Allow yaml2obj to order implicit sections for ELF

Summary:
This change allows yaml input to control the order of implicitly added sections
(`.symtab`, `.strtab`, `.shstrtab`). The order is controlled by adding a
placeholder section of the given name to the Sections field.

This change is to support changes in D39582, where it is desirable to control
the location of the `.dynsym` section.

Reviewers: compnerd, jakehehrlich

Reviewed By: jakehehrlich

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39749

llvm-svn: 317622

llvm/lib/ObjectYAML/ELFYAML.cpp

@@ -388,7 +388,7 @@ void ScalarEnumerationTraits<ELFYAML::ELF_SHT>::enumeration(
 #define ECase(X) IO.enumCase(Value, #X, ELF::X)
   ECase(SHT_NULL);
   ECase(SHT_PROGBITS);
-  // No SHT_SYMTAB. Use the top-level `Symbols` key instead.
+  ECase(SHT_SYMTAB);
   // FIXME: Issue a diagnostic with this information.
   ECase(SHT_STRTAB);
   ECase(SHT_RELA);

llvm/test/tools/yaml2obj/section-ordering.yaml

@@ -0,0 +1,29 @@
+# Ensures that implicitly added sections can be ordered within Sections.
+# RUN: yaml2obj %s -o %t
+# RUN: llvm-readobj -sections %t | FileCheck %s
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+  - Name:            .symtab
+    Type:            SHT_SYMTAB
+  - Name:            .data
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_WRITE ]
+  - Name:            .shstrtab
+    Type:            SHT_STRTAB
+  - Name:            .strtab
+    Type:            SHT_STRTAB
+
+# CHECK: Name: .text
+# CHECK: Name: .symtab
+# CHECK: Name: .data
+# CHECK: Name: .shstrtab
+# CHECK: Name: .strtab

llvm/tools/yaml2obj/yaml2elf.cpp

@@ -74,6 +74,13 @@ class NameToIdxMap {
     Idx = I->getValue();
     return false;
   }
+  /// asserts if name is not present in the map
+  unsigned get(StringRef Name) const {
+    unsigned Idx;
+    assert(!lookup(Name, Idx) && "Expected section not found in index");
+    return Idx;
+  }
+  unsigned size() const { return Map.size(); }
 };
 } // end anonymous namespace
 
@@ -144,19 +151,21 @@ class ELFState {
                            ContiguousBlobAccumulator &CBA);
 
   // - SHT_NULL entry (placed first, i.e. 0'th entry)
-  // - symbol table (.symtab) (placed third to last)
-  // - string table (.strtab) (placed second to last)
-  // - section header string table (.shstrtab) (placed last)
-  unsigned getDotSymTabSecNo() const { return Doc.Sections.size() + 1; }
-  unsigned getDotStrTabSecNo() const { return Doc.Sections.size() + 2; }
-  unsigned getDotShStrTabSecNo() const { return Doc.Sections.size() + 3; }
-  unsigned getSectionCount() const { return Doc.Sections.size() + 4; }
+  // - symbol table (.symtab) (defaults to third to last)
+  // - string table (.strtab) (defaults to second to last)
+  // - section header string table (.shstrtab) (defaults to last)
+  unsigned getDotSymTabSecNo() const { return SN2I.get(".symtab"); }
+  unsigned getDotStrTabSecNo() const { return SN2I.get(".strtab"); }
+  unsigned getDotShStrTabSecNo() const { return SN2I.get(".shstrtab"); }
+  unsigned getSectionCount() const { return SN2I.size() + 1; }
 
   ELFState(const ELFYAML::Object &D) : Doc(D) {}
 
 public:
   static int writeELF(raw_ostream &OS, const ELFYAML::Object &Doc);
 };
+
+static const char * const ImplicitSecNames[] = {".symtab", ".strtab", ".shstrtab"};
 } // end anonymous namespace
 
 template <class ELFT>
@@ -211,10 +220,6 @@ bool ELFState<ELFT>::initSectionHeaders(std::vector<Elf_Shdr> &SHeaders,
   zero(SHeader);
   SHeaders.push_back(SHeader);
 
