[ELF] - Added partial support for --emit-relocs (no --gc-section case, no /DISCARD/ support) #3

[grimar]

Summary

-q, --emit-relocs - Generate relocations in output

This patch is extracted from D28612/D29636,

Simplest implementation: no GC case, no "/DISCARD/" linkerscript command support.

Relative to PR31579.

[grimar]

• Addressed review commetns.

[grimar]

• Removed forgotten input,

lld/trunk/ELF/Config.h

struct Configuration {
   bool AsNeeded = false;
   bool Bsymbolic;
   bool BsymbolicFunctions;
   bool ColorDiagnostics = false;
   bool DefineCommon;
   bool Demangle = true;
   bool DisableVerify;
   bool EhFrameHdr;
+  bool EmitRelocs;
   bool EnableNewDtags;
   bool ExportDynamic;
   bool FatalWarnings;
   bool GcSections;
   bool GdbIndex;
   bool GnuHash = false;
   bool ICF;
   bool Mips64EL = false;
struct Configuration {
   uint64_t ErrorLimit = 20;
   uint64_t ImageBase;
   uint64_t MaxPageSize;
   uint64_t ZStackSize;
   unsigned LTOPartitions;
   unsigned LTOO;
   unsigned Optimize;
   unsigned ThinLTOJobs;
+  bool copyRelocs() { return Relocatable || EmitRelocs; };
 };
 
 // The only instance of Configuration struct.
 extern Configuration *Config;
 
 } // namespace elf
 } // namespace lld
 
 #endif

lld/trunk/ELF/Driver.cpp

if (auto *Arg = Args.getLastArg(OPT_m)) {
     Config->Emulation = S;
   }
 
   Config->AllowMultipleDefinition = Args.hasArg(OPT_allow_multiple_definition);
   Config->Bsymbolic = Args.hasArg(OPT_Bsymbolic);
   Config->BsymbolicFunctions = Args.hasArg(OPT_Bsymbolic_functions);
   Config->Demangle = getArg(Args, OPT_demangle, OPT_no_demangle, true);
   Config->DisableVerify = Args.hasArg(OPT_disable_verify);
+  Config->EmitRelocs = Args.hasArg(OPT_emit_relocs);
   Config->EhFrameHdr = Args.hasArg(OPT_eh_frame_hdr);
   Config->EnableNewDtags = !Args.hasArg(OPT_disable_new_dtags);
   Config->ExportDynamic =
       getArg(Args, OPT_export_dynamic, OPT_no_export_dynamic, false);
   Config->FatalWarnings =
       getArg(Args, OPT_fatal_warnings, OPT_no_fatal_warnings, false);
   Config->GcSections = getArg(Args, OPT_gc_sections, OPT_no_gc_sections, false);
   Config->GdbIndex = Args.hasArg(OPT_gdb_index);

lld/trunk/ELF/InputFiles.cpp

case SHT_REL: {
     } else {
       ArrayRef<Elf_Rel> Rels = check(this->getObj().rels(&Sec));
       Target->FirstRelocation = Rels.begin();
       NumRelocations = Rels.size();
       Target->AreRelocsRela = false;
     }
     assert(isUInt<31>(NumRelocations));
     Target->NumRelocations = NumRelocations;
+
+    // Relocation sections processed by the linker are usually removed
+    // from the output, so returning `nullptr` for the normal case.
+    // However, if -emit-relocs is given, we need to leave them in the output.
+    // (Some post link analysis tools need this information.)
+    if (Config->EmitRelocs)
+      return make<InputSection<ELFT>>(this, &Sec, Name);
     return nullptr;
   }
   }
 
   // .note.GNU-stack is a marker section to control the presence of
   // PT_GNU_STACK segment in outputs. Since the presence of the segment
   // is controlled only by the command line option (-z execstack) in LLD,
   // .note.GNU-stack is ignored.

lld/trunk/ELF/InputSection.cpp

 
 template <class ELFT>
 InputSectionBase<ELFT> *InputSection<ELFT>::getRelocatedSection() {
   assert(this->Type == SHT_RELA || this->Type == SHT_REL);
   ArrayRef<InputSectionBase<ELFT> *> Sections = this->File->getSections();
   return Sections[this->Info];
 }
 
