src/lib/elflib/elflib.h

// Copyright 2018 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef SRC_LIB_ELFLIB_ELFLIB_H_
#define SRC_LIB_ELFLIB_ELFLIB_H_

#include <stdio.h>

#include <map>
#include <memory>
#include <optional>
#include <string>
#include <utility>
#include <vector>

#include <fbl/macros.h>

#include "llvm/BinaryFormat/ELF.h"

namespace elflib {

using namespace llvm::ELF;

class ElfLib {
 public:
  friend class PltEntryBufferX86;
  friend class PltEntryBufferArm;
  class MemoryAccessor;

  // Essentially just a pointer with a bound.
  struct MemoryRegion {
    const uint8_t* ptr;
    size_t size;
  };

  // How do we expect the ELF structures to be mapped? Are they packed in a
  // file or mapped as they would be in a running process?
  enum class AddressMode { kFile, kProcess };

  // Whether we should take ownership of the FILE handle given to our Create
  // method.
  enum class Ownership { kTakeOwnership, kDontTakeOwnership };

  virtual ~ElfLib();

  // Attach a second ElfLib to this one which contains debug info. This second
  // object will be treated as authoritative on section headers.
  //
  // Returns true on success. Returns false if either this or the given debug
  // data already have debug data associated, or if this has sections
  // associated already.
  bool SetDebugData(std::unique_ptr<ElfLib> debug);

  // Get the contents of a section by its name. Return nullptr if there is no
  // section by that name.
  MemoryRegion GetSectionData(const std::string& name);

  // Get a list of all segement headers.
  const std::vector<Elf64_Phdr>& GetSegmentHeaders();

  // Get the contents of a segment by its index. Return nullptr if the index is
  // invalid.
  MemoryRegion GetSegmentData(size_t segment);

  // Get a note from the notes section.
  std::optional<std::vector<uint8_t>> GetNote(const std::string& name, uint64_t type);

  // Get the NT_GNU_BUILD_ID note as a hex string. Return empty string if we
  // don't have that note.
  std::string GetGNUBuildID();

  // Get a symbol from the symbol table. Return nullptr if there is no such
  // symbol. Pointer should live as long as the memory accessor.
  const Elf64_Sym* GetSymbol(const std::string& name);

  // Get a map of all symbols and their string names. Returns nullopt if the
  // symbols could not be loaded.
  std::optional<std::map<std::string, Elf64_Sym>> GetAllSymbols();

  // Get a symbol from the symbol table. Return nullptr if there is no such
  // symbol. Pointer should live as long as the memory accessor.
  const Elf64_Sym* GetDynamicSymbol(const std::string& name);

  // Get a map of all symbols and their string names. Returns nullopt if the
  // symbols could not be loaded.
  std::optional<std::map<std::string, Elf64_Sym>> GetAllDynamicSymbols();

  // Attempt to discern whether this file has debug symbols (otherwise it is
  // presumably stripped).
  //
  // There are different types of debug information and a file could contain an
  // arbitrary subset of it. This function specifically probes for a
  // ".debug_info" section which contains the main DWARF symbol information. But
  // a file could lack this but still contain certain names or unwind
  // information. If you need to tell if a file has this other information, a
  // different probe function should be added for the specific thing you need.
  bool ProbeHasDebugInfo();

  // Attempt to discern whether this file has the actual program contents. It
  // may not if it is a split debug info file.
  bool ProbeHasProgramBits();

  // Create a new ElfLib object for reading a file. If take_ownership is set to
  // true, the given handle will be closed when the ElfLib object is destroyed.
  static std::unique_ptr<ElfLib> Create(FILE* fp, Ownership owned);

  // Create a new ElfLib object for reading a file. ElfLib will attempt to open
  // the file and retain a handle to it until the object is destroyed.
  static std::unique_ptr<ElfLib> Create(const std::string& path);

  // Create a new ElfLib object for accessing an ELF file mapped into memory.
  // This is expected to be a file, not an address space, and will be addressed
  // accordingly.
  static std::unique_ptr<ElfLib> Create(const uint8_t* mem, size_t size);

