shared_memory.h

// Copyright (c) 2012 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.

#ifndef BASE_MEMORY_SHARED_MEMORY_H_
#define BASE_MEMORY_SHARED_MEMORY_H_

#include <stddef.h>

#include <string>

#include "base/base_export.h"
#include "base/hash.h"
#include "base/macros.h"
#include "base/memory/shared_memory_handle.h"
#include "base/process/process_handle.h"
#include "base/strings/string16.h"
#include "build/build_config.h"

#if defined(OS_POSIX) || defined(OS_FUCHSIA)
#include <stdio.h>
#include <sys/types.h>
#include <semaphore.h>
#include "base/file_descriptor_posix.h"
#include "base/files/file_util.h"
#include "base/files/scoped_file.h"
#endif

#if defined(OS_WIN)
#include "base/win/scoped_handle.h"
#endif

namespace base {

class FilePath;

// Options for creating a shared memory object.
struct BASE_EXPORT SharedMemoryCreateOptions {
#if !defined(OS_FUCHSIA)
  // DEPRECATED (crbug.com/345734):
  // If NULL, the object is anonymous.  This pointer is owned by the caller
  // and must live through the call to Create().
  const std::string* name_deprecated = nullptr;

  // DEPRECATED (crbug.com/345734):
  // If true, and the shared memory already exists, Create() will open the
  // existing shared memory and ignore the size parameter.  If false,
  // shared memory must not exist.  This flag is meaningless unless
  // name_deprecated is non-NULL.
  bool open_existing_deprecated = false;
#endif

  // Size of the shared memory object to be created.
  // When opening an existing object, this has no effect.
  size_t size = 0;

  // If true, mappings might need to be made executable later.
  bool executable = false;

  // If true, the file can be shared read-only to a process.
  bool share_read_only = false;
};

// Platform abstraction for shared memory.
// SharedMemory consumes a SharedMemoryHandle [potentially one that it created]
// to map a shared memory OS resource into the virtual address space of the
// current process.
class BASE_EXPORT SharedMemory {
 public:
  SharedMemory();

#if defined(OS_WIN)
  // Similar to the default constructor, except that this allows for
  // calling LockDeprecated() to acquire the named mutex before either Create or
  // Open are called on Windows.
  explicit SharedMemory(const string16& name);
#endif

  // Create a new SharedMemory object from an existing, open
  // shared memory file.
  //
  // WARNING: This does not reduce the OS-level permissions on the handle; it
  // only affects how the SharedMemory will be mmapped. Use
  // GetReadOnlyHandle to drop permissions. TODO(jln,jyasskin): DCHECK
  // that |read_only| matches the permissions of the handle.
  SharedMemory(const SharedMemoryHandle& handle, bool read_only);

  // Closes any open files.
  ~SharedMemory();

  // Return true iff the given handle is valid (i.e. not the distingished
  // invalid value; NULL for a HANDLE and -1 for a file descriptor)
  static bool IsHandleValid(const SharedMemoryHandle& handle);

  // Closes a shared memory handle.
  static void CloseHandle(const SharedMemoryHandle& handle);

  // Returns the maximum number of handles that can be open at once per process.
  static size_t GetHandleLimit();

  // Duplicates The underlying OS primitive. Returns an invalid handle on
  // failure. The caller is responsible for destroying the duplicated OS
  // primitive.
  static SharedMemoryHandle DuplicateHandle(const SharedMemoryHandle& handle);

#if defined(OS_POSIX) && !(defined(OS_MACOSX) && !defined(OS_IOS))
  // This method requires that the SharedMemoryHandle is backed by a POSIX fd.
  static int GetFdFromSharedMemoryHandle(const SharedMemoryHandle& handle);
#endif

  // Creates a shared memory object as described by the options struct.
  // Returns true on success and false on failure.
  bool Create(const SharedMemoryCreateOptions& options);

  // Creates and maps an anonymous shared memory segment of size size.
  // Returns true on success and false on failure.
  bool CreateAndMapAnonymous(size_t size);

  // Creates an anonymous shared memory segment of size size.
  // Returns true on success and false on failure.
  bool CreateAnonymous(size_t size) {
    SharedMemoryCreateOptions options;
    options.size = size;
    return Create(options);
  }

