[Courgette] Make ELF section header sorting less intrusive.

courgette/disassembler_elf_32.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "courgette/disassembler_elf_32.h"
 
 #include <algorithm>
 
 #include "base/logging.h"
 #include "courgette/assembly_program.h"
 #include "courgette/courgette.h"
 
 namespace courgette {
 
 namespace {
 
-// Initializes |section_header_table| by copying |section_header_table_size|
-// entries from |section_header_table_raw|, and sorting by |sh_offset|.
-void SortSectionHeader(const Elf32_Shdr* section_header_table_raw,
-                       Elf32_Half section_header_table_size,
-                       std::vector<Elf32_Shdr>* section_header_table) {
-  section_header_table->assign(section_header_table_raw,
-      section_header_table_raw + section_header_table_size);
-  auto comp = [](const Elf32_Shdr& header1, const Elf32_Shdr& header2) {
-    return header1.sh_offset < header2.sh_offset;
+// Sorts |section_headers| by file offset and stores the resulting permutation
+// of section ids in |order|.
+std::vector<Elf32_Half> GetSectionHeaderFileOffsetOrder(
+    const std::vector<Elf32_Shdr>& section_headers) {
+  size_t size = section_headers.size();
+  std::vector<Elf32_Half> order(size);
+  for (size_t i = 0; i < size; ++i)
+    order[i] = static_cast<Elf32_Half>(i);
+
+  auto comp = [&](int idx1, int idx2) {
+    return section_headers[idx1].sh_offset < section_headers[idx2].sh_offset;
   };
-  std::stable_sort(
-      section_header_table->begin(), section_header_table->end(), comp);
+  std::stable_sort(order.begin(), order.end(), comp);
+  return order;
 }
 
 }  // namespace
 
 DisassemblerElf32::DisassemblerElf32(const void* start, size_t length)
     : Disassembler(start, length),
       header_(nullptr),
       section_header_table_size_(0),
       program_header_table_(nullptr),
       program_header_table_size_(0),
       default_string_section_(nullptr) {
 }
 
 RVA DisassemblerElf32::FileOffsetToRVA(FileOffset offset) const {
   // File offsets can be 64-bit values, but we are dealing with 32-bit
   // executables and so only need to support 32-bit file sizes.
   uint32_t offset32 = static_cast<uint32_t>(offset);
 
-  for (Elf32_Half section_id = 0; section_id < SectionHeaderCount();
-       ++section_id) {
+  // Visit section headers ordered by file offset.
+  for (Elf32_Half section_id : section_header_file_offset_order_) {
     const Elf32_Shdr* section_header = SectionHeader(section_id);
     // These can appear to have a size in the file, but don't.
     if (section_header->sh_type == SHT_NOBITS)
       continue;
 
     Elf32_Off section_begin = section_header->sh_offset;
     Elf32_Off section_end = section_begin + section_header->sh_size;
 
     if (offset32 >= section_begin && offset32 < section_end) {
       return section_header->sh_addr + (offset32 - section_begin);
@@ skipping 46 matching lines @@
 
   if (header_->e_version != 1)
     return Bad("Unknown file version");
 
   if (header_->e_shentsize != sizeof(Elf32_Shdr))
     return Bad("Unexpected section header size");
 
   if (!IsArrayInBounds(header_->e_shoff, header_->e_shnum, sizeof(Elf32_Shdr)))
     return Bad("Out of bounds section header table");
 
+  // Extract |section_header_table_|, ordered by section id.
   const Elf32_Shdr* section_header_table_raw =
       reinterpret_cast<const Elf32_Shdr*>(
           FileOffsetToPointer(header_->e_shoff));
   section_header_table_size_ = header_->e_shnum;
-  SortSectionHeader(section_header_table_raw, section_header_table_size_,
-                    &section_header_table_);
+  section_header_table_.assign(section_header_table_raw,
+      section_header_table_raw + section_header_table_size_);
+
+  section_header_file_offset_order_ =
+      GetSectionHeaderFileOffsetOrder(section_header_table_);
 
   if (!IsArrayInBounds(header_->e_phoff, header_->e_phnum, sizeof(Elf32_Phdr)))
     return Bad("Out of bounds program header table");
 
   program_header_table_ = reinterpret_cast<const Elf32_Phdr*>(
       FileOffsetToPointer(header_->e_phoff));
   program_header_table_size_ = header_->e_phnum;
 
   if (header_->e_shstrndx >= header_->e_shnum)
     return Bad("Out of bounds string section index");
@@ skipping 126 matching lines @@
 
   if (!RVAsToFileOffsets(&rel32_locations_))
     return false;
 
   std::vector<FileOffset>::iterator current_abs_offset = abs_offsets.begin();
   ScopedVector<TypedRVA>::iterator current_rel = rel32_locations_.begin();
 
   std::vector<FileOffset>::iterator end_abs_offset = abs_offsets.end();
   ScopedVector<TypedRVA>::iterator end_rel = rel32_locations_.end();
 
-  for (Elf32_Half section_id = 0; section_id < SectionHeaderCount();
-       ++section_id) {
+  // Visit section headers ordered by file offset.
+  for (Elf32_Half section_id : section_header_file_offset_order_) {
     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))
       return false;
 
     file_offset = section_header->sh_offset;
 
@@ skipping 220 matching lines @@
 
   std::sort(rel32_locations_.begin(),
             rel32_locations_.end(),
             TypedRVA::IsLessThan);
   DCHECK(rel32_locations_.empty() ||
          rel32_locations_.back()->rva() != kUnassignedRVA);
   return true;
 }
 
 }  // namespace courgette