Add debug fission support.

src/common/dwarf/elf_reader.cc

+// Copyright 2005 Google Inc. All Rights Reserved.
+// Author: chatham@google.com (Andrew Chatham)

[ivanpe, 2016/04/27 01:11:45]

Where does this file come from?  Has it been reviewed before?

[yunlian, 2016/04/27 16:35:29]

It comes from Google source tree and it has been reviewed.

[ivanpe, 2016/04/27 18:02:07]

Great.  Can you please, update the CL description with information on where this
code came from.  Also, in the description you should clarify which parts of the
code are new and must be reviewed more carefully.  This is a very large CL and
such information could speed up the review process and people can focus on the
more important parts.

+// Author: satorux@google.com (Satoru Takabayashi)
+//
+// Code for reading in ELF files.
+//
+// For information on the ELF format, see
+// http://www.x86.org/ftp/manuals/tools/elf.pdf
+//
+// I also liked:
+// http://www.caldera.com/developers/gabi/1998-04-29/contents.html
+//
+// A note about types: When dealing with the file format, we use types
+// like Elf32_Word, but in the public interfaces we treat all
+// addresses as uint64. As a result, we should be able to symbolize
+// 64-bit binaries from a 32-bit process (which we don't do,
+// anyway). size_t should therefore be avoided, except where required
+// by things like mmap().
+//
+// Although most of this code can deal with arbitrary ELF files of
+// either word size, the public ElfReader interface only examines
+// files loaded into the current address space, which must all match
+// __WORDSIZE. This code cannot handle ELF files with a non-native
+// byte ordering.
+//
+// TODO(chatham): It would be nice if we could accomplish this task
+// without using malloc(), so we could use it as the process is dying.
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE  // needed for pread()
+#endif
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <elf.h>
+#include <string.h>
+
+#include <algorithm>
+#include <map>
+#include <string>
+#include <vector>
+#include "zlib.h"
+
+#include "elf_reader.h"
+//#include "using_std_string.h"
+// EM_AARCH64 is not defined by elf.h of GRTE v3 on x86.
+// TODO(dougkwan): Remove this when v17 is retired.
+#if !defined(EM_AARCH64)
+#define EM_AARCH64      183             /* ARM AARCH64 */
+#endif
+
+// TODO(dthomson): Can be removed once all Java code is using the Google3
+// launcher. We need to avoid processing PLT functions as it causes memory
+// fragmentation in malloc, which is fixed in tcmalloc - and if the Google3
+// launcher is used the JVM will then use tcmalloc. b/13735638
+//DEFINE_bool(elfreader_process_dynsyms, true,
+//            "Activate PLT function processing");
+
+using std::string;
+using std::vector;
+
+namespace {
+
+// The lowest bit of an ARM symbol value is used to indicate a Thumb address.
+const int kARMThumbBitOffset = 0;
+
+// Converts an ARM Thumb symbol value to a true aligned address value.
+template <typename T>
+T AdjustARMThumbSymbolValue(const T& symbol_table_value) {
+  return symbol_table_value & ~(1 << kARMThumbBitOffset);
+}
+
+// Names of PLT-related sections.
+const char kElfPLTRelSectionName[] = ".rel.plt";      // Use Rel struct.
+const char kElfPLTRelaSectionName[] = ".rela.plt";    // Use Rela struct.
+const char kElfPLTSectionName[] = ".plt";
+const char kElfDynSymSectionName[] = ".dynsym";
+
+const int kX86PLTCodeSize = 0x10;  // Size of one x86 PLT function in bytes.
+const int kARMPLTCodeSize = 0xc;
+const int kAARCH64PLTCodeSize = 0x10;
+
+const int kX86PLT0Size = 0x10;  // Size of the special PLT0 entry.
+const int kARMPLT0Size = 0x14;
+const int kAARCH64PLT0Size = 0x20;
+
+// Suffix for PLT functions when it needs to be explicitly identified as such.
+const char kPLTFunctionSuffix[] = "@plt";
+
+}  // namespace
+
+namespace dwarf2reader {
+
+template <class ElfArch> class ElfReaderImpl;
+
+// 32-bit and 64-bit ELF files are processed exactly the same, except
+// for various field sizes. Elf32 and Elf64 encompass all of the
+// differences between the two formats, and all format-specific code
+// in this file is templated on one of them.
+class Elf32 {
+ public:
+  typedef Elf32_Ehdr Ehdr;
+  typedef Elf32_Shdr Shdr;
+  typedef Elf32_Phdr Phdr;
+  typedef Elf32_Word Word;
+  typedef Elf32_Sym Sym;
+  typedef Elf32_Rel Rel;
+  typedef Elf32_Rela Rela;
+
+  // What should be in the EI_CLASS header.
+  static const int kElfClass = ELFCLASS32;
+
+  // Given a symbol pointer, return the binding type (eg STB_WEAK).
+  static char Bind(const Elf32_Sym *sym) {
+    return ELF32_ST_BIND(sym->st_info);
+  }
+  // Given a symbol pointer, return the symbol type (eg STT_FUNC).
+  static char Type(const Elf32_Sym *sym) {
+    return ELF32_ST_TYPE(sym->st_info);
+  }
+
+  // Extract the symbol index from the r_info field of a relocation.
+  static int r_sym(const Elf32_Word r_info) {
+    return ELF32_R_SYM(r_info);
+  }
+};
+
+
+class Elf64 {
+ public:
+  typedef Elf64_Ehdr Ehdr;
+  typedef Elf64_Shdr Shdr;
+  typedef Elf64_Phdr Phdr;
+  typedef Elf64_Word Word;
+  typedef Elf64_Sym Sym;
+  typedef Elf64_Rel Rel;
+  typedef Elf64_Rela Rela;
+
+  // What should be in the EI_CLASS header.
+  static const int kElfClass = ELFCLASS64;
+
+  static char Bind(const Elf64_Sym *sym) {
+    return ELF64_ST_BIND(sym->st_info);
+  }
+  static char Type(const Elf64_Sym *sym) {
+    return ELF64_ST_TYPE(sym->st_info);
+  }
+  static int r_sym(const Elf64_Xword r_info) {
+    return ELF64_R_SYM(r_info);
+  }
+};
+
+
+// ElfSectionReader mmaps a section of an ELF file ("section" is ELF
+// terminology). The ElfReaderImpl object providing the section header
+// must exist for the lifetime of this object.