-// This is used for -r. We can't use memcpy to copy relocations because we need
-// to update symbol table offset and section index for each relocation. So we
-// copy relocations one by one.
+// This is used for -r and --emit-relocs. We can't use memcpy to copy
+// relocations because we need to update symbol table offset and section index
+// for each relocation. So we copy relocations one by one.
 template <class ELFT>
 template <class RelTy>
 void InputSection<ELFT>::copyRelocations(uint8_t *Buf, ArrayRef<RelTy> Rels) {
   InputSectionBase<ELFT> *RelocatedSection = getRelocatedSection();
 
   // Loop is slow and have complexity O(N*M), where N - amount of
   // relocations and M - amount of symbols in symbol table.
   // That happens because getSymbolIndex(...) call below performs
   // simple linear search.
   for (const RelTy &Rel : Rels) {
     uint32_t Type = Rel.getType(Config->Mips64EL);
     SymbolBody &Body = this->File->getRelocTargetSym(Rel);
 
     Elf_Rela *P = reinterpret_cast<Elf_Rela *>(Buf);
     Buf += sizeof(RelTy);
 
     if (Config->Rela)
       P->r_addend = getAddend<ELFT>(Rel);
-    P->r_offset = RelocatedSection->getOffset(Rel.r_offset);
+
+    // Output section VA is zero for -r, so r_offset is an offset within the
+    // section, but for --emit-relocs it is an virtual address.
+    P->r_offset = RelocatedSection->OutSec->Addr +
+                  RelocatedSection->getOffset(Rel.r_offset);
     P->setSymbolAndType(In<ELFT>::SymTab->getSymbolIndex(&Body), Type,
                         Config->Mips64EL);
   }
 }
 
 static uint32_t getARMUndefinedRelativeWeakVA(uint32_t Type, uint32_t A,
                                               uint32_t P) {
   switch (Type) {
template <class ELFT> void InputSection<ELFT>::writeTo(uint8_t *Buf) {
   if (this->Type == SHT_NOBITS)
     return;
 
   if (auto *S = dyn_cast<SyntheticSection<ELFT>>(this)) {
     S->writeTo(Buf + OutSecOff);
     return;
   }
 
-  // If -r is given, then an InputSection may be a relocation section.
+  // If -r or --emit-relocs is given, then an InputSection
+  // may be a relocation section.
   if (this->Type == SHT_RELA) {
     copyRelocations(Buf + OutSecOff, this->template getDataAs<Elf_Rela>());
     return;
   }
   if (this->Type == SHT_REL) {
     copyRelocations(Buf + OutSecOff, this->template getDataAs<Elf_Rel>());
     return;
   }

lld/trunk/ELF/Options.td

def dynamic_linker: S<"dynamic-linker">,
   HelpText<"Which dynamic linker to use">;
 
 def dynamic_list: S<"dynamic-list">,
   HelpText<"Read a list of dynamic symbols">;
 
 def eh_frame_hdr: F<"eh-frame-hdr">,
   HelpText<"Request creation of .eh_frame_hdr section and PT_GNU_EH_FRAME segment header">;
 
+def emit_relocs: F<"emit-relocs">, HelpText<"Generate relocations in output">;
+
 def enable_new_dtags: F<"enable-new-dtags">,
   HelpText<"Enable new dynamic tags">;
 
 def end_lib: F<"end-lib">,
   HelpText<"End a grouping of objects that should be treated as if they were together in an archive">;
 
 def entry: S<"entry">, MetaVarName<"<entry>">,
   HelpText<"Name of entry point symbol">;

 def alias_Bstatic_static: F<"static">, Alias<Bstatic>;
 def alias_L__library_path: J<"library-path=">, Alias<L>;
 def alias_define_common_d: Flag<["-"], "d">, Alias<define_common>;
 def alias_define_common_dc: F<"dc">, Alias<define_common>;
 def alias_define_common_dp: F<"dp">, Alias<define_common>;
 def alias_discard_all_x: Flag<["-"], "x">, Alias<discard_all>;
 def alias_discard_locals_X: Flag<["-"], "X">, Alias<discard_locals>;
 def alias_dynamic_list: J<"dynamic-list=">, Alias<dynamic_list>;
+def alias_emit_relocs: Flag<["-"], "q">, Alias<emit_relocs>;
 def alias_entry_e: JoinedOrSeparate<["-"], "e">, Alias<entry>;
 def alias_entry_entry: J<"entry=">, Alias<entry>;
 def alias_error_limit: J<"error-limit=">, Alias<error_limit>;
 def alias_export_dynamic_E: Flag<["-"], "E">, Alias<export_dynamic>;
 def alias_export_dynamic_symbol: J<"export-dynamic-symbol=">,
   Alias<export_dynamic_symbol>;
 def alias_fini_fini: J<"fini=">, Alias<fini>;
 def alias_format_b: S<"b">, Alias<format>;

