[Courgette] Fix AssemblyProgram parsing for ELF-ARM.

This fixes two problems:
(A) In DisassemblerElf32, RVAToFileOffset() used Program Segment Header,
  but FileOffsetToRVA() used Section Header. For consistency, both
  should use the same one. We choose the latter (for now).
(B) Even if a section has sh_type=SHT_PROGBITS, it can still have
  sh_addr=0. Extracting Rel32 address from these sections would add
  overlapping RVA chaos, and so should be avoided.

Also using elf-arm7 in a unit test. The test fails before the fix and
passes after.

BUG= 579206

Review URL: https://codereview.chromium.org/1658463002

Cr-Commit-Position: refs/heads/master@{#373639}

3 files changed, 20 insertions, 40 deletions

diff --git a/courgette/disassembler_elf_32.cc b/courgette/disassembler_elf_32.cc
index c0f42e6..84aa971 100644
--- a/courgette/disassembler_elf_32.cc
+++ b/courgette/disassembler_elf_32.cc
@@ -162,24 +162,21 @@ CheckBool DisassemblerElf32::IsValidRVA(RVA rva) const {
   return false;
 }
 
-// Returns RVA for an in memory address, or NULL.
-CheckBool DisassemblerElf32::RVAToFileOffset(Elf32_Addr addr,
-                                                size_t* result) const {
-
-  for (int i = 0; i < ProgramSegmentHeaderCount(); i++) {
-    Elf32_Addr begin = ProgramSegmentMemoryBegin(i);
-    Elf32_Addr end = begin + ProgramSegmentMemorySize(i);
-
-    if (addr >= begin  && addr < end) {
-      Elf32_Addr offset = addr - begin;
+CheckBool DisassemblerElf32::RVAToFileOffset(RVA rva,
+                                             size_t* result) const {
+  for (int i = 0; i < SectionHeaderCount(); i++) {
+    const Elf32_Shdr *section_header = SectionHeader(i);
+    // These can appear to have a size in the file, but don't.
+    if (section_header->sh_type == SHT_NOBITS)
+      continue;
+    Elf32_Addr begin = section_header->sh_addr;
+    Elf32_Addr end = begin + section_header->sh_size;
 
-      if (offset < ProgramSegmentFileSize(i)) {
-        *result = ProgramSegmentFileOffset(i) + offset;
-        return true;
-      }
+    if (rva >= begin && rva < end) {
+      *result = section_header->sh_offset + (rva - begin);
+      return true;
     }
   }
-
   return false;
 }
 
@@ -491,7 +488,9 @@ CheckBool DisassemblerElf32::ParseRel32RelocsFromSections() {
 
     const Elf32_Shdr *section_header = SectionHeader(section_id);
 
-    if (section_header->sh_type != SHT_PROGBITS)
+    // Some debug sections can have sh_type=SHT_PROGBITS but sh_addr=0.
+    if (section_header->sh_type != SHT_PROGBITS ||
+        section_header->sh_addr == 0)
       continue;
 
     if (!ParseRel32RelocsFromSection(section_header))
diff --git a/courgette/disassembler_elf_32.h b/courgette/disassembler_elf_32.h
index 110dd71..8483ce3 100644
--- a/courgette/disassembler_elf_32.h
+++ b/courgette/disassembler_elf_32.h
@@ -116,10 +116,6 @@ class DisassemblerElf32 : public Disassembler {
     return OffsetToPointer(SectionHeader(id)->sh_offset);
   }
 
-  Elf32_Word SectionBodySize(int id) const {
-    return SectionHeader(id)->sh_size;
-  }
-
   // Misc Segment Helpers
 
   Elf32_Half ProgramSegmentHeaderCount() const {
@@ -131,26 +127,6 @@ class DisassemblerElf32 : public Disassembler {
     return program_header_table_ + id;
   }
 
-  // The virtual memory address at which this program segment will be loaded
-  Elf32_Addr ProgramSegmentMemoryBegin(int id) const {
-    return ProgramSegmentHeader(id)->p_vaddr;
-  }
-
-  // The number of virtual memory bytes for this program segment
-  Elf32_Word ProgramSegmentMemorySize(int id) const {
-    return ProgramSegmentHeader(id)->p_memsz;
-  }
-
-  // Pointer into the source file for this program segment
-  Elf32_Addr ProgramSegmentFileOffset(int id) const {
-    return ProgramSegmentHeader(id)->p_offset;
-  }
-
-  // Number of file bytes for this program segment. Is <= ProgramMemorySize.
-  Elf32_Word ProgramSegmentFileSize(int id) const {
-    return ProgramSegmentHeader(id)->p_filesz;
-  }
-
   // Misc address space helpers
 
   CheckBool IsValidRVA(RVA rva) const WARN_UNUSED_RESULT;
diff --git a/courgette/encode_decode_unittest.cc b/courgette/encode_decode_unittest.cc
index 7494785..8c972a6 100644
--- a/courgette/encode_decode_unittest.cc
+++ b/courgette/encode_decode_unittest.cc
@@ -94,3 +94,8 @@ TEST_F(EncodeDecodeTest, Elf_HighBSS) {
   std::string file = FileContents("elf-32-high-bss");
   TestAssembleToStreamDisassemble(file, 7312);
 }
+
+TEST_F(EncodeDecodeTest, Elf_Arm) {
+  std::string file = FileContents("elf-armv7");
+  TestAssembleToStreamDisassemble(file, 8531);
+}