+//
+// The motivation for mmaping individual sections of the file is that
+// many Google executables are large enough when unstripped that we
+// have to worry about running out of virtual address space.
+//
+// For compressed sections we have no choice but to allocate memory.
+template<class ElfArch>
+class ElfSectionReader {
+ public:
+  ElfSectionReader(const char *name, const string &path, int fd,
+                   const typename ElfArch::Shdr &section_header)
+      : contents_aligned_(NULL),
+        contents_(NULL),
+        header_(section_header) {
+    // Back up to the beginning of the page we're interested in.
+    const size_t additional = header_.sh_offset % getpagesize();
+    const size_t offset_aligned = header_.sh_offset - additional;
+    section_size_ = header_.sh_size;
+    size_aligned_ = section_size_ + additional;
+    // If the section has been stripped or is empty, do not attempt
+    // to process its contents.
+    if (header_.sh_type == SHT_NOBITS || header_.sh_size == 0)
+      return;
+    contents_aligned_ = mmap(NULL, size_aligned_, PROT_READ, MAP_SHARED,
+                             fd, offset_aligned);
+    // Set where the offset really should begin.
+    contents_ = reinterpret_cast<char *>(contents_aligned_) +
+                (header_.sh_offset - offset_aligned);
+
+    // Check for and handle any compressed contents.
+    //if (strncmp(name, ".zdebug_", strlen(".zdebug_")) == 0)
+    //  DecompressZlibContents();
+    // TODO(saugustine): Add support for proposed elf-section flag
+    // "SHF_COMPRESS".
+  }
+
+  ~ElfSectionReader() {
+    if (contents_aligned_ != NULL)
+      munmap(contents_aligned_, size_aligned_);
+    else
+      delete[] contents_;
+  }
+
+  // Return the section header for this section.
+  typename ElfArch::Shdr const &header() const { return header_; }
+
+  // Return memory at the given offset within this section.
+  const char *GetOffset(typename ElfArch::Word bytes) const {
+    return contents_ + bytes;
+  }
+
+  const char *contents() const { return contents_; }
+  size_t section_size() const { return section_size_; }
+
+ private:
+  // page-aligned file contents
+  void *contents_aligned_;
+  // contents as usable by the client. For non-compressed sections,
+  // pointer within contents_aligned_ to where the section data
+  // begins; for compressed sections, pointer to the decompressed
+  // data.
+  char *contents_;
+  // size of contents_aligned_
+  size_t size_aligned_;
+  // size of contents.
+  size_t section_size_;
+  const typename ElfArch::Shdr header_;
+};
+
+// An iterator over symbols in a given section. It handles walking
+// through the entries in the specified section and mapping symbol
+// entries to their names in the appropriate string table (in
+// another section).
+template<class ElfArch>
+class SymbolIterator {
+ public:
+  SymbolIterator(ElfReaderImpl<ElfArch> *reader,
+                 typename ElfArch::Word section_type)
+      : symbol_section_(reader->GetSectionByType(section_type)),
+        string_section_(NULL),
+        num_symbols_in_section_(0),
+        symbol_within_section_(0) {
+
+    // If this section type doesn't exist, leave
+    // num_symbols_in_section_ as zero, so this iterator is already
+    // done().
+    if (symbol_section_ != NULL) {
+      num_symbols_in_section_ = symbol_section_->header().sh_size /
+                                symbol_section_->header().sh_entsize;
+
+      // Symbol sections have sh_link set to the section number of
+      // the string section containing the symbol names.
+      string_section_ = reader->GetSection(symbol_section_->header().sh_link);
+    }
+  }
+
+  // Return true iff we have passed all symbols in this section.
+  bool done() const {
+    return symbol_within_section_ >= num_symbols_in_section_;
+  }
+
+  // Advance to the next symbol in this section.
+  // REQUIRES: !done()
+  void Next() { ++symbol_within_section_; }
+
+  // Return a pointer to the current symbol.
+  // REQUIRES: !done()
+  const typename ElfArch::Sym *GetSymbol() const {
+    return reinterpret_cast<const typename ElfArch::Sym*>(
+        symbol_section_->GetOffset(symbol_within_section_ *
+                                   symbol_section_->header().sh_entsize));
+  }
+
+  // Return the name of the current symbol, NULL if it has none.
+  // REQUIRES: !done()
+  const char *GetSymbolName() const {
+    int name_offset = GetSymbol()->st_name;
+    if (name_offset == 0)
+      return NULL;
+    return string_section_->GetOffset(name_offset);
+  }
+
+  int GetCurrentSymbolIndex() const {
+    return symbol_within_section_;
+  }
+
+ private:
+  const ElfSectionReader<ElfArch> *const symbol_section_;
+  const ElfSectionReader<ElfArch> *string_section_;
+  int num_symbols_in_section_;
+  int symbol_within_section_;
+};
+
+
+// Copied from strings/strutil.h.  Per chatham,
+// this library should not depend on strings.
+
+static inline bool MyHasSuffixString(const string& str, const string& suffix) {
+  int len = str.length();
+  int suflen = suffix.length();
+  return (suflen <= len) && (str.compare(len-suflen, suflen, suffix) == 0);
+}
+
+
+// ElfReader loads an ELF binary and can provide information about its
+// contents. It is most useful for matching addresses to function
+// names. It does not understand debugging formats (eg dwarf2), so it
+// can't print line numbers. It takes a path to an elf file and a
+// readable file descriptor for that file, which it does not assume
+// ownership of.
+template<class ElfArch>
+class ElfReaderImpl {
+ public:
+  explicit ElfReaderImpl(const string &path, int fd)
+      : path_(path),
+        fd_(fd),
+        section_headers_(NULL),
+        program_headers_(NULL),
+        opd_section_(NULL),
+        base_for_text_(0),
+        plts_supported_(false),
+        plt_code_size_(0),
+        plt0_size_(0),
+        visited_relocation_entries_(false) {
+    string error;
+    is_dwp_ = MyHasSuffixString(path, ".dwp");
+    ParseHeaders(fd, path);
+    // Currently we need some extra information for PowerPC64 binaries
+    // including a way to read the .opd section for function descriptors and a
+    // way to find the linked base for function symbols.