#if (!defined(OS_MACOSX) || defined(OS_IOS)) && !defined(OS_FUCHSIA)
  // DEPRECATED (crbug.com/345734):
  // Creates or opens a shared memory segment based on a name.
  // If open_existing is true, and the shared memory already exists,
  // opens the existing shared memory and ignores the size parameter.
  // If open_existing is false, shared memory must not exist.
  // size is the size of the block to be created.
  // Returns true on success, false on failure.
  bool CreateNamedDeprecated(
      const std::string& name, bool open_existing, size_t size) {
    SharedMemoryCreateOptions options;
    options.name_deprecated = &name;
    options.open_existing_deprecated = open_existing;
    options.size = size;
    return Create(options);
  }

  // Deletes resources associated with a shared memory segment based on name.
  // Not all platforms require this call.
  bool Delete(const std::string& name);

  // Opens a shared memory segment based on a name.
  // If read_only is true, opens for read-only access.
  // Returns true on success, false on failure.
  bool Open(const std::string& name, bool read_only);
#endif  // !defined(OS_MACOSX) || defined(OS_IOS)

  // Maps the shared memory into the caller's address space.
  // Returns true on success, false otherwise.  The memory address
  // is accessed via the memory() accessor.  The mapped address is guaranteed to
  // have an alignment of at least MAP_MINIMUM_ALIGNMENT. This method will fail
  // if this object is currently mapped.
  bool Map(size_t bytes) {
    return MapAt(0, bytes);
  }

  // Same as above, but with |offset| to specify from begining of the shared
  // memory block to map.
  // |offset| must be alignent to value of |SysInfo::VMAllocationGranularity()|.
  bool MapAt(off_t offset, size_t bytes);
  enum { MAP_MINIMUM_ALIGNMENT = 32 };

  // Unmaps the shared memory from the caller's address space.
  // Returns true if successful; returns false on error or if the
  // memory is not mapped.
  bool Unmap();

  // The size requested when the map is first created.
  size_t requested_size() const { return requested_size_; }

  // The actual size of the mapped memory (may be larger than requested).
  size_t mapped_size() const { return mapped_size_; }

  // Gets a pointer to the opened memory space if it has been
  // Mapped via Map().  Returns NULL if it is not mapped.
  void* memory() const { return memory_; }

  // Returns the underlying OS handle for this segment.
  // Use of this handle for anything other than an opaque
  // identifier is not portable.
  SharedMemoryHandle handle() const;

  // Returns the underlying OS handle for this segment. The caller takes
  // ownership of the handle and memory is unmapped. This is equivalent to
  // duplicating the handle and then calling Unmap() and Close() on this object,
  // without the overhead of duplicating the handle.
  SharedMemoryHandle TakeHandle();

  // Closes the open shared memory segment. The memory will remain mapped if
  // it was previously mapped.
  // It is safe to call Close repeatedly.
  void Close();

  // Returns a read-only handle to this shared memory region. The caller takes
  // ownership of the handle. For POSIX handles, CHECK-fails if the region
  // wasn't Created or Opened with share_read_only=true, which is required to
  // make the handle read-only. When the handle is passed to the IPC subsystem,
  // that takes ownership of the handle. As such, it's not valid to pass the
  // sample handle to the IPC subsystem twice. Returns an invalid handle on
  // failure.
  SharedMemoryHandle GetReadOnlyHandle() const;

  // Returns an ID for the mapped region. This is ID of the SharedMemoryHandle
  // that was mapped. The ID is valid even after the SharedMemoryHandle is
  // Closed, as long as the region is not unmapped.
  const UnguessableToken& mapped_id() const { return mapped_id_; }

 private:
#if defined(OS_POSIX) && !defined(OS_NACL) && !defined(OS_ANDROID) && \
    (!defined(OS_MACOSX) || defined(OS_IOS))
  bool FilePathForMemoryName(const std::string& mem_name, FilePath* path);
#endif

#if defined(OS_WIN)
  // If true indicates this came from an external source so needs extra checks
  // before being mapped.
  bool external_section_ = false;
  string16 name_;
#elif !defined(OS_ANDROID) && !defined(OS_FUCHSIA)
  // If valid, points to the same memory region as shm_, but with readonly
  // permissions.
  SharedMemoryHandle readonly_shm_;
#endif

  // The OS primitive that backs the shared memory region.
  SharedMemoryHandle shm_;

  size_t mapped_size_ = 0;
  void* memory_ = nullptr;
  bool read_only_ = false;
  size_t requested_size_ = 0;
  base::UnguessableToken mapped_id_;

  DISALLOW_COPY_AND_ASSIGN(SharedMemory);
};

}  // namespace base

#endif  // BASE_MEMORY_SHARED_MEMORY_H_