[yaml2obj] Write the section header table after section contents

Linkers (ld.bfd/gold/lld) place the section header table at the very
end. This allows tools to strip it, which is optional in executable/shared objects.
In addition, if we add or section, the size of the section header table
will change. Placing the section header table in the end keeps section
offsets unchanged.

yaml2obj currently places the section header table immediately after the
program header. Follow what linkers do to make offset updating easier.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D67221

llvm-svn: 371074

1 files changed, 17 insertions, 16 deletions

diff --git a/llvm/lib/ObjectYAML/ELFEmitter.cpp b/llvm/lib/ObjectYAML/ELFEmitter.cpp
index cd3a0faa03d..362a9432568 100644
--- a/llvm/lib/ObjectYAML/ELFEmitter.cpp
+++ b/llvm/lib/ObjectYAML/ELFEmitter.cpp
@@ -116,7 +116,6 @@ template <class ELFT> class ELFState {
 
   bool buildSectionIndex();
   bool buildSymbolIndexes();
-  void initELFHeader(Elf_Ehdr &Header);
   void initProgramHeaders(std::vector<Elf_Phdr> &PHeaders);
   bool initImplicitHeader(ContiguousBlobAccumulator &CBA, Elf_Shdr &Header,
                           StringRef SecName, ELFYAML::Section *YAMLSec);
@@ -132,6 +131,7 @@ template <class ELFT> class ELFState {
   void setProgramHeaderLayout(std::vector<Elf_Phdr> &PHeaders,
                               std::vector<Elf_Shdr> &SHeaders);
   void finalizeStrings();
+  void writeELFHeader(ContiguousBlobAccumulator &CBA, raw_ostream &OS);
   bool writeSectionContent(Elf_Shdr &SHeader,
                            const ELFYAML::RawContentSection &Section,
                            ContiguousBlobAccumulator &CBA);
@@ -205,8 +205,11 @@ template <class ELFT> ELFState<ELFT>::ELFState(ELFYAML::Object &D) : Doc(D) {
   }
 }
 
-template <class ELFT> void ELFState<ELFT>::initELFHeader(Elf_Ehdr &Header) {
+template <class ELFT>
+void ELFState<ELFT>::writeELFHeader(ContiguousBlobAccumulator &CBA, raw_ostream &OS) {
   using namespace llvm::ELF;
+
+  Elf_Ehdr Header;
   zero(Header);
   Header.e_ident[EI_MAG0] = 0x7f;
   Header.e_ident[EI_MAG1] = 'E';
@@ -230,14 +233,18 @@ template <class ELFT> void ELFState<ELFT>::initELFHeader(Elf_Ehdr &Header) {
   Header.e_shentsize =
       Doc.Header.SHEntSize ? (uint16_t)*Doc.Header.SHEntSize : sizeof(Elf_Shdr);
   // Immediately following the ELF header and program headers.
-  Header.e_shoff =
-      Doc.Header.SHOffset
-          ? (typename ELFT::uint)(*Doc.Header.SHOffset)
-          : sizeof(Header) + sizeof(Elf_Phdr) * Doc.ProgramHeaders.size();
+  // Align the start of the section header and write the ELF header.
+  uint64_t ShOffset;
+  CBA.getOSAndAlignedOffset(ShOffset, sizeof(typename ELFT::uint));
+  Header.e_shoff = Doc.Header.SHOffset
+                       ? typename ELFT::uint(*Doc.Header.SHOffset)
+                       : ShOffset;
   Header.e_shnum =
       Doc.Header.SHNum ? (uint16_t)*Doc.Header.SHNum : Doc.Sections.size();
   Header.e_shstrndx = Doc.Header.SHStrNdx ? (uint16_t)*Doc.Header.SHStrNdx
                                           : SN2I.get(".shstrtab");
+
+  OS.write((const char *)&Header, sizeof(Header));
 }
 
 template <class ELFT>
@@ -1040,19 +1047,13 @@ int ELFState<ELFT>::writeELF(raw_ostream &OS, ELFYAML::Object &Doc) {
   if (!State.buildSectionIndex() || !State.buildSymbolIndexes())
     return 1;
 
-  Elf_Ehdr Header;
-  State.initELFHeader(Header);
-
-  // TODO: Flesh out section header support.
-
   std::vector<Elf_Phdr> PHeaders;
   State.initProgramHeaders(PHeaders);
 
   // XXX: This offset is tightly coupled with the order that we write
   // things to `OS`.
-  const size_t SectionContentBeginOffset = Header.e_ehsize +
-                                           Header.e_phentsize * Header.e_phnum +
-                                           Header.e_shentsize * Header.e_shnum;
+  const size_t SectionContentBeginOffset =
+      sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * Doc.ProgramHeaders.size();
   ContiguousBlobAccumulator CBA(SectionContentBeginOffset);
 
   std::vector<Elf_Shdr> SHeaders;
@@ -1062,10 +1063,10 @@ int ELFState<ELFT>::writeELF(raw_ostream &OS, ELFYAML::Object &Doc) {
   // Now we can decide segment offsets
   State.setProgramHeaderLayout(PHeaders, SHeaders);
 
-  OS.write((const char *)&Header, sizeof(Header));
+  State.writeELFHeader(CBA, OS);
   writeArrayData(OS, makeArrayRef(PHeaders));
-  writeArrayData(OS, makeArrayRef(SHeaders));
   CBA.writeBlobToStream(OS);
+  writeArrayData(OS, makeArrayRef(SHeaders));
   return 0;
 }