+    if (header_.e_machine == EM_PPC64) {
+      // "opd_section_" must always be checked for NULL before use.
+      opd_section_ = GetSectionInfoByName(".opd", &opd_info_);
+      for (unsigned int k = 0u; k < GetNumSections(); ++k) {
+        const char *name = GetSectionName(section_headers_[k].sh_name);
+        if (strncmp(name, ".text", strlen(".text")) == 0) {
+          base_for_text_ =
+              section_headers_[k].sh_addr - section_headers_[k].sh_offset;
+          break;
+        }
+      }
+    }
+    // Turn on PLTs.
+    if (header_.e_machine == EM_386 || header_.e_machine == EM_X86_64) {
+      plt_code_size_ = kX86PLTCodeSize;
+      plt0_size_ = kX86PLT0Size;
+      plts_supported_ = true;
+    } else if (header_.e_machine == EM_ARM) {
+      plt_code_size_ = kARMPLTCodeSize;
+      plt0_size_ = kARMPLT0Size;
+      plts_supported_ = true;
+    } else if (header_.e_machine == EM_AARCH64) {
+      plt_code_size_ = kAARCH64PLTCodeSize;
+      plt0_size_ = kAARCH64PLT0Size;
+      plts_supported_ = true;
+    }
+  }
+
+  ~ElfReaderImpl() {
+    for (unsigned int i = 0u; i < sections_.size(); ++i)
+      delete sections_[i];
+    delete [] section_headers_;
+    delete [] program_headers_;
+  }
+
+  // Examine the headers of the file and return whether the file looks
+  // like an ELF file for this architecture. Takes an already-open
+  // file descriptor for the candidate file, reading in the prologue
+  // to see if the ELF file appears to match the current
+  // architecture. If error is non-NULL, it will be set with a reason
+  // in case of failure.
+  static bool IsArchElfFile(int fd, string *error) {
+    unsigned char header[EI_NIDENT];
+    if (pread(fd, header, sizeof(header), 0) != sizeof(header)) {
+      if (error != NULL) *error = "Could not read header";
+      return false;
+    }
+
+    if (memcmp(header, ELFMAG, SELFMAG) != 0) {
+      if (error != NULL) *error = "Missing ELF magic";
+      return false;
+    }
+
+    if (header[EI_CLASS] != ElfArch::kElfClass) {
+      if (error != NULL) *error = "Different word size";
+      return false;
+    }
+
+    int endian = 0;
+    if (header[EI_DATA] == ELFDATA2LSB)
+      endian = __LITTLE_ENDIAN;
+    else if (header[EI_DATA] == ELFDATA2MSB)
+      endian = __BIG_ENDIAN;
+    if (endian != __BYTE_ORDER) {
+      if (error != NULL) *error = "Different byte order";
+      return false;
+    }
+
+    return true;
+  }
+
+  // Return true if we can use this symbol in Address-to-Symbol map.
+  bool CanUseSymbol(const char *name, const typename ElfArch::Sym *sym) {
+    // For now we only save FUNC and NOTYPE symbols. For now we just
+    // care about functions, but some functions written in assembler
+    // don't have a proper ELF type attached to them, so we store
+    // NOTYPE symbols as well. The remaining significant type is
+    // OBJECT (eg global variables), which represent about 25% of
+    // the symbols in a typical google3 binary.
+    if (ElfArch::Type(sym) != STT_FUNC &&
+        ElfArch::Type(sym) != STT_NOTYPE) {
+      return false;
+    }
+
+    // Target specific filtering.
+    switch (header_.e_machine) {
+    case EM_AARCH64:
+    case EM_ARM:
+      // Filter out '$x' special local symbols used by tools
+      return name[0] != '$' || ElfArch::Bind(sym) != STB_LOCAL;
+    case EM_X86_64:
+      // Filter out read-only constants like .LC123.
+      return name[0] != '.' || ElfArch::Bind(sym) != STB_LOCAL;
+    default:
+      return true;
+    }
+  }
+
+  // Iterate over the symbols in a section, either SHT_DYNSYM or
+  // SHT_SYMTAB. Add all symbols to the given SymbolMap.
+  /*
+  void GetSymbolPositions(SymbolMap *symbols,
+                          typename ElfArch::Word section_type,
+                          uint64 mem_offset,
+                          uint64 file_offset) {
+    // This map is used to filter out "nested" functions.
+    // See comment below.
+    AddrToSymMap addr_to_sym_map;
+    for (SymbolIterator<ElfArch> it(this, section_type);
+         !it.done(); it.Next()) {
+      const char *name = it.GetSymbolName();
+      if (name == NULL)
+        continue;
+      const typename ElfArch::Sym *sym = it.GetSymbol();
+      if (CanUseSymbol(name, sym)) {
+        const int sec = sym->st_shndx;
+
+        // We don't support special section indices. The most common
+        // is SHN_ABS, for absolute symbols used deep in the bowels of
+        // glibc. Also ignore any undefined symbols.
+        if (sec == SHN_UNDEF ||
+            (sec >= SHN_LORESERVE && sec <= SHN_HIRESERVE)) {
+          continue;
+        }
+
+        const typename ElfArch::Shdr& hdr = section_headers_[sec];
+
+        // Adjust for difference between where we expected to mmap
+        // this section, and where it was actually mmapped.
+        const int64 expected_base = hdr.sh_addr - hdr.sh_offset;
+        const int64 real_base = mem_offset - file_offset;
+        const int64 adjust = real_base - expected_base;
+
+        uint64 start = sym->st_value + adjust;
+
+        // Adjust function symbols for PowerPC64 by dereferencing and adjusting
+        // the function descriptor to get the function address.
+        if (header_.e_machine == EM_PPC64 && ElfArch::Type(sym) == STT_FUNC) {
+          const uint64 opd_addr =
+              AdjustPPC64FunctionDescriptorSymbolValue(sym->st_value);
+          // Only adjust the returned value if the function address was found.
+          if (opd_addr != sym->st_value) {
+            const int64 adjust_function_symbols =
+                real_base - base_for_text_;
+            start = opd_addr + adjust_function_symbols;
+          }
+        }
+
+        addr_to_sym_map.push_back(std::make_pair(start, sym));
+      }
+    }
+    std::sort(addr_to_sym_map.begin(), addr_to_sym_map.end(), &AddrToSymSorter);
+    addr_to_sym_map.erase(std::unique(addr_to_sym_map.begin(),
+                                      addr_to_sym_map.end(), &AddrToSymEquals),
+                          addr_to_sym_map.end());
+
+    // Squeeze out any "nested functions".
