[yaml2obj] - Allow empty SectionHeaderTable definitions.

Currently we don't allow the following definition:

```
Sections:
  - Type: SectionHeaderTable
  - Name: .foo
    Type: SHT_PROGBITS
```

We report an error: "SectionHeaderTable can't be empty. Use 'NoHeaders' key to drop the section header table".

It was implemented in this way earlier, when `SectionHeaderTable`
was a dedicated key outside of the `Sections` list. And we did not
allow to select where the table is written.

Currently it makes sense to allow it, because a user might
want to place the default section header table at an arbitrary position,
e.g. before other sections. In this case it is not convenient and error prone
to require specifying all sections:

```
Sections:
  - Type: SectionHeaderTable
    Sections:
      - Name: .foo
      - Name: .strtab
      - Name: .shstrtab
  - Name: .foo
    Type: SHT_PROGBITS
```

This patch allows empty SectionHeaderTable definitions.

Differential revision: https://reviews.llvm.org/D95341

Author: Georgii Rymar

llvm/include/llvm/ObjectYAML/ELFYAML.h

@@ -296,13 +296,15 @@ struct SectionHeaderTable : Chunk {
   Optional<bool> NoHeaders;
 
   size_t getNumHeaders(size_t SectionsNum) const {
-    if (IsImplicit)
+    if (IsImplicit || isDefault())
       return SectionsNum;
     if (NoHeaders)
       return (*NoHeaders) ? 0 : SectionsNum;
     return (Sections ? Sections->size() : 0) + /*Null section*/ 1;
   }
 
+  bool isDefault() const { return !Sections && !Excluded && !NoHeaders; }
+
   static constexpr StringRef TypeStr = "SectionHeaderTable";
 };
 

llvm/lib/ObjectYAML/ELFEmitter.cpp

@@ -555,7 +555,8 @@ unsigned ELFState<ELFT>::toSectionIndex(StringRef S, StringRef LocSec,
   const ELFYAML::SectionHeaderTable &SectionHeaders =
       Doc.getSectionHeaderTable();
   if (SectionHeaders.IsImplicit ||
-      (SectionHeaders.NoHeaders && !SectionHeaders.NoHeaders.getValue()))
+      (SectionHeaders.NoHeaders && !SectionHeaders.NoHeaders.getValue()) ||
+      SectionHeaders.isDefault())
     return Index;
 
   assert(!SectionHeaders.NoHeaders.getValueOr(false) ||
@@ -1744,7 +1745,8 @@ template <class ELFT>
 DenseMap<StringRef, size_t> ELFState<ELFT>::buildSectionHeaderReorderMap() {
   const ELFYAML::SectionHeaderTable &SectionHeaders =
       Doc.getSectionHeaderTable();
-  if (SectionHeaders.IsImplicit || SectionHeaders.NoHeaders)
+  if (SectionHeaders.IsImplicit || SectionHeaders.NoHeaders ||
+      SectionHeaders.isDefault())
     return DenseMap<StringRef, size_t>();
 
   DenseMap<StringRef, size_t> Ret;

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -1474,9 +1474,6 @@ std::string MappingTraits<std::unique_ptr<ELFYAML::Chunk>>::validate(
   if (const auto *SHT = dyn_cast<ELFYAML::SectionHeaderTable>(C.get())) {
     if (SHT->NoHeaders && (SHT->Sections || SHT->Excluded || SHT->Offset))
       return "NoHeaders can't be used together with Offset/Sections/Excluded";
-    if (!SHT->NoHeaders && !SHT->Sections && !SHT->Excluded)
-      return "SectionHeaderTable can't be empty. Use 'NoHeaders' key to drop "
-             "the section header table";
     return "";
   }
 

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

@@ -151,20 +151,28 @@ Sections:
     Sections:  []
     NoHeaders: [[NOHEADERS]]
 
-## Check that we do not allow an empty SectionHeaderTable tag and suggest to use an explicit syntax instead.
-# RUN: not yaml2obj %s --docnum=5 -o /dev/null 2>&1 | FileCheck %s --check-prefix=NO-VALUE
+## Check that we allow using an empty SectionHeaderTable definition.
+## It can be used to emit the default section header table at an arbitrary position.
 
-# NO-VALUE: SectionHeaderTable can't be empty. Use 'NoHeaders' key to drop the section header table
+# RUN: yaml2obj %s --docnum=5 -o %t5.novalues
+# RUN: llvm-readelf --sections %t5.novalues | \
+# RUN:   FileCheck %s --check-prefix=NO-VALUES
+
+## Check we placed the section header table before the .foo section.
+
+# NO-VALUES: There are 4 section headers, starting at offset 0x40:
+# NO-VALUES: [Nr] Name Type     Address          Off    Size
+# NO-VALUES: [ 1] .foo PROGBITS 0000000000000000 000140 000000
 
 --- !ELF
 FileHeader:
   Class: ELFCLASS64
   Data:  ELFDATA2LSB
   Type:  ET_REL
 Sections:
+  - Type: SectionHeaderTable
   - Name: .foo
     Type: SHT_PROGBITS
-  - Type: SectionHeaderTable
 
 ## Test that we are still able to override e_shoff, e_shnum and e_shstrndx
 ## fields even when we do not produce section headers.