   void writeBuildId();
   bool isDiscarded(InputSectionBase<ELFT> *IS) const;
   StringRef getOutputSectionName(InputSectionBase<ELFT> *S) const;
private:
   void addCopyRelSymbol(SharedSymbol<ELFT> *Sym);
 
   std::unique_ptr<llvm::FileOutputBuffer> Buffer;
 
   BumpPtrAllocator Alloc;
   std::vector<OutputSectionBase<ELFT> *> OutputSections;
   std::vector<std::unique_ptr<OutputSectionBase<ELFT>>> OwningSections;
 
-  // We create a section for the ELF header and one for the program headers.
-  ArrayRef<OutputSectionBase<ELFT> *> getSections() const {
-    return makeArrayRef(OutputSections).slice(dummySectionsNum());
-  }
-  unsigned getNumSections() const {
-    return OutputSections.size() + 1 - dummySectionsNum();
-  }
-
   void addRelIpltSymbols();
   void addStartEndSymbols();
   void addStartStopSymbols(OutputSectionBase<ELFT> *Sec);
 
   SymbolTable<ELFT> &Symtab;
   std::vector<Phdr> Phdrs;
 
   uintX_t FileSize;
template <class ELFT> void Writer<ELFT>::run() {
   createSections();
   if (HasError)
     return;
 
   if (Config->Relocatable) {
     assignFileOffsets();
   } else {
     createPhdrs();
+    fixHeaders();
     fixSectionAlignments();
     assignAddresses();
     assignFileOffsets();
     setPhdrs();
     fixAbsoluteSymbols();
   }
 
   openFile();

 // Sort input sections by the special rule for .ctors and .dtors.
 template <class ELFT> static void sortCtorsDtors(OutputSectionBase<ELFT> *S) {
   if (S)
     reinterpret_cast<OutputSection<ELFT> *>(S)->sortCtorsDtors();
 }
 
 // Create output section objects and add them to OutputSections.
 template <class ELFT> void Writer<ELFT>::createSections() {
-  OutputSections.push_back(Out<ELFT>::ElfHeader);
-  if (!Config->Relocatable)
-    OutputSections.push_back(Out<ELFT>::ProgramHeaders);
-
   // Add .interp first because some loaders want to see that section
   // on the first page of the executable file when loaded into memory.
   if (needsInterpSection())
     OutputSections.push_back(Out<ELFT>::Interp);
 
   // A core file does not usually contain unmodified segments except
   // the first page of the executable. Add the build ID section now
   // so that the section is included in the first page.
for (InputSectionBase<ELFT> *C : F->getSections()) {
       if (auto *S = dyn_cast<InputSection<ELFT>>(C))
         scanRelocs(*S);
       else if (auto *S = dyn_cast<EHInputSection<ELFT>>(C))
         if (S->RelocSection)
           scanRelocs(*S, *S->RelocSection);
     }
   }
 
-  for (OutputSectionBase<ELFT> *Sec : getSections())
+  for (OutputSectionBase<ELFT> *Sec : OutputSections)
     Sec->assignOffsets();
 
   // Now that we have defined all possible symbols including linker-
   // synthesized ones. Visit all symbols to give the finishing touches.
   std::vector<DefinedCommon *> CommonSymbols;
   for (auto &P : Symtab.getSymbols()) {
     SymbolBody *Body = P.second->Body;
     if (auto *U = dyn_cast<Undefined>(Body))
template <class ELFT> void Writer<ELFT>::createSections() {
 
   // So far we have added sections from input object files.
   // This function adds linker-created Out<ELFT>::* sections.
   addPredefinedSections();
 
   std::stable_sort(OutputSections.begin(), OutputSections.end(),
                    compareSections<ELFT>);
 
-  for (unsigned I = dummySectionsNum(), N = OutputSections.size(); I < N; ++I)
-    OutputSections[I]->SectionIndex = I + 1 - dummySectionsNum();
-
-  for (OutputSectionBase<ELFT> *Sec : getSections())
+  unsigned I = 1;
+  for (OutputSectionBase<ELFT> *Sec : OutputSections) {
+    Sec->SectionIndex = I++;
     Sec->setSHName(Out<ELFT>::ShStrTab->addString(Sec->getName()));
+  }
 
   // Finalizers fix each section's size.
   // .dynsym is finalized early since that may fill up .gnu.hash.
   if (isOutputDynamic())
     Out<ELFT>::DynSymTab->finalize();
 
   // Fill other section headers. The dynamic table is finalized
   // at the end because some tags like RELSZ depend on result
if (needsInterpSection()) {
     Phdr &Hdr = *AddHdr(PT_INTERP, toPhdrFlags(Out<ELFT>::Interp->getFlags()));
     AddSec(Hdr, Out<ELFT>::Interp);
   }
 
   // Add the first PT_LOAD segment for regular output sections.
   uintX_t Flags = PF_R;
   Phdr *Load = AddHdr(PT_LOAD, Flags);
   AddSec(*Load, Out<ELFT>::ElfHeader);
+  AddSec(*Load, Out<ELFT>::ProgramHeaders);
 