+    // Nested functions are not allowed in C, but libc plays tricks.
+    //
+    // For example, here is disassembly of /lib64/tls/libc-2.3.5.so:
+    //   0x00000000000aa380 <read+0>:             cmpl   $0x0,0x2781b9(%rip)
+    //   0x00000000000aa387 <read+7>:             jne    0xaa39b <read+27>
+    //   0x00000000000aa389 <__read_nocancel+0>:  mov    $0x0,%rax
+    //   0x00000000000aa390 <__read_nocancel+7>:  syscall
+    //   0x00000000000aa392 <__read_nocancel+9>:  cmp $0xfffffffffffff001,%rax
+    //   0x00000000000aa398 <__read_nocancel+15>: jae    0xaa3ef <read+111>
+    //   0x00000000000aa39a <__read_nocancel+17>: retq
+    //   0x00000000000aa39b <read+27>:            sub    $0x28,%rsp
+    //   0x00000000000aa39f <read+31>:            mov    %rdi,0x8(%rsp)
+    //   ...
+    // Without removing __read_nocancel, symbolizer will return NULL
+    // given e.g. 0xaa39f (because the lower bound is __read_nocancel,
+    // but 0xaa39f is beyond its end.
+    if (addr_to_sym_map.empty()) {
+      return;
+    }
+    const ElfSectionReader<ElfArch> *const symbol_section =
+        this->GetSectionByType(section_type);
+    const ElfSectionReader<ElfArch> *const string_section =
+        this->GetSection(symbol_section->header().sh_link);
+
+    typename AddrToSymMap::iterator curr = addr_to_sym_map.begin();
+    // Always insert the first symbol.
+    symbols->AddSymbol(string_section->GetOffset(curr->second->st_name),
+                       curr->first, curr->second->st_size);
+    typename AddrToSymMap::iterator prev = curr++;
+    for (; curr != addr_to_sym_map.end(); ++curr) {
+      const uint64 prev_addr = prev->first;
+      const uint64 curr_addr = curr->first;
+      const typename ElfArch::Sym *const prev_sym = prev->second;
+      const typename ElfArch::Sym *const curr_sym = curr->second;
+      if (prev_addr + prev_sym->st_size <= curr_addr ||
+          // The next condition is true if two symbols overlap like this:
+          //
+          //   Previous symbol  |----------------------------|
+          //   Current symbol     |-------------------------------|
+          //
+          // These symbols are not found in google3 codebase, but in
+          // jdk1.6.0_01_gg1/jre/lib/i386/server/libjvm.so.
+          //
+          // 0619e040 00000046 t CardTableModRefBS::write_region_work()
+          // 0619e070 00000046 t CardTableModRefBS::write_ref_array_work()
+          //
+          // We allow overlapped symbols rather than ignore these.
+          // Due to the way SymbolMap::GetSymbolAtPosition() works,
+          // lookup for any address in [curr_addr, curr_addr + its size)
+          // (e.g. 0619e071) will produce the current symbol,
+          // which is the desired outcome.
+          prev_addr + prev_sym->st_size < curr_addr + curr_sym->st_size) {
+        const char *name = string_section->GetOffset(curr_sym->st_name);
+        symbols->AddSymbol(name, curr_addr, curr_sym->st_size);
+        prev = curr;
+      } else {
+        // Current symbol is "nested" inside previous one like this:
+        //
+        //   Previous symbol  |----------------------------|
+        //   Current symbol     |---------------------|
+        //
+        // This happens within glibc, e.g. __read_nocancel is nested
+        // "inside" __read. Ignore "inner" symbol.
+        //DCHECK_LE(curr_addr + curr_sym->st_size,
+        //          prev_addr + prev_sym->st_size);
+        ;
+      }
+    }
+  }
+*/
+
+  void VisitSymbols(typename ElfArch::Word section_type,
+                    ElfReader::SymbolSink *sink) {
+    VisitSymbols(section_type, sink, -1, -1, false);
+  }
+
+  void VisitSymbols(typename ElfArch::Word section_type,
+                    ElfReader::SymbolSink *sink,
+                    int symbol_binding,
+                    int symbol_type,
+                    bool get_raw_symbol_values) {
+    for (SymbolIterator<ElfArch> it(this, section_type);
+         !it.done(); it.Next()) {
+      const char *name = it.GetSymbolName();
+      if (!name) continue;
+      const typename ElfArch::Sym *sym = it.GetSymbol();
+      if ((symbol_binding < 0 || ElfArch::Bind(sym) == symbol_binding) &&
+          (symbol_type < 0 || ElfArch::Type(sym) == symbol_type)) {
+        typename ElfArch::Sym symbol = *sym;
+        // Add a PLT symbol in addition to the main undefined symbol.
+        // Only do this for SHT_DYNSYM, because PLT symbols are dynamic.
+        int symbol_index = it.GetCurrentSymbolIndex();
+        // TODO(dthomson): Can be removed once all Java code is using the
+        // Google3 launcher.
+        if (section_type == SHT_DYNSYM &&
+            static_cast<unsigned int>(symbol_index) < symbols_plt_offsets_.size() &&
+            symbols_plt_offsets_[symbol_index] != 0) {
+          string plt_name = string(name) + kPLTFunctionSuffix;
+          if (plt_function_names_[symbol_index].empty()) {
+            plt_function_names_[symbol_index] = plt_name;
+          } else if (plt_function_names_[symbol_index] != plt_name) {
+                ;
+          }
+          sink->AddSymbol(plt_function_names_[symbol_index].c_str(),
+                          symbols_plt_offsets_[it.GetCurrentSymbolIndex()],
+                          plt_code_size_);
+        }
+        if (!get_raw_symbol_values)
+          AdjustSymbolValue(&symbol);
+        sink->AddSymbol(name, symbol.st_value, symbol.st_size);
+      }
+    }
+  }
+
+  void VisitRelocationEntries() {
+    if (visited_relocation_entries_) {
+      return;
+    }
+    visited_relocation_entries_ = true;
+
+    if (!plts_supported_) {
+      return;
+    }
+    // First determine if PLTs exist. If not, then there is nothing to do.
