[elf2] Add support for R_X86_64_TLSLD.

[Bigcheese]

Summary

leaq symbol@tlsld(%rip), %rdi
call __tls_get_addr@plt

symbol@tlsld (R_X86_64_TLSLD) instructs the linker to generate a tls_index entry (two GOT slots) in the GOT for the entire module (shared object or executable) with an offset of 0. The symbol for this GOT entry doesn't matter (as long as it's either local to the module or null), and gold doesn't put a symbol in the dynamic R_X86_64_DTPMOD64 relocation for the GOT entry.

All other platforms defined in http://www.akkadia.org/drepper/tls.pdf except for Itanium use a similar model where global and local dynamic GOT entries take up 2 contiguous GOT slots, so we can handle this in a unified manner if we don't care about Itanium.

While scanning relocations we need to identify local dynamic relocations and generate a single tls_index entry in the GOT for the module and store the address of it somewhere so we can later statically resolve the offset for R_X86_64_TLSLD relocations. We also need to generate a R_X86_64_DTPMOD64 relocation in the RelaDyn relocation section.

This implementation is a bit hacky. It side steps the issue of GotSection and RelocationSection only handling SymbolBody entries by relying on a specific relocation type. The alternative to this seemed to be completely rewriting how GotSection and RelocationSection work, or using a different hacky signaling method.

[• rafael]

rebased the patch and fixed the warnings.

• t.diff7 KBDownload

[• rafael]

test/elf2/tls-dynamic.s

3

When producing an executable gold produces no dynamic relocations. I understand that is just an optimization that we don't have yet. To avoid changing this test is the future, can you change it to use -shared? With that you can also remove

​ .text

		​  .globl  _start
		​_start:

[• rafael]

Please add a second " leaq b@tlsld(%rip), %rdi" to show that only a
single R_X86_64_DTPMOD64 is created.

[• rafael]

ELF/InputSection.cpp

96

Buf + RI.r_offset is now used 3 times is this function. Please factor it into a variable above, commit that and rebase.

The same is true for BaseAddr + RI.r_offset

ELF/OutputSections.cpp

83

Will this also be used for global tls? Change the name if so.

[ruiu]

I'm out of office today. I'll review this in a few days. Sorry for the
delay.

[Bigcheese]

ELF/OutputSections.cpp

83

No. Global Dynamic symbols can be handled by addEntry already. Just needs to check the symbol type.

test/elf2/tls-dynamic.s

3

It's already -shared, I'll remove the _start symbol.

[Bigcheese]

Strengthened test and removed redundancy.

[• rafael]

LGTM with the {} removed.

ELF/Target.h

69

No other entry uses {}

[Bigcheese]

ELF/Target.h

69

The other entries don't require a default value. This is only set for x86-64, and needs to be defaulted to zero otherwise. TlsModuleIndexReloc doesn't need it though.

[• rafael]

Use = 0.

[ruiu]

LGTM

ELF/InputSection.cpp

void InputSection<ELFT>::relocate(
     uint8_t *Buf, uint8_t *BufEnd,
     iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels,
     const ObjectFile<ELFT> &File, uintX_t BaseAddr) {
   typedef Elf_Rel_Impl<ELFT, isRela> RelType;
   for (const RelType &RI : Rels) {
     uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
     uint32_t Type = RI.getType(Config->Mips64EL);
 
+    if (Type == Target->getTlsLocalDynamicReloc()) {
+      Target->relocateOne(
+          Buf + RI.r_offset, BufEnd, Type, BaseAddr + RI.r_offset,
+          Out<ELFT>::Got->getVA() + Out<ELFT>::LocalModuleTlsIndexOffset +
+              getAddend<ELFT>(RI));
+      continue;
+    }
+
     // Handle relocations for local symbols -- they never get
     // resolved so we don't allocate a SymbolBody.
     const Elf_Shdr *SymTab = File.getSymbolTable();
     if (SymIndex < SymTab->sh_info) {
       uintX_t SymVA = getLocalRelTarget(File, RI);
       Target->relocateOne(Buf + RI.r_offset, BufEnd, Type,
                           BaseAddr + RI.r_offset, SymVA);
       continue;

