[yaml2obj] - Add NBucket and NChain fields for the SHT_HASH section.

[grimar]

Summary

These fields allows to override nchain and nbucket fields of a SHT_HASH section.
I have a plan for a patch where I'd like to use this approach to produce broken inputs.
Overriding of fields is something we already support for a "Section".
An alternative is to set raw "Content", but it doesn't look nice.

[jhenderson]

Looks fine to me. I'd like to see an example of it being used in a real test case, so I'm holding off on the LGTM until that point.

llvm/include/llvm/ObjectYAML/ELFYAML.h

285

fields which -> fields. This

llvm/test/tools/yaml2obj/ELF/hash-section.yaml

280

This line should probabbly be wrapped.

[grimar]

• Addressed review comments.

Going to work and post a patch that uses this functionality.

[grimar]

In D76834#1943679, @grimar wrote:

Going to work and post a patch that uses this functionality.

I need D76920 and D76924 to be landed for the patch mentioned.

[grimar]

• Fixed a gcc compilation issue .

[jhenderson]

LGTM.

llvm/include/llvm/ObjectYAML/ELFYAML.h

 };
 
 struct HashSection : Section {
   Optional<yaml::BinaryRef> Content;
   Optional<llvm::yaml::Hex64> Size;
   Optional<std::vector<uint32_t>> Bucket;
   Optional<std::vector<uint32_t>> Chain;
 
+  // The following members are used to override section fields.
+  // This is useful for creating invalid objects.
+  Optional<llvm::yaml::Hex64> NBucket;
+  Optional<llvm::yaml::Hex64> NChain;
+
   HashSection() : Section(ChunkKind::Hash) {}
 
   static bool classof(const Chunk *S) { return S->Kind == ChunkKind::Hash; }
 };
 
 struct GnuHashHeader {
   // The number of hash buckets.
   // Not used when dumping the object, but can be used to override

llvm/lib/ObjectYAML/ELFEmitter.cpp

void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
   if (Section.Link.empty() && SN2I.lookup(".dynsym", Link))
     SHeader.sh_link = Link;
 
   if (Section.Content || Section.Size) {
     SHeader.sh_size = writeContent(OS, Section.Content, Section.Size);
     return;
   }
 
-  support::endian::write<uint32_t>(OS, Section.Bucket->size(),
-                                   ELFT::TargetEndianness);
-  support::endian::write<uint32_t>(OS, Section.Chain->size(),
-                                   ELFT::TargetEndianness);
+  support::endian::write<uint32_t>(
+      OS, Section.NBucket.getValueOr(llvm::yaml::Hex64(Section.Bucket->size())),
+  support::endian::write<uint32_t>(
+      OS, Section.NChain.getValueOr(llvm::yaml::Hex64(Section.Chain->size())),
+
   for (uint32_t Val : *Section.Bucket)
     support::endian::write<uint32_t>(OS, Val, ELFT::TargetEndianness);
   for (uint32_t Val : *Section.Chain)
     support::endian::write<uint32_t>(OS, Val, ELFT::TargetEndianness);
 
   SHeader.sh_size = (2 + Section.Bucket->size() + Section.Chain->size()) * 4;
 }
 

llvm/lib/ObjectYAML/ELFYAML.cpp

 }
 
 static void sectionMapping(IO &IO, ELFYAML::HashSection &Section) {
   commonSectionMapping(IO, Section);
   IO.mapOptional("Content", Section.Content);
   IO.mapOptional("Bucket", Section.Bucket);
   IO.mapOptional("Chain", Section.Chain);
   IO.mapOptional("Size", Section.Size);
+
+  // obj2yaml does not dump these fields. They can be used to override nchain
+  // and nbucket values for creating broken sections.
+  assert(!IO.outputting() ||
+         (!Section.NBucket.hasValue() && !Section.NChain.hasValue()));
+  IO.mapOptional("NChain", Section.NChain);
+  IO.mapOptional("NBucket", Section.NBucket);
 }
 
 static void sectionMapping(IO &IO, ELFYAML::NoteSection &Section) {
   commonSectionMapping(IO, Section);
   IO.mapOptional("Content", Section.Content);
   IO.mapOptional("Size", Section.Size);
   IO.mapOptional("Notes", Section.Notes);
 }

llvm/test/tools/yaml2obj/ELF/hash-section.yaml

FileHeader:
   Data:    ELFDATA2LSB
   Type:    ET_EXEC
   Machine: EM_X86_64
 Sections:
   - Name: .hash
     Type: SHT_HASH
     Size: 0x1
     Chain: [ 1 ]
+
+## Check we can override "nbucket" and "nchain" values of a SHT_HASH section using "NBucket"
+## and "NChain" tags. Check that the section size is unaffected when we do this.
+
+# RUN: yaml2obj --docnum=14 %s -o %t14
+# RUN: llvm-readobj --sections --section-data %t14 | FileCheck %s --check-prefix=OVERRIDE
+
+# OVERRIDE:      Name: .hash
+# OVERRIDE-NEXT: Type: SHT_HASH
+# OVERRIDE-NEXT: Flags [
+# OVERRIDE-NEXT: ]
+# OVERRIDE-NEXT: Address: 0x0
+# OVERRIDE-NEXT: Offset: 0x34
+# OVERRIDE-NEXT: Size: 28
+# OVERRIDE-NEXT: Link: 0
+# OVERRIDE-NEXT: Info: 0
+# OVERRIDE-NEXT: AddressAlignment: 0
+# OVERRIDE-NEXT: EntrySize: 0
+# OVERRIDE-NEXT: SectionData (
+# OVERRIDE-NEXT:   0000: AA000000 BB000000 01000000 02000000
+# OVERRIDE-NEXT:   0010: 03000000 04000000 05000000
+# OVERRIDE-NEXT: )
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_386
+Sections:
+  - Name:   .hash
+    Type:   SHT_HASH
+    Bucket: [ 1, 2 ]
+    Chain:  [ 3, 4, 5 ]
+    NBucket: 0xAA
+    NChain:  0xBB