ElfLoader.h

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef ElfLoader_h
#define ElfLoader_h

#include <vector>
#include <dlfcn.h>
#include <signal.h>
#include "mozilla/Atomics.h"
#include "mozilla/RefCounted.h"
#include "mozilla/RefPtr.h"
#include "mozilla/UniquePtr.h"
#include "Elfxx.h"
#include "Mappable.h"

/**
 * dlfcn.h replacement functions
 */
extern "C" {
void* __wrap_dlopen(const char* path, int flags);
const char* __wrap_dlerror(void);
void* __wrap_dlsym(void* handle, const char* symbol);
int __wrap_dlclose(void* handle);

#ifndef HAVE_DLADDR
typedef struct {
  const char* dli_fname;
  void* dli_fbase;
  const char* dli_sname;
  void* dli_saddr;
} Dl_info;
#endif
int __wrap_dladdr(const void* addr, Dl_info* info);

typedef int (*dl_phdr_cb)(struct dl_phdr_info*, size_t, void*);
int __wrap_dl_iterate_phdr(dl_phdr_cb callback, void* data);

#ifdef __ARM_EABI__
const void* __wrap___gnu_Unwind_Find_exidx(void* pc, int* pcount);
#endif
}

/* Forward declarations for use in LibHandle */
class BaseElf;
class CustomElf;
class SystemElf;

/**
 * Specialize RefCounted template for LibHandle. We may get references to
 * LibHandles during the execution of their destructor, so we need
 * RefCounted<LibHandle>::Release to support some reentrancy. See further
 * below.
 */
class LibHandle;

namespace mozilla {
namespace detail {

template <>
inline void RefCounted<LibHandle, AtomicRefCount>::Release() const;

#ifdef DEBUG
template <>
inline RefCounted<LibHandle, AtomicRefCount>::~RefCounted() {
  MOZ_ASSERT(mRefCnt == 0x7fffdead);
}
#endif

} /* namespace detail */
} /* namespace mozilla */

/**
 * Abstract class for loaded libraries. Libraries may be loaded through the
 * system linker or this linker, both cases will be derived from this class.
 */
class LibHandle : public mozilla::external::AtomicRefCounted<LibHandle> {
 public:
  MOZ_DECLARE_REFCOUNTED_TYPENAME(LibHandle)
  /**
   * Constructor. Takes the path of the loaded library and will store a copy
   * of the leaf name.
   */
  LibHandle(const char* path)
      : directRefCnt(0),
        path(path ? strdup(path) : nullptr),
        mappable(nullptr) {}

  /**
   * Destructor.
   */
  virtual ~LibHandle();

  /**
   * Returns the pointer to the address to which the given symbol resolves
   * inside the library. It is not supposed to resolve the symbol in other
   * libraries, although in practice, it will for system libraries.
   */
  virtual void* GetSymbolPtr(const char* symbol) const = 0;

  /**
   * Returns whether the given address is part of the virtual address space
   * covered by the loaded library.
   */
  virtual bool Contains(void* addr) const = 0;

  /**
   * Returns the base address of the loaded library.
   */
  virtual void* GetBase() const = 0;

  /**
   * Returns the file name of the library without the containing directory.
   */
  const char* GetName() const;

  /**
   * Returns the full path of the library, when available. Otherwise, returns
   * the file name.
   */
  const char* GetPath() const { return path; }

  /**
   * Library handles can be referenced from other library handles or
   * externally (when dlopen()ing using this linker). We need to be
   * able to distinguish between the two kind of referencing for better
   * bookkeeping.
   */
  void AddDirectRef() {
    mozilla::external::AtomicRefCounted<LibHandle>::AddRef();
    ++directRefCnt;
  }

  /**
   * Releases a direct reference, and returns whether there are any direct
   * references left.
   */
  bool ReleaseDirectRef() {
    const MozRefCountType count = --directRefCnt;
    MOZ_ASSERT(count + 1 > 0);
    MOZ_ASSERT(count + 1 <=
               mozilla::external::AtomicRefCounted<LibHandle>::refCount());
    mozilla::external::AtomicRefCounted<LibHandle>::Release();
    return !!count;
  }