ELF/OutputSections.h

 
 public:
   GotSection();
   void finalize() override {
     this->Header.sh_size = Entries.size() * sizeof(uintX_t);
   }
   void writeTo(uint8_t *Buf) override;
   void addEntry(SymbolBody *Sym);
+  uint32_t addLocalModuleTlsIndex();
   bool empty() const { return Entries.empty(); }
   uintX_t getEntryAddr(const SymbolBody &B) const;
 
 private:
   std::vector<const SymbolBody *> Entries;
 };
 
 template <class ELFT>
template <class ELFT> struct Out {
   static RelocationSection<ELFT> *RelaDyn;
   static RelocationSection<ELFT> *RelaPlt;
   static StringTableSection<ELFT> *DynStrTab;
   static StringTableSection<ELFT> *ShStrTab;
   static StringTableSection<ELFT> *StrTab;
   static SymbolTableSection<ELFT> *DynSymTab;
   static SymbolTableSection<ELFT> *SymTab;
   static uintX_t TlsInitImageVA;
+  static uint32_t LocalModuleTlsIndexOffset;
 };
 
 template <class ELFT> DynamicSection<ELFT> *Out<ELFT>::Dynamic;
 template <class ELFT> GnuHashTableSection<ELFT> *Out<ELFT>::GnuHashTab;
 template <class ELFT> GotPltSection<ELFT> *Out<ELFT>::GotPlt;
 template <class ELFT> GotSection<ELFT> *Out<ELFT>::Got;
 template <class ELFT> HashTableSection<ELFT> *Out<ELFT>::HashTab;
 template <class ELFT> InterpSection<ELFT> *Out<ELFT>::Interp;
 template <class ELFT> OutputSection<ELFT> *Out<ELFT>::Bss;
 template <class ELFT> OutputSectionBase<ELFT> *Out<ELFT>::Opd;
 template <class ELFT> uint8_t *Out<ELFT>::OpdBuf;
 template <class ELFT> PltSection<ELFT> *Out<ELFT>::Plt;
 template <class ELFT> RelocationSection<ELFT> *Out<ELFT>::RelaDyn;
 template <class ELFT> RelocationSection<ELFT> *Out<ELFT>::RelaPlt;
 template <class ELFT> StringTableSection<ELFT> *Out<ELFT>::DynStrTab;
 template <class ELFT> StringTableSection<ELFT> *Out<ELFT>::ShStrTab;
 template <class ELFT> StringTableSection<ELFT> *Out<ELFT>::StrTab;
 template <class ELFT> SymbolTableSection<ELFT> *Out<ELFT>::DynSymTab;
 template <class ELFT> SymbolTableSection<ELFT> *Out<ELFT>::SymTab;
 template <class ELFT> typename Out<ELFT>::uintX_t Out<ELFT>::TlsInitImageVA;
+template <class ELFT> uint32_t Out<ELFT>::LocalModuleTlsIndexOffset = -1;
 
 } // namespace elf2
 } // namespace lld
 
 #endif // LLD_ELF_OUTPUT_SECTIONS_H

ELF/OutputSections.cpp

GotSection<ELFT>::GotSection()
   this->Header.sh_addralign = sizeof(uintX_t);
 }
 
 template <class ELFT> void GotSection<ELFT>::addEntry(SymbolBody *Sym) {
   Sym->GotIndex = Entries.size();
   Entries.push_back(Sym);
 }
 
+template <class ELFT> uint32_t GotSection<ELFT>::addLocalModuleTlsIndex() {
+  Entries.push_back(nullptr);
+  Entries.push_back(nullptr);
+  return (Entries.size() - 2) * sizeof(uintX_t);
+}
+
 template <class ELFT>
 typename GotSection<ELFT>::uintX_t
 GotSection<ELFT>::getEntryAddr(const SymbolBody &B) const {
   return this->getVA() + B.GotIndex * sizeof(uintX_t);
 }
 
