[llvm-readobj] Extend the output of -elf-section-groups with the field Link

[alexander-shaposhnikov]

Summary

This diff extends the output of -elf-section-groups (llvm style, gnu unchanged (since it's meant to be compatible with binutils readelf) with the field Link. This change will enable us to use llvm-readobj -elf-section-groups for testing llvm-objcopy's support for groups (instead of -section-data -sections).
Test plan: make check-all

[jhenderson]

I'm thinking that we should really either have both the sh_info and the sh_link fields printed, or neither. At the moment, they're kind of implicit in the "Signature" field, since that is the symbol name for the corresponding symbol. If the name is wrong, then we're pointing at either the wrong symbol table or the wrong entry in that table. I suppose there's not technically anything stopping there being multiple symbols with the same name though, under the right circumstances, so having both would allow us to uniquely identify it. However, printing only the link doesn't uniquely identify it.

[alexander-shaposhnikov]

Add sh_info as well.

[jhenderson]

LGTM.

[jakehehrlich]

LGTM as well

test/tools/llvm-readobj/broken-group.test

 # RUN:   %t.o >%t2out.log 2>%t2err.log %t.o
 # RUN: FileCheck %s -check-prefix=LLVM-STDERR < %t2err.log
 # LLVM-STDERR: Error: .foo (3) in a group .group1 (4) is already in a group .group (2)
 # RUN: FileCheck %s -check-prefix=LLVM-STDOUT < %t2out.log
 # LLVM-STDOUT:      Groups {
 # LLVM-STDOUT-NEXT:   Group {
 # LLVM-STDOUT-NEXT:     Name: .group
 # LLVM-STDOUT-NEXT:     Index: 2
+# LLVM-STDOUT-NEXT:     Link: 5
+# LLVM-STDOUT-NEXT:     Info: 1
 # LLVM-STDOUT-NEXT:     Type: COMDAT
 # LLVM-STDOUT-NEXT:     Signature: bar
 # LLVM-STDOUT-NEXT:     Section(s) in group [
 # LLVM-STDOUT-NEXT:       .foo (3)
 # LLVM-STDOUT-NEXT:     ]
 # LLVM-STDOUT-NEXT:   }
 # LLVM-STDOUT-NEXT:   Group {
 # LLVM-STDOUT-NEXT:     Name: .group1
 # LLVM-STDOUT-NEXT:     Index: 4
+# LLVM-STDOUT-NEXT:     Link: 5
+# LLVM-STDOUT-NEXT:     Info: 2
 # LLVM-STDOUT-NEXT:     Type: COMDAT
 # LLVM-STDOUT-NEXT:     Signature: zed
 # LLVM-STDOUT-NEXT:     Section(s) in group [
 # LLVM-STDOUT-NEXT:     ]
 # LLVM-STDOUT-NEXT:   }
 # LLVM-STDOUT-NEXT: }
 
 !ELF

test/tools/llvm-readobj/elf-groups.test

 RUN: llvm-readobj -g %p/Inputs/elf-groups.x86_64 \
 RUN:   | FileCheck %s
 RUN: llvm-readobj -g -elf-output-style=GNU %p/Inputs/elf-groups.x86_64 \
 RUN:   | FileCheck --check-prefix="GNU" %s
 CHECK:         Groups {
 CHECK-NEXT:      Group {
 CHECK-NEXT:        Name: .group (92)
 CHECK-NEXT:        Index: 1
+CHECK-NEXT:        Link: 23
+CHECK-NEXT:        Info: 17
 CHECK-NEXT:        Type: COMDAT (0x1)
 CHECK-NEXT:        Signature: _Z3fooIcEvT_
 CHECK-NEXT:        Section(s) in group [
 CHECK-NEXT:          .text._Z3fooIcEvT_ (10)
 CHECK-NEXT:          .rela.text._Z3fooIcEvT_ (11)
 CHECK-NEXT:        ]
 CHECK-NEXT:      }
 CHECK-NEXT:      Group {
 CHECK-NEXT:        Name: .group (92)
 CHECK-NEXT:        Index: 2
+CHECK-NEXT:        Link: 23
+CHECK-NEXT:        Info: 18
 CHECK-NEXT:        Type: COMDAT (0x1)
 CHECK-NEXT:        Signature: _Z3fooIiEvT_
 CHECK-NEXT:        Section(s) in group [
 CHECK-NEXT:          .text._Z3fooIiEvT_ (12)
 CHECK-NEXT:          .rela.text._Z3fooIiEvT_ (13)
 CHECK-NEXT:        ]
 CHECK-NEXT:      }
 

tools/llvm-readobj/ELFDumper.cpp

struct GroupMember {
   uint64_t Index;
 };
 
 struct GroupSection {
   StringRef Name;
   StringRef Signature;
   uint64_t ShName;
   uint64_t Index;
+  uint32_t Link;
+  uint32_t Info;
   uint32_t Type;
   std::vector<GroupMember> Members;
 };
 
 template <class ELFT>
 std::vector<GroupSection> getGroups(const ELFFile<ELFT> *Obj) {
   using Elf_Shdr = typename ELFT::Shdr;
   using Elf_Sym = typename ELFT::Sym;
for (const Elf_Shdr &Sec : unwrapOrError(Obj->sections())) {
     StringRef StrTable = unwrapOrError(Obj->getStringTableForSymtab(*Symtab));
     const Elf_Sym *Sym =
         unwrapOrError(Obj->template getEntry<Elf_Sym>(Symtab, Sec.sh_info));
     auto Data =
         unwrapOrError(Obj->template getSectionContentsAsArray<Elf_Word>(&Sec));
 
     StringRef Name = unwrapOrError(Obj->getSectionName(&Sec));
     StringRef Signature = StrTable.data() + Sym->st_name;
-    Ret.push_back({Name, Signature, Sec.sh_name, I - 1, Data[0], {}});
+    Ret.push_back({Name,
+                   Signature,
+                   Sec.sh_name,
+                   I - 1,
+                   Sec.sh_link,
+                   Sec.sh_info,
+                   Data[0],
+                   {}});
 
     std::vector<GroupMember> &GM = Ret.back().Members;
     for (uint32_t Ndx : Data.slice(1)) {
       auto Sec = unwrapOrError(Obj->getSection(Ndx));
       const StringRef Name = unwrapOrError(Obj->getSectionName(Sec));
       GM.push_back({Name, Ndx});
     }
   }

 void LLVMStyle<ELFT>::printGroupSections(const ELFO *Obj) {
   DictScope Lists(W, "Groups");
   std::vector<GroupSection> V = getGroups<ELFT>(Obj);
   DenseMap<uint64_t, const GroupSection *> Map = mapSectionsToGroups(V);
   for (const GroupSection &G : V) {
     DictScope D(W, "Group");
     W.printNumber("Name", G.Name, G.ShName);
     W.printNumber("Index", G.Index);
+    W.printNumber("Link", G.Link);
+    W.printNumber("Info", G.Info);
     W.printHex("Type", getGroupType(G.Type), G.Type);
     W.startLine() << "Signature: " << G.Signature << "\n";
 
     ListScope L(W, "Section(s) in group");
     for (const GroupMember &GM : G.Members) {
       const GroupSection *MainGroup = Map[GM.Index];
       if (MainGroup != &G) {
         W.flush();