  /**
   * Returns the number of direct references
   */
  MozRefCountType DirectRefCount() { return directRefCnt; }

#ifdef __ARM_EABI__
  /**
   * Find the address and entry count of the ARM.exidx section
   * associated with the library
   */
  virtual const void* FindExidx(int* pcount) const = 0;
#endif

 protected:
  /**
   * Returns the instance, casted as the wanted type. Returns nullptr if
   * that's not the actual type. (short of a better way to do this without
   * RTTI)
   */
  friend class ElfLoader;
  friend class CustomElf;
  friend class SEGVHandler;
  friend int __wrap_dl_iterate_phdr(dl_phdr_cb callback, void* data);
  virtual BaseElf* AsBaseElf() { return nullptr; }
  virtual SystemElf* AsSystemElf() { return nullptr; }

 private:
  mozilla::Atomic<MozRefCountType> directRefCnt;
  char* path;

  /* Mappable object keeping the result of GetMappable() */
  mutable RefPtr<Mappable> mappable;
};

/**
 * Specialized RefCounted<LibHandle>::Release. Under normal operation, when
 * mRefCnt reaches 0, the LibHandle is deleted. Its mRefCnt is however
 * increased to 1 on normal builds, and 0x7fffdead on debug builds so that the
 * LibHandle can still be referenced while the destructor is executing. The
 * mRefCnt is allowed to grow > 0x7fffdead, but not to decrease under that
 * value, which would mean too many Releases from within the destructor.
 */
namespace mozilla {
namespace detail {

template <>
inline void RefCounted<LibHandle, AtomicRefCount>::Release() const {
#ifdef DEBUG
  if (mRefCnt > 0x7fff0000) MOZ_ASSERT(mRefCnt > 0x7fffdead);
#endif
  MOZ_ASSERT(mRefCnt > 0);
  if (mRefCnt > 0) {
    if (0 == --mRefCnt) {
#ifdef DEBUG
      mRefCnt = 0x7fffdead;
#else
      ++mRefCnt;
#endif
      delete static_cast<const LibHandle*>(this);
    }
  }
}

} /* namespace detail */
} /* namespace mozilla */

/**
 * Class handling libraries loaded by the system linker
 */
class SystemElf : public LibHandle {
 public:
  /**
   * Returns a new SystemElf for the given path. The given flags are passed
   * to dlopen().
   */
  static already_AddRefed<LibHandle> Load(const char* path, int flags);

  /**
   * Inherited from LibHandle
   */
  virtual ~SystemElf();
  virtual void* GetSymbolPtr(const char* symbol) const;
  virtual bool Contains(void* addr) const { return false; /* UNIMPLEMENTED */ }
  virtual void* GetBase() const { return nullptr; /* UNIMPLEMENTED */ }

#ifdef __ARM_EABI__
  virtual const void* FindExidx(int* pcount) const;
#endif

 protected:
  /**
   * Returns the instance, casted as SystemElf. (short of a better way to do
   * this without RTTI)
   */
  friend class ElfLoader;
  virtual SystemElf* AsSystemElf() { return this; }

  /**
   * Remove the reference to the system linker handle. This avoids dlclose()
   * being called when the instance is destroyed.
   */
  void Forget() { dlhandle = nullptr; }

 private:
  /**
   * Private constructor
   */
  SystemElf(const char* path, void* handle)
      : LibHandle(path), dlhandle(handle) {}

  /* Handle as returned by system dlopen() */
  void* dlhandle;
};

/**
 * The ElfLoader registers its own SIGSEGV handler to handle segmentation
 * faults within the address space of the loaded libraries. It however
 * allows a handler to be set for faults in other places, and redispatches
 * to the handler set through signal() or sigaction().
 */
class SEGVHandler {
 public:
  bool hasRegisteredHandler() {
    if (!initialized) FinishInitialization();
    return registeredHandler;
  }

  static int __wrap_sigaction(int signum, const struct sigaction* act,
                              struct sigaction* oldact);

 protected:
  SEGVHandler();
  ~SEGVHandler();

 private:
  /**
   * The constructor doesn't do all initialization, and the tail is done
   * at a later time.
   */
  void FinishInitialization();