  // Create an ElfLib object for reading ELF structures via a read callback.
  // The offsets will assume either an ELF file or an ELF mapped address space
  // depending on the value of the address_mode argument.
  static std::unique_ptr<ElfLib> Create(std::function<bool(uint64_t, std::vector<uint8_t>*)> fetch,
                                        AddressMode address_mode = AddressMode::kProcess);

  // Returns a map from symbol names to the locations of their PLT entries.
  // Returns an empty map if the data is inaccessible.
  //
  // Getting this information is architecture-specific and involves reading and
  // decoding the actual jump table instructions in the .plt section. Once
  // we've done that decoding we can quickly get relocation indices and then
  // symbol table mappings.
  std::map<std::string, uint64_t> GetPLTOffsets();

  // ElfLib may notice inconsistencies as it parses the ELF file or address
  // space, but may be able to continue. In such cases it will log a warning
  // message internally. This method will retrieve those messages and clear
  // them from the internal list.
  std::vector<std::string> GetAndClearWarnings() {
    auto ret = std::move(warnings_);
    warnings_.clear();
    return ret;
  }

 private:
  explicit ElfLib(std::unique_ptr<MemoryAccessor>&& memory, AddressMode address_mode);

  // Add a warning to this instance. See GetAndClearWarnings.
  void Warn(const std::string&& m) { warnings_.push_back(m); }

  // Location of a section specified by data gleaned from the dynamic segment.
  struct DynamicSection {
    std::optional<uint64_t> offset;
    std::optional<size_t> size;

    bool IsValid() { return offset && size; }
  };

  // Create a new ElfLib object.
  static std::unique_ptr<ElfLib> Create(std::unique_ptr<MemoryAccessor>&& memory,
                                        AddressMode address_mode);

  // See the definition of this class in elflib.cc for details.
  class PltEntryBuffer;

  std::map<std::string, uint64_t> GetPLTOffsetsCommon(PltEntryBuffer& adapter);

  // Get the header for a section by its index. Return nullptr if the index is
  // invalid.
  const Elf64_Shdr* GetSectionHeader(size_t section);

  // Load the program header table into the cache in segments_. Return true
  // unless a read error occurred.
  bool LoadProgramHeaders();

  // Load the section name-to-index mappings and cache them in section_names_.
  bool LoadSectionNames();

  // Get the contents of a section by its index. Return nullptr if the index is
  // invalid.
  MemoryRegion GetSectionData(size_t section);

  const AddressMode address_mode_;

  // Get the contents of the symbol table. Return nullptr if it is not present
  // or we do not have the means to locate it. Size is number of structs, not
  // number of bytes.
  std::pair<const Elf64_Sym*, size_t> GetSymtab();

  // Get the contents of the dynamic symbol table. Return nullptr if it is not
  // present or we do not have the means to locate it. Size is number of
  // structs, not number of bytes.
  std::pair<const Elf64_Sym*, size_t> GetDynamicSymtab();

  // Get a string from the .strtab section. Return nullptr if the offset is
  // invalid.
  std::optional<std::string> GetString(size_t offset);

  // Get a string from the .dynstr section. Return nullptr if the offset is
  // invalid.
  std::optional<std::string> GetDynamicString(size_t offset);

  // Load symbols from the dynamic segment of the target. We only do this when
  // the section data isn't available and we can't use the regular .symtab
  // information. Returns true unless an error occurred.
  bool LoadDynamicSymbols();

  bool did_load_dynamic_symbols_ = false;

  std::unique_ptr<MemoryAccessor> memory_;
  Elf64_Ehdr header_;
  std::optional<bool> dynamic_plt_use_rela_;

  DynamicSection dynsym_;
  DynamicSection dynstr_;
  std::vector<Elf64_Shdr> sections_;
  std::vector<Elf64_Phdr> segments_;
  std::map<std::string, size_t> section_names_;
  std::unique_ptr<ElfLib> debug_;

  std::vector<std::string> warnings_;

  DISALLOW_COPY_ASSIGN_AND_MOVE(ElfLib);
};

}  // namespace elflib

#endif  // SRC_LIB_ELFLIB_ELFLIB_H_