4.3 binder 驱动文件操作

上文已经提到，所有的操作定义在 binder_fops 结构体中，下面讲述这些操作。

设备的打开 - binder_open() 函数

用户空间在打开 /dev/binder 设备时，驱动会出发 binder_open() 函数的响应。

static int binder_open(struct inode *nodp, struct file *filp)
{
  struct binder_proc *proc;

  // 分配 binder_proc 数据结构内存
  proc = kzalloc(sizeof(*proc), GFP_KERNEL);
  if (proc == NULL)
    return -ENOMEM;
  
  // 增加当前线程/进程的引用计数并赋值给 tsk
  get_task_struct(current);
  proc->tsk = current;
  // 初始化队列
  INIT_LIST_HEAD(&proc->todo);
  init_waitqueue_head(&proc->wait);
  proc->default_priority = task_nice(current);

  binder_lock(__func__);

  // 增加 BINDER_STAT_PROC 的对象计数
  binder_stats_created(BINDER_STAT_PROC);
  // 添加 proc_node 到 binder_procs 全局列表中，这样任何进程就可以访问到其他进程的 binder_proc 对象了
  hlist_add_head(&proc->proc_node, &binder_procs);
  // 保存进程 id
  proc->pid = current->group_leader->pid;
  INIT_LIST_HEAD(&proc->delivered_death);
  // 驱动文件 private_data 指向 proc
  filp->private_data = proc;

  binder_unlock(__func__);
  
  return 0;
}

驱动文件释放 - binder_release() 函数

在用户空间关闭驱动设备文件时，会调用 binder_release() 函数，清理 binder_proc 对象，释放占用的内存。

static int binder_release(struct inode *nodp, struct file *filp)
{
  struct binder_proc *proc = filp->private_data;
  binder_defer_work(proc, BINDER_DEFERRED_RELEASE);

  return 0;
}

static void
binder_defer_work(struct binder_proc *proc, enum binder_deferred_state defer)
{
  mutex_lock(&binder_deferred_lock);
  proc->deferred_work |= defer;
  if (hlist_unhashed(&proc->deferred_work_node)) {
    // 添加到释放队列中
    hlist_add_head(&proc->deferred_work_node,
        &binder_deferred_list);
    queue_work(binder_deferred_workqueue, &binder_deferred_work);
  }
  mutex_unlock(&binder_deferred_lock);
}

内存映射 - binder_mmap() 函数

binder_mmap() 函数把设备内存映射到用户进程地址空间中，这样就可以像操作用户内存那样操作设备内存。

static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
{
  int ret;
  struct vm_struct *area;
  // 获得 binder_proc 对象
  struct binder_proc *proc = filp->private_data;
  const char *failure_string;
  struct binder_buffer *buffer;

  // 最多只分配 4M 的内存
  if ((vma->vm_end - vma->vm_start) > SZ_4M)
    vma->vm_end = vma->vm_start + SZ_4M;

  // 检查 flags
  if (vma->vm_flags & FORBIDDEN_MMAP_FLAGS) {
    ret = -EPERM;
    failure_string = "bad vm_flags";
    goto err_bad_arg;
  }
  vma->vm_flags = (vma->vm_flags | VM_DONTCOPY) & ~VM_MAYWRITE;

  mutex_lock(&binder_mmap_lock);
  // 检查是否已经映射
  if (proc->buffer) {
    ret = -EBUSY;
    failure_string = "already mapped";
    goto err_already_mapped;
  }

  // 申请内核虚拟内存空间
  area = get_vm_area(vma->vm_end - vma->vm_start, VM_IOREMAP);
  if (area == NULL) {
    ret = -ENOMEM;
    failure_string = "get_vm_area";
    goto err_get_vm_area_failed;
  }
  // 将申请到的内存地址保存到 binder_proc 对象中
  proc->buffer = area->addr;
  proc->user_buffer_offset = vma->vm_start - (uintptr_t)proc->buffer;
  mutex_unlock(&binder_mmap_lock);

  // 根据请求到的内存空间大小，分配给 binder_proc 对象的 pages， 用于保存指向物理页的指针
  proc->pages = kzalloc(sizeof(proc->pages[0]) * ((vma->vm_end - vma->vm_start) / PAGE_SIZE), GFP_KERNEL);
  if (proc->pages == NULL) {
    ret = -ENOMEM;
    failure_string = "alloc page array";
    goto err_alloc_pages_failed;
  }
  proc->buffer_size = vma->vm_end - vma->vm_start;

  vma->vm_ops = &binder_vm_ops;
  vma->vm_private_data = proc;