  /**
   * SIGSEGV handler registered with __wrap_signal or __wrap_sigaction.
   */
  struct sigaction action;

  /**
   * ElfLoader SIGSEGV handler.
   */
  static void handler(int signum, siginfo_t* info, void* context);

  /**
   * Temporary test handler.
   */
  static void test_handler(int signum, siginfo_t* info, void* context);

  /**
   * Size of the alternative stack. The printf family requires more than 8KB
   * of stack, and our signal handler may print a few things.
   */
  static const size_t stackSize = 12 * 1024;

  /**
   * Alternative stack information used before initialization.
   */
  stack_t oldStack;

  /**
   * Pointer to an alternative stack for signals. Only set if oldStack is
   * not set or not big enough.
   */
  MappedPtr stackPtr;

  bool initialized;
  bool registeredHandler;
  bool signalHandlingSlow;
};

/**
 * Elf Loader class in charge of loading and bookkeeping libraries.
 */
class ElfLoader : public SEGVHandler {
 public:
  /**
   * The Elf Loader instance
   */
  static ElfLoader Singleton;

  /**
   * Loads the given library with the given flags. Equivalent to dlopen()
   * The extra "parent" argument optionally gives the handle of the library
   * requesting the given library to be loaded. The loader may look in the
   * directory containing that parent library for the library to load.
   */
  already_AddRefed<LibHandle> Load(const char* path, int flags,
                                   LibHandle* parent = nullptr);

  /**
   * Returns the handle of the library containing the given address in
   * its virtual address space, i.e. the library handle for which
   * LibHandle::Contains returns true. Its purpose is to allow to
   * implement dladdr().
   */
  already_AddRefed<LibHandle> GetHandleByPtr(void* addr);

  /**
   * Returns a Mappable object for the path. Paths in the form
   *   /foo/bar/baz/archive!/directory/lib.so
   * try to load the directory/lib.so in /foo/bar/baz/archive, provided
   * that file is a Zip archive.
   */
  static Mappable* GetMappableFromPath(const char* path);

  void ExpectShutdown(bool val) { expect_shutdown = val; }
  bool IsShutdownExpected() { return expect_shutdown; }

 private:
  bool expect_shutdown;

 protected:
  /**
   * Registers the given handle. This method is meant to be called by
   * LibHandle subclass creators.
   */
  void Register(LibHandle* handle);
  void Register(CustomElf* handle);

  /**
   * Forget about the given handle. This method is meant to be called by
   * LibHandle subclass destructors.
   */
  void Forget(LibHandle* handle);
  void Forget(CustomElf* handle);

  friend class SystemElf;
  friend const char* __wrap_dlerror(void);
  friend void* __wrap_dlsym(void* handle, const char* symbol);
  friend int __wrap_dlclose(void* handle);
  /* __wrap_dlerror() returns this custom last error if non-null or the system
   * dlerror() value if this is null. Must refer to a string constant. */
  mozilla::Atomic<const char*, mozilla::Relaxed> lastError;

 private:
  ElfLoader() : expect_shutdown(true), lastError(nullptr) {
    pthread_mutex_init(&handlesMutex, nullptr);
  }

  ~ElfLoader();

  /* Initialization code that can't run during static initialization. */
  void Init();

  /* System loader handle for the library/program containing our code. This
   * is used to resolve wrapped functions. */
  RefPtr<LibHandle> self_elf;

  /* Bookkeeping */
  typedef std::vector<LibHandle*> LibHandleList;
  LibHandleList handles;

  pthread_mutex_t handlesMutex;

 protected:
  friend class CustomElf;
  friend class LoadedElf;

  /* Definition of static destructors as to be used for C++ ABI compatibility */
  typedef void (*Destructor)(void* object);