   Phdr TlsHdr(PT_TLS, PF_R);
   Phdr RelRo(PT_GNU_RELRO, PF_R);
   Phdr Note(PT_NOTE, PF_R);
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (!(Sec->getFlags() & SHF_ALLOC))
       break;
 
for (const Phdr &P : Phdrs) {
     if (I == End || (I + 1) == End)
       continue;
     OutputSectionBase<ELFT> *Sec = *(I + 1);
     if (needsPtLoad(Sec))
       Sec->PageAlign = true;
   }
 }
 
+// We should set file offsets and VAs for elf header and program headers
+// sections. These are special, we do not include them into output sections
+// list, but have them to simplify the code.
+template <class ELFT> void Writer<ELFT>::fixHeaders() {
+  Out<ELFT>::ElfHeader->setVA(Target->getVAStart());
+  Out<ELFT>::ElfHeader->setFileOffset(0);
+  uintX_t Off = Out<ELFT>::ElfHeader->getSize();
+  Out<ELFT>::ProgramHeaders->setVA(Off + Target->getVAStart());
+  Out<ELFT>::ProgramHeaders->setFileOffset(Off);
+}
+
 // Assign VAs (addresses at run-time) to output sections.
 template <class ELFT> void Writer<ELFT>::assignAddresses() {
-  uintX_t ThreadBssOffset = 0;
-  uintX_t VA = Target->getVAStart();
+  uintX_t VA = Target->getVAStart() + Out<ELFT>::ElfHeader->getSize() +
+               Out<ELFT>::ProgramHeaders->getSize();
 
+  uintX_t ThreadBssOffset = 0;
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     uintX_t Align = Sec->getAlign();
     if (Sec->PageAlign)
       Align = std::max<uintX_t>(Align, Target->PageSize);
 
     // We only assign VAs to allocated sections.
     if (needsPtLoad<ELFT>(Sec)) {
       VA = alignTo(VA, Align);
       Sec->setVA(VA);
       VA += Sec->getSize();
     } else if (Sec->getFlags() & SHF_TLS && Sec->getType() == SHT_NOBITS) {
       uintX_t TVA = VA + ThreadBssOffset;
       TVA = alignTo(TVA, Align);
       Sec->setVA(TVA);
       ThreadBssOffset = TVA - VA + Sec->getSize();
     }
   }
 }
 
 // Assign file offsets to output sections.
 template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
-  uintX_t Off = 0;
+  uintX_t Off =
+      Out<ELFT>::ElfHeader->getSize() + Out<ELFT>::ProgramHeaders->getSize();
+
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (Sec->getType() == SHT_NOBITS) {
       Sec->setFileOffset(Off);
       continue;
     }
     uintX_t Align = Sec->getAlign();
     if (Sec->PageAlign)
       Align = std::max<uintX_t>(Align, Target->PageSize);
     Off = alignTo(Off, Align);
     Sec->setFileOffset(Off);
     Off += Sec->getSize();
   }
   SectionHeaderOff = alignTo(Off, sizeof(uintX_t));
-  FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
+  FileSize = SectionHeaderOff + (OutputSections.size() + 1) * sizeof(Elf_Shdr);
 }
 
 // Finalize the program headers. We call this function after we assign
 // file offsets and VAs to all sections.
 template <class ELFT> void Writer<ELFT>::setPhdrs() {
   for (Phdr &P : Phdrs) {
     Elf_Phdr &H = P.H;
     OutputSectionBase<ELFT> *First = P.First;
template <class ELFT> void Writer<ELFT>::writeHeader() {
   EHdr->e_type = getELFType();
   EHdr->e_machine = FirstObj.getEMachine();
   EHdr->e_version = EV_CURRENT;
   EHdr->e_entry = getEntryAddr<ELFT>();
   EHdr->e_shoff = SectionHeaderOff;
   EHdr->e_ehsize = sizeof(Elf_Ehdr);
   EHdr->e_phnum = Phdrs.size();
   EHdr->e_shentsize = sizeof(Elf_Shdr);
-  EHdr->e_shnum = getNumSections();
+  EHdr->e_shnum = OutputSections.size() + 1;
   EHdr->e_shstrndx = Out<ELFT>::ShStrTab->SectionIndex;
 
   if (Config->EMachine == EM_MIPS)
     EHdr->e_flags = getMipsEFlags();
 
   if (!Config->Relocatable) {
     EHdr->e_phoff = sizeof(Elf_Ehdr);
     EHdr->e_phentsize = sizeof(Elf_Phdr);
   }
 
   // Write the program header table.
   auto *HBuf = reinterpret_cast<Elf_Phdr *>(Buf + EHdr->e_phoff);
   for (Phdr &P : Phdrs)
     *HBuf++ = P.H;
 
   // Write the section header table. Note that the first table entry is null.
   auto *SHdrs = reinterpret_cast<Elf_Shdr *>(Buf + EHdr->e_shoff);
-  for (OutputSectionBase<ELFT> *Sec : getSections())
+  for (OutputSectionBase<ELFT> *Sec : OutputSections)
     Sec->writeHeaderTo(++SHdrs);
 }
 
 template <class ELFT> void Writer<ELFT>::openFile() {
   ErrorOr<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
       FileOutputBuffer::create(Config->OutputFile, FileSize,
                                FileOutputBuffer::F_executable);
   if (BufferOrErr)