Fix courgette ELF x86 dissembler

Courgette cannot parse files that have out of order NOBITS sections and instead
will crash. The solution to this is to skip them and encode them as raw bytes.

Also, courgette has a bug where it incorrectly emits ElfRelocationInstruction
instructions even when there are no valid R_386_RELATIVE relocations in the
file.

Added a test file that exhibits both of these symptoms.

BUG=424820,423925
TEST=courgette_unittests
R=dgarrett@chromium.org, tommi@chromium.org

Review URL: https://codereview.chromium.org/664803002

Cr-Commit-Position: refs/heads/master@{#318764}

courgette/disassembler_elf_32.cc

@@ -262,6 +262,9 @@ CheckBool DisassemblerElf32::ParseFile(AssemblyProgram* program) {
 
     const Elf32_Shdr *section_header = SectionHeader(section_id);
 
+    if (section_header->sh_type == SHT_NOBITS)
+      continue;
+
     if (!ParseSimpleRegion(file_offset,
                            section_header->sh_offset,
                            program))
@@ -282,8 +285,6 @@ CheckBool DisassemblerElf32::ParseFile(AssemblyProgram* program) {
           return false;
         file_offset = section_header->sh_offset + section_header->sh_size;
         break;
-      case SHT_NOBITS:
-        // Fall through
       case SHT_INIT_ARRAY:
         // Fall through
       case SHT_FINI_ARRAY:

courgette/disassembler_elf_32_x86.cc

@@ -91,6 +91,9 @@ CheckBool DisassemblerElf32X86::ParseRelocationSection(
   uint32 section_relocs_count = section_header->sh_size /
                                 section_header->sh_entsize;
 
+  if (abs32_locations_.empty())
+    match = false;
+
   if (abs32_locations_.size() > section_relocs_count)
     match = false;
 

courgette/encode_decode_unittest.cc

@@ -82,3 +82,8 @@ TEST_F(EncodeDecodeTest, Elf_Small) {
   std::string file = FileContents("elf-32-1");
   TestAssembleToStreamDisassemble(file, 135988);
 }
+
+TEST_F(EncodeDecodeTest, Elf_HighBSS) {
+  std::string file = FileContents("elf-32-high-bss");
+  TestAssembleToStreamDisassemble(file, 7308);
+}