[yaml2elf/obj2yaml] - Allow normal parsing/dumping of the .rela.dyn section.

[grimar]

Summary

.rela.dyn is a section that has sh_info normally set to zero.
And Info is an optional field in the description of the relocation section in YAML.

But currently, yaml2obj would fail to produce the object when
Info is not explicitly listed.

The patch fixes the issue.

[jhenderson]

LGTM, with two nits.

test/tools/obj2yaml/elf-reladyn-section-shinfo.yaml

2

There's some discussion about removing support for long single-dash options in llvm-readobj, so it's probably best to use double dash here (i.e. --sections).

7

need -> needing

41

Nit: needs a trailing full stop.

test/tools/obj2yaml/elf-reladyn-section-shinfo.yaml

+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj -sections %t | FileCheck %s
+# RUN: obj2yaml %t | FileCheck %s --check-prefix=YAML
+
+## .rela.dyn is a dynamic relocation section that normally has
+## no value in sh_info field. Check we are able to use
+## yaml2obj/obj2yaml without need to explicitly set it.
+
+# CHECK:      Name: .rela.dyn
+# CHECK-NEXT: Type: SHT_RELA
+# CHECK-NEXT: Flags [
+# CHECK-NEXT:   SHF_ALLOC
+# CHECK-NEXT: ]
+# CHECK-NEXT: Address:
+# CHECK-NEXT: Offset:
+# CHECK-NEXT: Size:
+# CHECK-NEXT: Link:
+# CHECK-NEXT: Info: 0
+# CHECK-NEXT: AddressAlignment:
+# CHECK-NEXT: EntrySize:
+
+# YAML:      - Name:    .rela.dyn
+# YAML-NEXT:   Type:    SHT_RELA
+# YAML-NEXT:   Flags:   [ SHF_ALLOC ]
+# YAML-NEXT:   Link:    .dynsym
+# YAML-NEXT:   EntSize: 0x0000000000000018
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+  Entry:   0x0000000000001000
+Sections:
+  - Name:    .rela.dyn
+    Type:    SHT_RELA
+    Flags:   [ SHF_ALLOC ]
+    Link:    .dynsym
+    EntSize: 0x0000000000000018
+# Add at least one symbol to trigger the .dynsym emission
+DynamicSymbols:
+  - Name:    bar
+    Binding: STB_GLOBAL

tools/yaml2obj/yaml2elf.cpp

for (const auto &Sec : Doc.Sections) {
 
     if (auto S = dyn_cast<ELFYAML::RawContentSection>(Sec.get()))
       writeSectionContent(SHeader, *S, CBA);
     else if (auto S = dyn_cast<ELFYAML::RelocationSection>(Sec.get())) {
       if (S->Link.empty())
         // For relocation section set link to .symtab by default.
         SHeader.sh_link = getDotSymTabSecNo();
 
-      unsigned Index;
-      if (!convertSectionIndex(SN2I, S->Name, S->RelocatableSec, Index))
+      unsigned Index = 0;
+      if (!S->RelocatableSec.empty() &&
+          !convertSectionIndex(SN2I, S->Name, S->RelocatableSec, Index))
         return false;
       SHeader.sh_info = Index;
       if (!writeSectionContent(SHeader, *S, CBA))
         return false;
     } else if (auto S = dyn_cast<ELFYAML::Group>(Sec.get())) {
       unsigned SymIdx;
       if (SymN2I.lookup(S->Signature, SymIdx) &&
           !to_integer(S->Signature, SymIdx)) {