+    ElfReader::SectionInfo plt_section_info;
+    const char* plt_section =
+        GetSectionInfoByName(kElfPLTSectionName, &plt_section_info);
+    if (!plt_section) {
+      return;
+    }
+    if (plt_section_info.size == 0) {
+      return;
+    }
+
+    // The PLTs could be referenced by either a Rel or Rela (Rel with Addend)
+    // section.
+    ElfReader::SectionInfo rel_section_info;
+    ElfReader::SectionInfo rela_section_info;
+    const char* rel_section =
+        GetSectionInfoByName(kElfPLTRelSectionName, &rel_section_info);
+    const char* rela_section =
+        GetSectionInfoByName(kElfPLTRelaSectionName, &rela_section_info);
+
+    const typename ElfArch::Rel* rel =
+        reinterpret_cast<const typename ElfArch::Rel*>(rel_section);
+    const typename ElfArch::Rela* rela =
+        reinterpret_cast<const typename ElfArch::Rela*>(rela_section);
+
+    if (!rel_section && !rela_section) {
+      return;
+    }
+
+    // Use either Rel or Rela section, depending on which one exists.
+    size_t section_size = rel_section ? rel_section_info.size
+                                      : rela_section_info.size;
+    size_t entry_size = rel_section ? sizeof(typename ElfArch::Rel)
+                                    : sizeof(typename ElfArch::Rela);
+
+    // Determine the number of entries in the dynamic symbol table.
+    ElfReader::SectionInfo dynsym_section_info;
+    const char* dynsym_section =
+        GetSectionInfoByName(kElfDynSymSectionName, &dynsym_section_info);
+    // The dynsym section might not exist, or it might be empty. In either case
+    // there is nothing to be done so return.
+    if (!dynsym_section || dynsym_section_info.size == 0) {
+      return;
+    }
+    size_t num_dynamic_symbols =
+        dynsym_section_info.size / dynsym_section_info.entsize;
+    symbols_plt_offsets_.resize(num_dynamic_symbols, 0);
+
+    // TODO(dthomson): Can be removed once all Java code is using the
+    // Google3 launcher.
+    // Make storage room for PLT function name strings.
+    plt_function_names_.resize(num_dynamic_symbols);
+
+    for (size_t i = 0; i < section_size / entry_size; ++i) {
+      // Determine symbol index from the |r_info| field.
+      int sym_index = ElfArch::r_sym(rel_section ? rel[i].r_info
+                                                 : rela[i].r_info);
+      if (static_cast<unsigned int>(sym_index) >= symbols_plt_offsets_.size()) {
+        continue;
+      }
+      symbols_plt_offsets_[sym_index] =
+          plt_section_info.addr + plt0_size_ + i * plt_code_size_;
+    }
+  }
+
+  // Return an ElfSectionReader for the first section of the given
+  // type by iterating through all section headers. Returns NULL if
+  // the section type is not found.
+  const ElfSectionReader<ElfArch> *GetSectionByType(
+      typename ElfArch::Word section_type) {
+    for (unsigned int k = 0u; k < GetNumSections(); ++k) {
+      if (section_headers_[k].sh_type == section_type) {
+        return GetSection(k);
+      }
+    }
+    return NULL;
+  }
+
+  // Return the name of section "shndx".  Returns NULL if the section
+  // is not found.
+  const char *GetSectionNameByIndex(int shndx) {
+    return GetSectionName(section_headers_[shndx].sh_name);
+  }
+
+  // Return a pointer to section "shndx", and store the size in
+  // "size".  Returns NULL if the section is not found.
+  const char *GetSectionContentsByIndex(int shndx, size_t *size) {
+    const ElfSectionReader<ElfArch> *section = GetSection(shndx);
+    if (section != NULL) {
+      *size = section->section_size();
+      return section->contents();
+    }
+    return NULL;
+  }
+
+  // Return a pointer to the first section of the given name by
+  // iterating through all section headers, and store the size in
+  // "size".  Returns NULL if the section name is not found.
+  const char *GetSectionContentsByName(const string &section_name,
+                                       size_t *size) {
+    for (unsigned int k = 0u; k < GetNumSections(); ++k) {
+      // When searching for sections in a .dwp file, the sections
+      // we're looking for will always be at the end of the section
+      // table, so reverse the direction of iteration.
+      int shndx = is_dwp_ ? GetNumSections() - k - 1 : k;
+      const char *name = GetSectionName(section_headers_[shndx].sh_name);
+      if (name != NULL && ElfReader::SectionNamesMatch(section_name, name)) {
+        const ElfSectionReader<ElfArch> *section = GetSection(shndx);
+        if (section == NULL) {
+          return NULL;
+        } else {
+          *size = section->section_size();
+          return section->contents();
+        }
+      }
+    }
+    return NULL;
+  }
+
+  // This is like GetSectionContentsByName() but it returns a lot of extra
+  // information about the section.
+  const char *GetSectionInfoByName(const string &section_name,
+                                   ElfReader::SectionInfo *info) {
+    for (unsigned int k = 0u; k < GetNumSections(); ++k) {
+      // When searching for sections in a .dwp file, the sections
+      // we're looking for will always be at the end of the section
+      // table, so reverse the direction of iteration.
+      int shndx = is_dwp_ ? GetNumSections() - k - 1 : k;
+      const char *name = GetSectionName(section_headers_[shndx].sh_name);
+      if (name != NULL && ElfReader::SectionNamesMatch(section_name, name)) {
+        const ElfSectionReader<ElfArch> *section = GetSection(shndx);
+        if (section == NULL) {
+          return NULL;
+        } else {
+          info->type = section->header().sh_type;
+          info->flags = section->header().sh_flags;
+          info->addr = section->header().sh_addr;
+          info->offset = section->header().sh_offset;
+          info->size = section->header().sh_size;
+          info->link = section->header().sh_link;
+          info->info = section->header().sh_info;
+          info->addralign = section->header().sh_addralign;
+          info->entsize = section->header().sh_entsize;
+          return section->contents();
+        }
+      }
+    }
+    return NULL;
+  }
+
+  // p_vaddr of the first PT_LOAD segment (if any), or 0 if no PT_LOAD
+  // segments are present. This is the address an ELF image was linked
+  // (by static linker) to be loaded at. Usually (but not always) 0 for
+  // shared libraries and position-independent executables.