  // 分配一个页的物理内存
  if (binder_update_page_range(proc, 1, proc->buffer, proc->buffer + PAGE_SIZE, vma)) {
    ret = -ENOMEM;
    failure_string = "alloc small buf";
    goto err_alloc_small_buf_failed;
  }
  // 内存提供给 binder_buffer
  buffer = proc->buffer;
  // 初始化 proc->buffers 链表
  INIT_LIST_HEAD(&proc->buffers);
  // 将 binder_buffer 对象放入到 proc->buffers 链表中
  list_add(&buffer->entry, &proc->buffers);
  buffer->free = 1;
  binder_insert_free_buffer(proc, buffer);
  proc->free_async_space = proc->buffer_size / 2;
  barrier();
  proc->files = get_files_struct(proc->tsk);
  proc->vma = vma;
  proc->vma_vm_mm = vma->vm_mm;

  return 0;
}

驱动命令接口 - binder_ioctl() 函数

用户态程序调用 ioctl 系统函数向 /dev/binder 设备发送数据时，会触发 binder_ioctl() 函数响应。

上文数据结构中已经提到了 binder_ioctl 可以处理的 命令

// 核心命令，数据的读写
#define BINDER_WRITE_READ     _IOWR('b', 1, struct binder_write_read)
// 设置最大线程数
#define BINDER_SET_MAX_THREADS    _IOW('b', 5, size_t)
// 设置 context manager
#define BINDER_SET_CONTEXT_MGR    _IOW('b', 7, int)
// 线程退出命令
#define BINDER_THREAD_EXIT    _IOW('b', 8, int)
// binder 驱动的版本
#define BINDER_VERSION      _IOWR('b', 9, struct binder_version)

static long binder_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
{
  int ret;
  struct binder_proc *proc = filp->private_data;
  struct binder_thread *thread;
  unsigned int size = _IOC_SIZE(cmd);
  void __user *ubuf = (void __user *)arg;

  // 检查是否有错误
  ret = wait_event_interruptible(binder_user_error_wait, binder_stop_on_user_error < 2);
  if (ret)
    goto err_unlocked;

  binder_lock(__func__);
  // 获取 binder_thread 对象
  thread = binder_get_thread(proc);
  if (thread == NULL) {
    ret = -ENOMEM;
    goto err;
  }

  switch (cmd) {
  case BINDER_WRITE_READ: {
    struct binder_write_read bwr;
    if (size != sizeof(struct binder_write_read)) {
      ret = -EINVAL;
      goto err;
    }
    // 从用户空间拷贝 binder_write_read 到 binder 驱动，储存在 bwr
    if (copy_from_user(&bwr, ubuf, sizeof(bwr))) {
      ret = -EFAULT;
      goto err;
    }