-  for (const auto &Sec : Doc.Sections)
-    DotShStrtab.add(Sec->Name);
-  DotShStrtab.finalize();
-
   for (const auto &Sec : Doc.Sections) {
     zero(SHeader);
     SHeader.sh_name = DotShStrtab.getOffset(Sec->Name);
@@ -547,10 +552,6 @@ bool ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
 }
 
 template <class ELFT> bool ELFState<ELFT>::buildSectionIndex() {
-  SN2I.addName(".symtab", getDotSymTabSecNo());
-  SN2I.addName(".strtab", getDotStrTabSecNo());
-  SN2I.addName(".shstrtab", getDotShStrTabSecNo());
-
   for (unsigned i = 0, e = Doc.Sections.size(); i != e; ++i) {
     StringRef Name = Doc.Sections[i]->Name;
     if (Name.empty())
@@ -561,7 +562,19 @@ template <class ELFT> bool ELFState<ELFT>::buildSectionIndex() {
              << "' at YAML section number " << i << ".\n";
       return false;
     }
+    DotShStrtab.add(Name);
   }
+
+  auto SecNo = 1 + Doc.Sections.size();
+  // Add special sections after input sections, if necessary.
+  for (const auto &Name : ImplicitSecNames)
+    if (!SN2I.addName(Name, SecNo)) {
+      // Account for this section, since it wasn't in the Doc
+      ++SecNo;
+      DotShStrtab.add(Name);
+    }
+
+  DotShStrtab.finalize();
   return true;
 }
 
@@ -608,32 +621,23 @@ int ELFState<ELFT>::writeELF(raw_ostream &OS, const ELFYAML::Object &Doc) {
                                            Header.e_shentsize * Header.e_shnum;
   ContiguousBlobAccumulator CBA(SectionContentBeginOffset);
 
-  // Doc might not contain .symtab, .strtab and .shstrtab sections,
-  // but we will emit them, so make sure to add them to ShStrTabSHeader.
-  State.DotShStrtab.add(".symtab");
-  State.DotShStrtab.add(".strtab");
-  State.DotShStrtab.add(".shstrtab");
-
   std::vector<Elf_Shdr> SHeaders;
+  SHeaders.reserve(State.SN2I.size());
   if(!State.initSectionHeaders(SHeaders, CBA))
     return 1;
 
-  // .symtab section.
-  Elf_Shdr SymtabSHeader;
-  State.initSymtabSectionHeader(SymtabSHeader, CBA);
-  SHeaders.push_back(SymtabSHeader);
-
-  // .strtab string table header.
-  Elf_Shdr DotStrTabSHeader;
-  State.initStrtabSectionHeader(DotStrTabSHeader, ".strtab", State.DotStrtab,
-                                CBA);
-  SHeaders.push_back(DotStrTabSHeader);
-
-  // .shstrtab string table header.
-  Elf_Shdr ShStrTabSHeader;
-  State.initStrtabSectionHeader(ShStrTabSHeader, ".shstrtab", State.DotShStrtab,
-                                CBA);
-  SHeaders.push_back(ShStrTabSHeader);
+  // Populate SHeaders with implicit sections not present in the Doc
+  for (const auto &Name : ImplicitSecNames)
+    if (State.SN2I.get(Name) >= SHeaders.size())
+      SHeaders.push_back({});
+
+  // Initialize the implicit sections
+  auto Index = State.SN2I.get(".symtab");
+  State.initSymtabSectionHeader(SHeaders[Index], CBA);
+  Index = State.SN2I.get(".strtab");
+  State.initStrtabSectionHeader(SHeaders[Index], ".strtab", State.DotStrtab, CBA);
+  Index = State.SN2I.get(".shstrtab");
+  State.initStrtabSectionHeader(SHeaders[Index], ".shstrtab", State.DotShStrtab, CBA);
 
   // Now we can decide segment offsets
   State.setProgramHeaderLayout(PHeaders, SHeaders);