+  uint64 VaddrOfFirstLoadSegment() const {
+    // Relocatable objects (of type ET_REL) do not have LOAD segments.
+    if (header_.e_type == ET_REL) {
+      return 0;
+    }
+    for (int i = 0; i < GetNumProgramHeaders(); ++i) {
+      if (program_headers_[i].p_type == PT_LOAD) {
+        return program_headers_[i].p_vaddr;
+      }
+    }
+    return 0;
+  }
+
+  // According to the LSB ("ELF special sections"), sections with debug
+  // info are prefixed by ".debug".  The names are not specified, but they
+  // look like ".debug_line", ".debug_info", etc.
+  bool HasDebugSections() {
+    // Debug sections are likely to be near the end, so reverse the
+    // direction of iteration.
+    for (int k = GetNumSections() - 1; k >= 0; --k) {
+      const char *name = GetSectionName(section_headers_[k].sh_name);
+      if (strncmp(name, ".debug", strlen(".debug")) == 0) return true;
+      if (strncmp(name, ".zdebug", strlen(".zdebug")) == 0) return true;
+    }
+    return false;
+  }
+
+  bool IsDynamicSharedObject() const {
+    return header_.e_type == ET_DYN;
+  }
+
+  // Return the number of sections.
+  uint64_t GetNumSections() const {
+    if (HasManySections())
+      return first_section_header_.sh_size;
+    return header_.e_shnum;
+  }
+
+ private:
+  typedef vector<pair<uint64, const typename ElfArch::Sym *> > AddrToSymMap;
+
+  static bool AddrToSymSorter(const typename AddrToSymMap::value_type& lhs,
+                              const typename AddrToSymMap::value_type& rhs) {
+    return lhs.first < rhs.first;
+  }
+
+  static bool AddrToSymEquals(const typename AddrToSymMap::value_type& lhs,
+                              const typename AddrToSymMap::value_type& rhs) {
+    return lhs.first == rhs.first;
+  }
+
+  // Does this ELF file have too many sections to fit in the program header?
+  bool HasManySections() const {
+    return header_.e_shnum == SHN_UNDEF;
+  }
+
+  // Return the number of program headers.
+  int GetNumProgramHeaders() const {
+    if (HasManySections() && header_.e_phnum == 0xffff &&
+        first_section_header_.sh_info != 0)
+      return first_section_header_.sh_info;
+    return header_.e_phnum;
+  }
+
+  // Return the index of the string table.
+  int GetStringTableIndex() const {
+    if (HasManySections()) {
+      if (header_.e_shstrndx == 0xffff)
+        return first_section_header_.sh_link;
+      else if (header_.e_shstrndx >= GetNumSections())
+        return 0;
+    }
+    return header_.e_shstrndx;
+  }
+
+  // Given an offset into the section header string table, return the
+  // section name.
+  const char *GetSectionName(typename ElfArch::Word sh_name) {
+    const ElfSectionReader<ElfArch> *shstrtab =
+        GetSection(GetStringTableIndex());
+    if (shstrtab != NULL) {
+      return shstrtab->GetOffset(sh_name);
+    }
+    return NULL;
+  }
+
+  // Return an ElfSectionReader for the given section. The reader will
+  // be freed when this object is destroyed.
+  const ElfSectionReader<ElfArch> *GetSection(int num) {
+    const char *name;
+    // Hard-coding the name for the section-name string table prevents
+    // infinite recursion.
+    if (num == GetStringTableIndex())
+      name = ".shstrtab";
+    else
+      name = GetSectionNameByIndex(num);
+    ElfSectionReader<ElfArch> *& reader = sections_[num];
+    if (reader == NULL)
+      reader = new ElfSectionReader<ElfArch>(name, path_, fd_,
+                                             section_headers_[num]);
+    return reader;
+  }
+
+  // Parse out the overall header information from the file and assert
+  // that it looks sane. This contains information like the magic
+  // number and target architecture.
+  bool ParseHeaders(int fd, const string &path) {
+    // Read in the global ELF header.
+    if (pread(fd, &header_, sizeof(header_), 0) != sizeof(header_)) {
+      return false;
+    }
+
+    // Must be an executable, dynamic shared object or relocatable object
+    if (header_.e_type != ET_EXEC &&
+        header_.e_type != ET_DYN &&
+        header_.e_type != ET_REL) {
+      return false;
+    }
+    // Need a section header.
+    if (header_.e_shoff == 0) {
+      return false;
+    }
+
+    if (header_.e_shnum == SHN_UNDEF) {
+      // The number of sections in the program header is only a 16-bit value. In
+      // the event of overflow (greater than SHN_LORESERVE sections), e_shnum
+      // will read SHN_UNDEF and the true number of section header table entries
+      // is found in the sh_size field of the first section header.
+      // See: http://www.sco.com/developers/gabi/2003-12-17/ch4.sheader.html
+      if (pread(fd, &first_section_header_, sizeof(first_section_header_),
+                header_.e_shoff) != sizeof(first_section_header_)) {
+        return false;
+      }
+    }
+
+    // Dynamically allocate enough space to store the section headers
+    // and read them out of the file.
+    const int section_headers_size =
+        GetNumSections() * sizeof(*section_headers_);
+    section_headers_ = new typename ElfArch::Shdr[section_headers_size];
+    if (pread(fd, section_headers_, section_headers_size, header_.e_shoff) !=
+        section_headers_size) {
+      return false;
+    }
+
+    // Dynamically allocate enough space to store the program headers
+    // and read them out of the file.
+    //const int program_headers_size =
+    //    GetNumProgramHeaders() * sizeof(*program_headers_);
+    program_headers_ = new typename ElfArch::Phdr[GetNumProgramHeaders()];
+
+    // Presize the sections array for efficiency.
+    sections_.resize(GetNumSections(), NULL);
+    return true;
+  }
+
+  // Given the "value" of a function descriptor return the address of the
+  // function (i.e. the dereferenced value). Otherwise return "value".
