mozglue/linker/ElfLoader.h
author Jim Blandy <jimb@mozilla.com>
Tue, 24 Jun 2014 22:12:07 -0700
changeset 199513 69d61e42d5dfbf4588b72449249ff3e7f2125304
parent 188626 4cb70de5fd6aea678aa5985a1eb92d4af1eab153
child 196386 ec230387fad2651d9efa2e434cd4a0b27232ac05
child 202427 91d7659bb35145e10ff737753688e0972c1a79b5
permissions -rw-r--r--
Bug 914753: Make Emacs file variable header lines correct, or at least consistent. DONTBUILD r=ehsan The -*- file variable lines -*- establish per-file settings that Emacs will pick up. This patch makes the following changes to those lines (and touches nothing else): - Never set the buffer's mode. Years ago, Emacs did not have a good JavaScript mode, so it made sense to use Java or C++ mode in .js files. However, Emacs has had js-mode for years now; it's perfectly serviceable, and is available and enabled by default in all major Emacs packagings. Selecting a mode in the -*- file variable line -*- is almost always the wrong thing to do anyway. It overrides Emacs's default choice, which is (now) reasonable; and even worse, it overrides settings the user might have made in their '.emacs' file for that file extension. It's only useful when there's something specific about that particular file that makes a particular mode appropriate. - Correctly propagate settings that establish the correct indentation level for this file: c-basic-offset and js2-basic-offset should be js-indent-level. Whatever value they're given should be preserved; different parts of our tree use different indentation styles. - We don't use tabs in Mozilla JS code. Always set indent-tabs-mode: nil. Remove tab-width: settings, at least in files that don't contain tab characters. - Remove js2-mode settings that belong in the user's .emacs file, like js2-skip-preprocessor-directives.

/* 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/RefPtr.h"
#include "Zip.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(void *addr, Dl_info *info);

  struct dl_phdr_info {
    Elf::Addr dlpi_addr;
    const char *dlpi_name;
    const Elf::Phdr *dlpi_phdr;
    Elf::Half dlpi_phnum;
  };

  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

/**
 * faulty.lib public API
 */
MFBT_API size_t
__dl_get_mappable_length(void *handle);

MFBT_API void *
__dl_mmap(void *handle, void *addr, size_t length, off_t offset);

MFBT_API void
__dl_munmap(void *handle, void *addr, size_t length);

MFBT_API bool
IsSignalHandlingBroken();

}

/**
 * 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;

template <> inline RefCounted<LibHandle, AtomicRefCount>::~RefCounted()
{
  MOZ_ASSERT(refCnt == 0x7fffdead);
}

} /* 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 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()
  {
    ++directRefCnt;
    mozilla::external::AtomicRefCounted<LibHandle>::AddRef();
  }

  /**
   * Releases a direct reference, and returns whether there are any direct
   * references left.
   */
  bool ReleaseDirectRef()
  {
    bool ret = false;
    if (directRefCnt) {
      MOZ_ASSERT(directRefCnt <=
                 mozilla::external::AtomicRefCounted<LibHandle>::refCount());
      if (--directRefCnt)
        ret = true;
      mozilla::external::AtomicRefCounted<LibHandle>::Release();
    }
    return ret;
  }

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

  /**
   * Returns the complete size of the file or stream behind the library
   * handle.
   */
  size_t GetMappableLength() const;

  /**
   * Returns a memory mapping of the file or stream behind the library
   * handle.
   */
  void *MappableMMap(void *addr, size_t length, off_t offset) const;

  /**
   * Unmaps a memory mapping of the file or stream behind the library
   * handle.
   */
  void MappableMUnmap(void *addr, size_t length) const;

#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 a mappable object for use by MappableMMap and related functions.
   */
  virtual Mappable *GetMappable() const = 0;

  /**
   * Returns whether the handle is a SystemElf or not. (short of a better way
   * to do this without RTTI)
   */
  friend class ElfLoader;
  friend class CustomElf;
  friend class SEGVHandler;
  virtual bool IsSystemElf() const { return false; }

private:
  MozRefCountType directRefCnt;
  char *path;

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

/**
 * Specialized RefCounted<LibHandle>::Release. Under normal operation, when
 * refCnt reaches 0, the LibHandle is deleted. Its refCnt 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 refCnt 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 (refCnt > 0x7fff0000)
    MOZ_ASSERT(refCnt > 0x7fffdead);
#endif
  MOZ_ASSERT(refCnt > 0);
  if (refCnt > 0) {
    if (0 == --refCnt) {
#ifdef DEBUG
      refCnt = 0x7fffdead;
#else
      refCnt = 1;
#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 mozilla::TemporaryRef<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 */ }

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

protected:
  virtual Mappable *GetMappable() const;

  /**
   * Returns whether the handle is a SystemElf or not. (short of a better way
   * to do this without RTTI)
   */
  friend class ElfLoader;
  virtual bool IsSystemElf() const { return true; }

  /**
   * 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() {
    return registeredHandler;
  }

  bool isSignalHandlingBroken() {
    return signalHandlingBroken;
  }

protected:
  SEGVHandler();
  ~SEGVHandler();

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

  /**
   * 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 registeredHandler;
  bool signalHandlingBroken;
  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.
   */
  mozilla::TemporaryRef<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().
   */
  mozilla::TemporaryRef<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);

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

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

  /* Last error. Used for dlerror() */
  friend class SystemElf;
  friend const char *__wrap_dlerror(void);
  friend void *__wrap_dlsym(void *handle, const char *symbol);
  friend int __wrap_dlclose(void *handle);
  const char *lastError;

private:
  ~ElfLoader();

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

protected:
  friend class CustomElf;
  /**
   * Show some stats about Mappables in CustomElfs. The when argument is to
   * be used by the caller to give an identifier of the when the stats call
   * is made.
   */
  static void stats(const char *when);

  /* 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;
  };

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

    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;
      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 */