lld/trunk/ELF/OutputSections.cpp

if ((this->Flags & SHF_LINK_ORDER) && !this->Sections.empty()) {
     // SHF_LINK_ORDER flag. The dependency is indicated by the sh_link field. We
     // need to translate the InputSection sh_link to the OutputSection sh_link,
     // all InputSections in the OutputSection have the same dependency.
     if (auto *D = this->Sections.front()->getLinkOrderDep())
       this->Link = D->OutSec->SectionIndex;
   }
 
   uint32_t Type = this->Type;
-  if (!Config->Relocatable || (Type != SHT_RELA && Type != SHT_REL))
+  if (!Config->copyRelocs() || (Type != SHT_RELA && Type != SHT_REL))
     return;
 
   this->Link = In<ELFT>::SymTab->OutSec->SectionIndex;
   // sh_info for SHT_REL[A] sections should contain the section header index of
   // the section to which the relocation applies.
   InputSectionBase<ELFT> *S = Sections[0]->getRelocatedSection();
   this->Info = S->OutSec->SectionIndex;
 }

lld/trunk/ELF/Writer.cpp

 }
 
 template <class ELFT>
 static bool shouldKeepInSymtab(InputSectionBase<ELFT> *Sec, StringRef SymName,
                                const SymbolBody &B) {
   if (B.isFile())
     return false;
 
-  // We keep sections in symtab for relocatable output.
+  // We keep sections in symtab for relocatable output and --emit-reloc.
   if (B.isSection())
-    return Config->Relocatable;
+    return Config->copyRelocs();
 
   // If sym references a section in a discarded group, don't keep it.
   if (Sec == &InputSection<ELFT>::Discarded)
     return false;
 
   if (Config->Discard == DiscardPolicy::None)
     return true;
 

lld/trunk/test/ELF/emit-relocs.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1.o
+# RUN: ld.lld --emit-relocs %t1.o -o %t
+# RUN: llvm-readobj -t -r %t1 | FileCheck %s
+
+## Check single dash form.
+# RUN: ld.lld -emit-relocs %t1.o -o %t1
+# RUN: llvm-readobj -t -r %t1 | FileCheck %s
+
+## Check alias.
+# RUN: ld.lld -q %t1.o -o %t2
+# RUN: llvm-readobj -t -r %t2 | FileCheck %s
+
+# CHECK:      Relocations [
+# CHECK-NEXT:   Section ({{.*}}) .rela.text {
+# CHECK-NEXT:     0x201002 R_X86_64_32 .text 0x1
+# CHECK-NEXT:     0x201007 R_X86_64_PLT32 fn 0xFFFFFFFFFFFFFFFC
+# CHECK-NEXT:     0x20100E R_X86_64_32 .text 0x1
+# CHECK-NEXT:     0x201013 R_X86_64_PLT32 fn2 0xFFFFFFFFFFFFFFFC
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+# CHECK-NEXT: Symbols [
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name:
+# CHECK-NEXT:     Value: 0x0
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: None
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: Undefined
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: bar
+# CHECK-NEXT:     Value: 0x201001
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: None
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: foo
+# CHECK-NEXT:     Value: 0x20100D
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: None
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name:
+# CHECK-NEXT:     Value: 0x201000
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: Section
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name:
+# CHECK-NEXT:     Value: 0x20100C
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Local
+# CHECK-NEXT:     Type: Section
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: fn
+# CHECK-NEXT:     Value: 0x201000
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Global
+# CHECK-NEXT:     Type: Function
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Symbol {
+# CHECK-NEXT:     Name: fn2
+# CHECK-NEXT:     Value: 0x20100C
+# CHECK-NEXT:     Size: 0
+# CHECK-NEXT:     Binding: Global
+# CHECK-NEXT:     Type: Function
+# CHECK-NEXT:     Other: 0
+# CHECK-NEXT:     Section: .text
+# CHECK-NEXT:   }
+# CHECK-NEXT: ]
+
+.section .text,"ax",@progbits,unique,0
+.globl fn
+.type fn,@function
+fn:
+ nop
+
+bar:
+  movl $bar, %edx
+  callq fn@PLT
+  nop
+
+.section .text,"ax",@progbits,unique,1
+.globl fn2
+.type fn2,@function
+fn2:
+ nop
+
+foo:
+  movl $foo, %edx
+  callq fn2@PLT
+  nop