+  uint64 AdjustPPC64FunctionDescriptorSymbolValue(uint64 value) {
+    if (opd_section_ != NULL &&
+        opd_info_.addr <= value &&
+        value < opd_info_.addr + opd_info_.size) {
+      uint64 offset = value - opd_info_.addr;
+      return (*reinterpret_cast<const uint64*>(opd_section_ + offset));
+    }
+    return value;
+  }
+
+  void AdjustSymbolValue(typename ElfArch::Sym* sym) {
+    switch (header_.e_machine) {
+    case EM_ARM:
+      // For ARM architecture, if the LSB of the function symbol offset is set,
+      // it indicates a Thumb function.  This bit should not be taken literally.
+      // Clear it.
+      if (ElfArch::Type(sym) == STT_FUNC)
+        sym->st_value = AdjustARMThumbSymbolValue(sym->st_value);
+      break;
+    case EM_386:
+      // No adjustment needed for Intel x86 architecture.  However, explicitly
+      // define this case as we use it quite often.
+      break;
+    case EM_PPC64:
+      // PowerPC64 currently has function descriptors as part of the ABI.
+      // Function symbols need to be adjusted accordingly.
+      if (ElfArch::Type(sym) == STT_FUNC)
+        sym->st_value = AdjustPPC64FunctionDescriptorSymbolValue(sym->st_value);
+      break;
+    default:
+      break;
+    }
+  }
+
+  friend class SymbolIterator<ElfArch>;
+
+  // The file we're reading.
+  const string path_;
+  // Open file descriptor for path_. Not owned by this object.
+  const int fd_;
+
+  // The global header of the ELF file.
+  typename ElfArch::Ehdr header_;
+
+  // The header of the first section. This may be used to supplement the ELF
+  // file header.
+  typename ElfArch::Shdr first_section_header_;
+
+  // Array of GetNumSections() section headers, allocated when we read
+  // in the global header.
+  typename ElfArch::Shdr *section_headers_;
+
+  // Array of GetNumProgramHeaders() program headers, allocated when we read
+  // in the global header.
+  typename ElfArch::Phdr *program_headers_;
+
+  // An array of pointers to ElfSectionReaders. Sections are
+  // mmaped as they're needed and not released until this object is
+  // destroyed.
+  vector<ElfSectionReader<ElfArch>*> sections_;
+
+  // For PowerPC64 we need to keep track of function descriptors when looking up
+  // values for funtion symbols values. Function descriptors are kept in the
+  // .opd section and are dereferenced to find the function address.
+  ElfReader::SectionInfo opd_info_;
+  const char *opd_section_;  // Must be checked for NULL before use.
+  int64 base_for_text_;
+
+  // Read PLT-related sections for the current architecture.
+  bool plts_supported_;
+  // Code size of each PLT function for the current architecture.
+  size_t plt_code_size_;
+  // Size of the special first entry in the .plt section that calls the runtime
+  // loader resolution routine, and that all other entries jump to when doing
+  // lazy symbol binding.
+  size_t plt0_size_;
+
+  // Maps a dynamic symbol index to a PLT offset.
+  // The vector entry index is the dynamic symbol index.
+  std::vector<uint64> symbols_plt_offsets_;
+
+  // Container for PLT function name strings. These strings are passed by
+  // reference to SymbolSink::AddSymbol() so they need to be stored somewhere.
+  std::vector<string> plt_function_names_;
+
+  bool visited_relocation_entries_;
+
+  // True if this is a .dwp file.
+  bool is_dwp_;
+};
+
+ElfReader::ElfReader(const string &path)
+    : path_(path), fd_(-1), impl32_(NULL), impl64_(NULL) {
+  // linux 2.6.XX kernel can show deleted files like this:
+  //   /var/run/nscd/dbYLJYaE (deleted)
+  // and the kernel-supplied vdso and vsyscall mappings like this:
+  //   [vdso]
+  //   [vsyscall]
+  if (MyHasSuffixString(path, " (deleted)"))
+    return;
+  if (path == "[vdso]")
+    return;
+  if (path == "[vsyscall]")
+    return;
+
+  fd_ = open(path.c_str(), O_RDONLY);
+}
+
+ElfReader::~ElfReader() {
+  if (fd_ != -1)
+    close(fd_);
+  if (impl32_ != NULL)
+    delete impl32_;
+  if (impl64_ != NULL)
+    delete impl64_;
+}
+
+
+// The only word-size specific part of this file is IsNativeElfFile().
+#if __WORDSIZE == 32
+#define NATIVE_ELF_ARCH Elf32
+#elif __WORDSIZE == 64
+#define NATIVE_ELF_ARCH Elf64
+#else
+#error "Invalid word size"
+#endif
+
+template <typename ElfArch>
+static bool IsElfFile(const int fd, const string &path) {
+  if (fd < 0)
+    return false;
+  if (!ElfReaderImpl<ElfArch>::IsArchElfFile(fd, NULL)) {
+    // No error message here.  IsElfFile gets called many times.
+    return false;
+  }
+  return true;
+}
+
+bool ElfReader::IsNativeElfFile() const {
+  return IsElfFile<NATIVE_ELF_ARCH>(fd_, path_);
+}
+
+bool ElfReader::IsElf32File() const {
+  return IsElfFile<Elf32>(fd_, path_);
+}
+
+bool ElfReader::IsElf64File() const {
+  return IsElfFile<Elf64>(fd_, path_);
+}
+
+/*
+void ElfReader::AddSymbols(SymbolMap *symbols,
+                           uint64 mem_offset, uint64 file_offset,
+                           uint64 length) {
+  if (fd_ < 0)
+    return;
+  // TODO(chatham): Actually use the information about file offset and
+  // the length of the mapped section. On some machines the data
+  // section gets mapped as executable, and we'll end up reading the
+  // file twice and getting some of the offsets wrong.
