[ELF] Initialize Target before it may be referenced by findAux when reporting "duplicate symbol" error

[MaskRay]

Summary

for (InputFile *F : Files)
  Symtab->addFile<ELFT>(F); // if there is a duplicate symbol error

...

Target = getTarget();

When parsing .debug_info in the object file (for better diagnostics),
DWARF.cpp findAux may dereference the null pointer Target

auto *DR = dyn_cast<Defined>(&File->getRelocTargetSym(Rel));
if (!DR) { /// Some asan object files contain R_X86_64_NONE
  RelType Type = Rel.getType(Config->IsMips64EL);
  if (Type != Target->NoneRel) /// Target is null

Move the assignment of Target to an earlier place to fix this.

[ruiu]

LGTM

[grimar]

lld/trunk/test/ELF/undef-broken-debug.test

11

nit: I would not refer to the DWARF.cpp:findAux in the test cases comment.
Simply because both the file name and function name might change.

lld/trunk/ELF/Driver.cpp

if (errorCount())
     return;
 
   inferMachineType();
   setConfigs(Args);
   checkOptions();
   if (errorCount())
     return;
 
+  // The Target instance handles target-specific stuff, such as applying
+  // relocations or writing a PLT section. It also contains target-dependent
+  // values such as a default image base address.
+  Target = getTarget();
+
   switch (Config->EKind) {
   case ELF32LEKind:
     link<ELF32LE>(Args);
     return;
   case ELF32BEKind:
     link<ELF32BE>(Args);
     return;
   case ELF64LEKind:
for (BinaryFile *F : BinaryFiles)
     for (InputSectionBase *S : F->getSections())
       InputSections.push_back(cast<InputSection>(S));
 
   // We do not want to emit debug sections if --strip-all
   // or -strip-debug are given.
   if (Config->Strip != StripPolicy::None)
     llvm::erase_if(InputSections, [](InputSectionBase *S) { return S->Debug; });
 
-  // The Target instance handles target-specific stuff, such as applying
-  // relocations or writing a PLT section. It also contains target-dependent
-  // values such as a default image base address.
-  Target = getTarget();
-
   Config->EFlags = Target->calcEFlags();
   Config->MaxPageSize = getMaxPageSize(Args);
   Config->ImageBase = getImageBase(Args);
 
   if (Config->EMachine == EM_ARM) {
     // FIXME: These warnings can be removed when lld only uses these features
     // when the input objects have been compiled with an architecture that
     // supports them.

lld/trunk/test/ELF/undef-broken-debug.test

 # REQUIRES: x86
 # RUN: yaml2obj %s -o %t.o
 # RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
 
 # The debug info has a broken relocation. Check that we don't crash
 # and still report the undefined symbol.
 
 # CHECK: error: {{.*}}.o: relocation R_X86_64_64 at 0x29 has unsupported target
 # CHECK: error: undefined symbol: bar
 
+# We used to dereference null Target in DWARF.cpp:findAux while reporting a duplicate symbol error,
+# because Target wasn't initialized yet.
+# RUN: not ld.lld %t.o %t.o -o /dev/null
+
 --- !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64
 Sections:
   - Name:            .text