  /**
   * C++ ABI makes static initializers register destructors through a specific
   * atexit interface. On glibc/linux systems, the dso_handle is a pointer
   * within a given library. On bionic/android systems, it is an undefined
   * symbol. Making sense of the value is not really important, and all that
   * is really important is that it is different for each loaded library, so
   * that they can be discriminated when shutting down. For convenience, on
   * systems where the dso handle is a symbol, that symbol is resolved to
   * point at corresponding CustomElf.
   *
   * Destructors are registered with __*_atexit with an associated object to
   * be passed as argument when it is called.
   *
   * When __cxa_finalize is called, destructors registered for the given
   * DSO handle are called in the reverse order they were registered.
   */
#ifdef __ARM_EABI__
  static int __wrap_aeabi_atexit(void* that, Destructor destructor,
                                 void* dso_handle);
#else
  static int __wrap_cxa_atexit(Destructor destructor, void* that,
                               void* dso_handle);
#endif

  static void __wrap_cxa_finalize(void* dso_handle);

  /**
   * Registered destructor. Keeps track of the destructor function pointer,
   * associated object to call it with, and DSO handle.
   */
  class DestructorCaller {
   public:
    DestructorCaller(Destructor destructor, void* object, void* dso_handle)
        : destructor(destructor), object(object), dso_handle(dso_handle) {}

    /**
     * Call the destructor function with the associated object.
     * Call only once, see CustomElf::~CustomElf.
     */
    void Call();

    /**
     * Returns whether the destructor is associated to the given DSO handle
     */
    bool IsForHandle(void* handle) const { return handle == dso_handle; }

   private:
    Destructor destructor;
    void* object;
    void* dso_handle;
  };

 private:
  /* Keep track of all registered destructors */
  std::vector<DestructorCaller> destructors;

  /* Forward declaration, see further below */
  class DebuggerHelper;

 public:
  /* Loaded object descriptor for the debugger interface below*/
  struct link_map {
    /* Base address of the loaded object. */
    const void* l_addr;
    /* File name */
    const char* l_name;
    /* Address of the PT_DYNAMIC segment. */
    const void* l_ld;

   private:
    friend class ElfLoader::DebuggerHelper;
    /* Double linked list of loaded objects. */
    link_map *l_next, *l_prev;
  };

 private:
  /* Data structure used by the linker to give details about shared objects it
   * loaded to debuggers. This is normally defined in link.h, but Android
   * headers lack this file. */
  struct r_debug {
    /* Version number of the protocol. */
    int r_version;

    /* Head of the linked list of loaded objects. */
    link_map* r_map;

    /* Function to be called when updates to the linked list of loaded objects
     * are going to occur. The function is to be called before and after
     * changes. */
    void (*r_brk)(void);

    /* Indicates to the debugger what state the linked list of loaded objects
     * is in when the function above is called. */
    enum {
      RT_CONSISTENT, /* Changes are complete */
      RT_ADD,        /* Beginning to add a new object */
      RT_DELETE      /* Beginning to remove an object */
    } r_state;
  };

  /* Memory representation of ELF Auxiliary Vectors */
  struct AuxVector {
    Elf::Addr type;
    Elf::Addr value;
  };

  /* Helper class used to integrate libraries loaded by this linker in
   * r_debug */
  class DebuggerHelper {
   public:
    DebuggerHelper();

    void Init(AuxVector* auvx);

    explicit operator bool() { return dbg; }

    /* Make the debugger aware of a new loaded object */
    void Add(link_map* map);

    /* Make the debugger aware of the unloading of an object */
    void Remove(link_map* map);

    /* Iterates over all link_maps */
    class iterator {
     public:
      const link_map* operator->() const { return item; }

      const link_map& operator++() {
        item = item->l_next;
        return *item;
      }

      bool operator<(const iterator& other) const {
        if (other.item == nullptr) return item ? true : false;
        MOZ_CRASH(
            "DebuggerHelper::iterator::operator< called with something else "
            "than DebuggerHelper::end()");
      }

     protected:
      friend class DebuggerHelper;
      explicit iterator(const link_map* item) : item(item) {}

     private:
      const link_map* item;
    };

    iterator begin() const { return iterator(dbg ? dbg->r_map : nullptr); }

    iterator end() const { return iterator(nullptr); }

   private:
    r_debug* dbg;
    link_map* firstAdded;
  };
  friend int __wrap_dl_iterate_phdr(dl_phdr_cb callback, void* data);
  DebuggerHelper dbg;
};

#endif /* ElfLoader_h */