+  if (IsElf32File()) {
+    GetImpl32()->GetSymbolPositions(symbols, SHT_SYMTAB,
+                                    mem_offset, file_offset);
+    GetImpl32()->GetSymbolPositions(symbols, SHT_DYNSYM,
+                                    mem_offset, file_offset);
+  } else if (IsElf64File()) {
+    GetImpl64()->GetSymbolPositions(symbols, SHT_SYMTAB,
+                                    mem_offset, file_offset);
+    GetImpl64()->GetSymbolPositions(symbols, SHT_DYNSYM,
+                                    mem_offset, file_offset);
+  }
+}
+*/
+
+void ElfReader::VisitSymbols(ElfReader::SymbolSink *sink) {
+  VisitSymbols(sink, -1, -1);
+}
+
+void ElfReader::VisitSymbols(ElfReader::SymbolSink *sink,
+                             int symbol_binding,
+                             int symbol_type) {
+  VisitSymbols(sink, symbol_binding, symbol_type, false);
+}
+
+void ElfReader::VisitSymbols(ElfReader::SymbolSink *sink,
+                             int symbol_binding,
+                             int symbol_type,
+                             bool get_raw_symbol_values) {
+  if (IsElf32File()) {
+    GetImpl32()->VisitRelocationEntries();
+    GetImpl32()->VisitSymbols(SHT_SYMTAB, sink, symbol_binding, symbol_type,
+                              get_raw_symbol_values);
+    GetImpl32()->VisitSymbols(SHT_DYNSYM, sink, symbol_binding, symbol_type,
+                              get_raw_symbol_values);
+  } else if (IsElf64File()) {
+    GetImpl64()->VisitRelocationEntries();
+    GetImpl64()->VisitSymbols(SHT_SYMTAB, sink, symbol_binding, symbol_type,
+                              get_raw_symbol_values);
+    GetImpl64()->VisitSymbols(SHT_DYNSYM, sink, symbol_binding, symbol_type,
+                              get_raw_symbol_values);
+  }
+}
+
+uint64 ElfReader::VaddrOfFirstLoadSegment() {
+  if (IsElf32File()) {
+    return GetImpl32()->VaddrOfFirstLoadSegment();
+  } else if (IsElf64File()) {
+    return GetImpl64()->VaddrOfFirstLoadSegment();
+  } else {
+    return 0;
+  }
+}
+
+const char *ElfReader::GetSectionName(int shndx) {
+  if (shndx < 0 || static_cast<unsigned int>(shndx) >= GetNumSections()) return NULL;
+  if (IsElf32File()) {
+    return GetImpl32()->GetSectionNameByIndex(shndx);
+  } else if (IsElf64File()) {
+    return GetImpl64()->GetSectionNameByIndex(shndx);
+  } else {
+    return NULL;
+  }
+}
+
+uint64 ElfReader::GetNumSections() {
+  if (IsElf32File()) {
+    return GetImpl32()->GetNumSections();
+  } else if (IsElf64File()) {
+    return GetImpl64()->GetNumSections();
+  } else {
+    return 0;
+  }
+}
+
+const char *ElfReader::GetSectionByIndex(int shndx, size_t *size) {
+  if (IsElf32File()) {
+    return GetImpl32()->GetSectionContentsByIndex(shndx, size);
+  } else if (IsElf64File()) {
+    return GetImpl64()->GetSectionContentsByIndex(shndx, size);
+  } else {
+    return NULL;
+  }
+}
+
+const char *ElfReader::GetSectionByName(const string &section_name,
+                                        size_t *size) {
+  if (IsElf32File()) {
+    return GetImpl32()->GetSectionContentsByName(section_name, size);
+  } else if (IsElf64File()) {
+    return GetImpl64()->GetSectionContentsByName(section_name, size);
+  } else {
+    return NULL;
+  }
+}
+
+const char *ElfReader::GetSectionInfoByName(const string &section_name,
+                                            SectionInfo *info) {
+  if (IsElf32File()) {
+    return GetImpl32()->GetSectionInfoByName(section_name, info);
+  } else if (IsElf64File()) {
+    return GetImpl64()->GetSectionInfoByName(section_name, info);
+  } else {
+    return NULL;
+  }
+}
+
+bool ElfReader::SectionNamesMatch(const string &name, const string &sh_name) {
+  if ((name.find(".debug_", 0) == 0) && (sh_name.find(".zdebug_", 0) == 0)) {
+    const string name_suffix(name, strlen(".debug_"));
+    const string sh_name_suffix(sh_name, strlen(".zdebug_"));
+    return name_suffix == sh_name_suffix;
+  }
+  return name == sh_name;
+}
+
+bool ElfReader::IsDynamicSharedObject() {
+  if (IsElf32File()) {
+    return GetImpl32()->IsDynamicSharedObject();
+  } else if (IsElf64File()) {
+    return GetImpl64()->IsDynamicSharedObject();
+  } else {
+    return false;
+  }
+}
+
+ElfReaderImpl<Elf32> *ElfReader::GetImpl32() {
+  if (impl32_ == NULL) {
+    impl32_ = new ElfReaderImpl<Elf32>(path_, fd_);
+  }
+  return impl32_;
+}
+
+ElfReaderImpl<Elf64> *ElfReader::GetImpl64() {
+  if (impl64_ == NULL) {
+    impl64_ = new ElfReaderImpl<Elf64>(path_, fd_);
+  }
+  return impl64_;
+}
+
+// Return true if file is an ELF binary of ElfArch, with unstripped
+// debug info (debug_only=true) or symbol table (debug_only=false).
+// Otherwise, return false.
+template <typename ElfArch>
+static bool IsNonStrippedELFBinaryImpl(const string &path, const int fd,
+                                       bool debug_only) {
+  if (!ElfReaderImpl<ElfArch>::IsArchElfFile(fd, NULL)) return false;
+  ElfReaderImpl<ElfArch> elf_reader(path, fd);
+  return debug_only ?
+      elf_reader.HasDebugSections()
+      : (elf_reader.GetSectionByType(SHT_SYMTAB) != NULL);
+}
+
+// Helper for the IsNon[Debug]StrippedELFBinary functions.
+static bool IsNonStrippedELFBinaryHelper(const string &path,
+                                         bool debug_only) {
+  const int fd = open(path.c_str(), O_RDONLY);
+  if (fd == -1) {
+    return false;
+  }
+
+  if (IsNonStrippedELFBinaryImpl<Elf32>(path, fd, debug_only) ||
+      IsNonStrippedELFBinaryImpl<Elf64>(path, fd, debug_only)) {
+    close(fd);
+    return true;
+  }
+  close(fd);
+  return false;
+}
+
+bool ElfReader::IsNonStrippedELFBinary(const string &path) {
+  return IsNonStrippedELFBinaryHelper(path, false);
+}
+
+bool ElfReader::IsNonDebugStrippedELFBinary(const string &path) {
+  return IsNonStrippedELFBinaryHelper(path, true);
+}
+}  // namespace dwarf2reader