 template <class ELFT> void GotSection<ELFT>::writeTo(uint8_t *Buf) {
   for (const SymbolBody *B : Entries) {
     uint8_t *Entry = Buf;
     Buf += sizeof(uintX_t);
+    if (!B)
+      continue;
     if (canBePreempted(B, false))
       continue; // The dynamic linker will take care of it.
     uintX_t VA = getSymVA<ELFT>(*B);
     write<uintX_t, ELFT::TargetEndianness, sizeof(uintX_t)>(Entry, VA);
   }
 }
 
 template <class ELFT>
for (const DynamicReloc<ELFT> &Rel : Relocs) {
     const Elf_Rel &RI = Rel.RI;
     uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
     const ObjectFile<ELFT> &File = *C.getFile();
     SymbolBody *Body = File.getSymbolBody(SymIndex);
     if (Body)
       Body = Body->repl();
 
     uint32_t Type = RI.getType(Config->Mips64EL);
+
+    if (Type == Target->getTlsLocalDynamicReloc()) {
+      P->setSymbolAndType(0, Target->getTlsModuleIndexReloc(),
+                          Config->Mips64EL);
+      P->r_offset =
+          Out<ELFT>::Got->getVA() + Out<ELFT>::LocalModuleTlsIndexOffset;
+      continue;
+    }
+
     bool NeedsCopy = Body && Target->relocNeedsCopy(Type, *Body);
     bool NeedsGot = Body && Target->relocNeedsGot(Type, *Body);
     bool CanBePreempted = canBePreempted(Body, NeedsGot);
     bool LazyReloc = Body && Target->supportsLazyRelocations() &&
                      Target->relocNeedsPlt(Type, *Body);
 
     if (CanBePreempted) {
       if (NeedsGot)

ELF/Target.h

public:
   unsigned getPageSize() const { return PageSize; }
   uint64_t getVAStart() const { return VAStart; }
   unsigned getCopyReloc() const { return CopyReloc; }
   unsigned getPCRelReloc() const { return PCRelReloc; }
   unsigned getGotReloc() const { return GotReloc; }
   unsigned getPltReloc() const { return PltReloc; }
   unsigned getGotRefReloc() const { return GotRefReloc; }
   unsigned getRelativeReloc() const { return RelativeReloc; }
+  unsigned getTlsLocalDynamicReloc() const { return TlsLocalDynamicReloc; }
+  unsigned getTlsModuleIndexReloc() const { return TlsModuleIndexReloc; }
   unsigned getPltZeroEntrySize() const { return PltZeroEntrySize; }
   unsigned getPltEntrySize() const { return PltEntrySize; }
   bool supportsLazyRelocations() const { return LazyRelocations; }
   virtual unsigned getPLTRefReloc(unsigned Type) const;
   virtual void writeGotPltEntry(uint8_t *Buf, uint64_t Plt) const = 0;
   virtual void writePltZeroEntry(uint8_t *Buf, uint64_t GotEntryAddr,
                                  uint64_t PltEntryAddr) const = 0;
   virtual void writePltEntry(uint8_t *Buf, uint64_t GotEntryAddr,
protected:
   uint64_t VAStart = 0x10000;
 
   unsigned CopyReloc;
   unsigned PCRelReloc;
   unsigned GotRefReloc;
   unsigned GotReloc;
   unsigned PltReloc;
   unsigned RelativeReloc;
+  unsigned TlsLocalDynamicReloc{};
+  unsigned TlsModuleIndexReloc{};
   unsigned PltEntrySize = 8;
   unsigned PltZeroEntrySize = 0;
   bool LazyRelocations = false;
 };
 
 uint64_t getPPC64TocBase();
 
 extern std::unique_ptr<TargetInfo> Target;
 TargetInfo *createTarget();
 }
 }
 
 #endif

ELF/Target.cpp

 
 X86_64TargetInfo::X86_64TargetInfo() {
   CopyReloc = R_X86_64_COPY;
   PCRelReloc = R_X86_64_PC32;
   GotReloc = R_X86_64_GLOB_DAT;
   GotRefReloc = R_X86_64_PC32;
   PltReloc = R_X86_64_JUMP_SLOT;
   RelativeReloc = R_X86_64_RELATIVE;
+  TlsLocalDynamicReloc = R_X86_64_TLSLD;
+  TlsModuleIndexReloc = R_X86_64_DTPMOD64;
   LazyRelocations = true;
   PltEntrySize = 16;
   PltZeroEntrySize = 16;
 }
 
 void X86_64TargetInfo::writeGotPltEntry(uint8_t *Buf, uint64_t Plt) const {
   // Skip 6 bytes of "jmpq *got(%rip)"
   write32le(Buf, Plt + 6);

