[ELF] Fallback to sh_link=0 if neither .dynsym nor .symtab exists

[MaskRay]

Summary

.rela.plt may only contain R_*_{,I}RELATIVE relocations and not need a symbol table link. bfd/gold fallbacks to sh_link=0 in this case. Without this patch, ld.lld --strip-all caused lld to dereference a null pointer.

[MaskRay]

Update comment

[ruiu]

ELF/SyntheticSections.cpp

1496–1498

If ?: is nested, plain if ~ else if is perhaps easier to read.

[MaskRay]

Use if .. else if ... else

[ruiu]

LGTM

ELF/SyntheticSections.cpp

 
 void RelocationBaseSection::addReloc(const DynamicReloc &Reloc) {
   if (Reloc.Type == Target->RelativeRel)
     ++NumRelativeRelocs;
   Relocs.push_back(Reloc);
 }
 
 void RelocationBaseSection::finalizeContents() {
-  // If all relocations are R_*_RELATIVE they don't refer to any
-  // dynamic symbol and we don't need a dynamic symbol table. If that
-  // is the case, just use the index of the regular symbol table section.
-  getParent()->Link = In.DynSymTab ? In.DynSymTab->getParent()->SectionIndex
-                                   : In.SymTab->getParent()->SectionIndex;
+  // If all relocations are R_*_{,I}RELATIVE they don't refer to any dynamic
+  // symbol and we don't need a dynamic symbol table. If that is the case, use
+  // the index of the regular symbol table section (if exists) or 0.
+  if (In.DynSymTab)
+    getParent()->Link = In.DynSymTab->getParent()->SectionIndex;
+  else if (In.SymTab)
+    getParent()->Link = In.SymTab->getParent()->SectionIndex;
+  else
+    getParent()->Link = 0;
 
   if (In.RelaIplt == this || In.RelaPlt == this)
     getParent()->Info = In.GotPlt->getParent()->SectionIndex;
 }
 
 RelrBaseSection::RelrBaseSection()
     : SyntheticSection(SHF_ALLOC,
                        Config->UseAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,

test/ELF/gnu-ifunc-relative.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-// RUN: ld.lld -static %t.o -o %tout
-// RUN: llvm-readobj -r -t %tout | FileCheck %s
+// RUN: ld.lld --strip-all %t.o -o %t
+// RUN: llvm-readobj -r %t | FileCheck %s
+// RUN: ld.lld %t.o -o %t
+// RUN: llvm-readobj -r -t %t | FileCheck %s --check-prefixes=CHECK,SYM
 
 .type foo STT_GNU_IFUNC
 .globl foo
 foo:
  ret
 
 .globl _start
 _start:
  call foo
 
 // CHECK:      Section ({{.*}}) .rela.plt {
 // CHECK-NEXT:   R_X86_64_IRELATIVE - 0x[[ADDR:.*]]
 // CHECK-NEXT: }
 
-// CHECK:      Name: foo
-// CHECK-NEXT: Value: 0x[[ADDR]]
-// CHECK-NEXT: Size: 0
-// CHECK-NEXT: Binding: Global
-// CHECK-NEXT: Type: GNU_IFunc
+// SYM:      Name: foo
+// SYM-NEXT: Value: 0x[[ADDR]]
+// SYM-NEXT: Size: 0
+// SYM-NEXT: Binding: Global
+// SYM-NEXT: Type: GNU_IFunc