 }
 
 void X86_64TargetInfo::relocateOne(uint8_t *Loc, uint8_t *BufEnd, uint32_t Type,
                                    uint64_t P, uint64_t SA) const {
   switch (Type) {
   case R_X86_64_PC32:
   case R_X86_64_GOTPCREL:
   case R_X86_64_PLT32:
+  case R_X86_64_TLSLD:
     write32le(Loc, SA - P);
     break;
   case R_X86_64_64:
     write64le(Loc, SA);
     break;
   case R_X86_64_32:
   case R_X86_64_32S:
     if (Type == R_X86_64_32 && !isUInt<32>(SA))

ELF/Writer.cpp

void Writer<ELFT>::scanRelocs(
     iterator_range<const Elf_Rel_Impl<ELFT, isRela> *> Rels) {
   typedef Elf_Rel_Impl<ELFT, isRela> RelType;
   const ObjectFile<ELFT> &File = *C.getFile();
   for (const RelType &RI : Rels) {
     uint32_t SymIndex = RI.getSymbol(Config->Mips64EL);
     SymbolBody *Body = File.getSymbolBody(SymIndex);
     uint32_t Type = RI.getType(Config->Mips64EL);
 
+    if (Type == Target->getTlsLocalDynamicReloc()) {
+      if (Out<ELFT>::LocalModuleTlsIndexOffset == -1) {
+        Out<ELFT>::LocalModuleTlsIndexOffset =
+            Out<ELFT>::Got->addLocalModuleTlsIndex();
+        Out<ELFT>::RelaDyn->addReloc({C, RI});
+      }
+      continue;
+    }
+
     // Set "used" bit for --as-needed.
     if (Body && Body->isUndefined() && !Body->isWeak())
       if (auto *S = dyn_cast<SharedSymbol<ELFT>>(Body->repl()))
         S->File->IsUsed = true;
 
     if (Body)
       Body = Body->repl();
     bool NeedsGot = false;

test/elf2/tls-dynamic.s

+// REQUIRES: x86
+// RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+// RUN: ld.lld2 -shared %t -o %tout
+// RUN: llvm-readobj -sections -relocations %tout | FileCheck %s
+// RUN: llvm-objdump -d %tout | FileCheck %s --check-prefix=DIS
+
+  .text
+  .globl  _start
+_start:
+  leaq  a@tlsld(%rip), %rdi
+  callq __tls_get_addr@PLT
+
+  .global a
+  .section  .tbss,"awT",@nobits
+  .align 4
+a:
+  .long 0
+
+// Get the address of the got, and check that it has two entries.
+
+// CHECK:      Sections [
+// CHECK:          Name: .got
+// CHECK-NEXT:     Type: SHT_PROGBITS
+// CHECK-NEXT:     Flags [
+// CHECK-NEXT:       SHF_ALLOC
+// CHECK-NEXT:       SHF_WRITE
+// CHECK-NEXT:     ]
+// CHECK-NEXT:     Address: 0x20D0
+// CHECK-NEXT:     Offset:
+// CHECK-NEXT:     Size: 16
+
+// CHECK:      Relocations [
+// CHECK:        Section ({{.+}}) .rela.dyn {
+// CHECK-NEXT:     0x20D0 R_X86_64_DTPMOD64 - 0x0
+// CHECK-NEXT:   }
+
+// 4297 = (0x20D0 + -4) - (0x1000 + 3) // PC relative offset to got entry.
+
+// DIS:      Disassembly of section .text:
+// DIS-NEXT: _start:
+// DIS-NEXT:     1000: {{.+}} leaq    4297(%rip), %rdi