    if (bwr.write_size > 0) {
      // 执行写入操作
      ret = binder_thread_write(proc, thread, (void __user *)bwr.write_buffer, bwr.write_size, &bwr.write_consumed);
      if (ret < 0) {
        bwr.read_consumed = 0;
        if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
          ret = -EFAULT;
        goto err;
      }
    }
    if (bwr.read_size > 0) {
      // 执行读取操作
      ret = binder_thread_read(proc, thread, (void __user *)bwr.read_buffer, bwr.read_size, &bwr.read_consumed, filp->f_flags & O_NONBLOCK);
      if (!list_empty(&proc->todo))
        wake_up_interruptible(&proc->wait);
      if (ret < 0) {
        if (copy_to_user(ubuf, &bwr, sizeof(bwr)))
          ret = -EFAULT;
        goto err;
      }
    }
    // 操作完成后将数据返回给用户空间
    if (copy_to_user(ubuf, &bwr, sizeof(bwr))) {
      ret = -EFAULT;
      goto err;
    }
    break;
  }
  case BINDER_SET_MAX_THREADS:
    // 设置最大线程，从用户空间拷贝数据到 proc->max_threads
    if (copy_from_user(&proc->max_threads, ubuf, sizeof(proc->max_threads))) {
      ret = -EINVAL;
      goto err;
    }
    break;
  case BINDER_SET_CONTEXT_MGR:
    // 检查是否已经设置
    if (binder_context_mgr_node != NULL) {
      ret = -EBUSY;
      goto err;
    }
    // 设置 context manager
    ret = security_binder_set_context_mgr(proc->tsk);
    if (ret < 0)
      goto err;
    if (binder_context_mgr_uid != -1) {
      if (binder_context_mgr_uid != current->cred->euid) {
        ret = -EPERM;
        goto err;
      }
    } else
      binder_context_mgr_uid = current->cred->euid;
    // 创建 binder_context_mgr_node 节点
    binder_context_mgr_node = binder_new_node(proc, NULL, NULL);
    if (binder_context_mgr_node == NULL) {
      ret = -ENOMEM;
      goto err;
    }
    // 初始化节点数据
    binder_context_mgr_node->local_weak_refs++;
    binder_context_mgr_node->local_strong_refs++;
    binder_context_mgr_node->has_strong_ref = 1;
    binder_context_mgr_node->has_weak_ref = 1;
    break;
  case BINDER_THREAD_EXIT:
    // 线程退出，释放资源
    binder_free_thread(proc, thread);
    thread = NULL;
    break;
  case BINDER_VERSION:
    // 将 binder 驱动版本号写入到用户空间 ubuf->protocol_version 中
    if (put_user(BINDER_CURRENT_PROTOCOL_VERSION, &((struct binder_version *)ubuf)->protocol_version)) {
      ret = -EINVAL;
      goto err;
    }
    break;
  default:
    ret = -EINVAL;
    goto err;
  }
  ret = 0;
...
}

static struct binder_node *binder_new_node(struct binder_proc *proc,
             void __user *ptr,
             void __user *cookie)
{
  struct rb_node **p = &proc->nodes.rb_node;
  struct rb_node *parent = NULL;
  struct binder_node *node;

  // 查找要插入节点的父节点
  while (*p) {
    parent = *p;
    node = rb_entry(parent, struct binder_node, rb_node);

    if (ptr < node->ptr)
      p = &(*p)->rb_left;
    else if (ptr > node->ptr)
      p = &(*p)->rb_right;
    else
      return NULL;
  }

  // 为要插入节点分配内存空间
  node = kzalloc(sizeof(*node), GFP_KERNEL);
  if (node == NULL)
    return NULL;
  binder_stats_created(BINDER_STAT_NODE);
  // 插入节点
  rb_link_node(&node->rb_node, parent, p);
  rb_insert_color(&node->rb_node, &proc->nodes);
  // 初始化
  node->debug_id = ++binder_last_id;
  node->proc = proc;
  node->ptr = ptr;
  node->cookie = cookie;
  node->work.type = BINDER_WORK_NODE;
  INIT_LIST_HEAD(&node->work.entry);
  INIT_LIST_HEAD(&node->async_todo);
  return node;
}

BINDER_WRITE_READ 处理过程

在 binder 本地层中，我们看到在 IPCThreadState::talkWithDriver() 函数中， binder 本地层通过 ioctl()(mProcess->mDriverFD, BINDER_WRITE_READ, &bwr) 命令的形式，与 binder 驱动交互。

可以看出 ioctl() 的第三个参数是一个 binder_write_read 结构体

binder.h 头文件中定义了两个数据类型，一个是 binder_write_read

struct binder_write_read {
  signed long write_size; /* bytes to write */
  signed long write_consumed; /* bytes consumed by driver */
  unsigned long   write_buffer;
  signed long read_size;  /* bytes to read */
  signed long read_consumed;  /* bytes consumed by driver */
  unsigned long   read_buffer;
};

其中 write_size 和 read_size 表示需要被读写的字节数， write_consumed 和 read_consumed 表示已经被 binder 驱动读写的字节数， write_buffer 和 read_buffer 则是指向被读写数据的指针。

具体的读写操作被 binder_thread_write 和 binder_thread_read 实现。

数据写入 - binder_thread_write() 函数

将用户空间数据写入到 binder 驱动，从驱动角度来看是读取的操作。

int binder_thread_write(struct binder_proc *proc, struct binder_thread *thread,
      void __user *buffer, int size, signed long *consumed)
{
  uint32_t cmd;
  // 用户空间数据，起始地址和结束地址
  void __user *ptr = buffer + *consumed;
  void __user *end = buffer + size;

  // 循环读取
  while (ptr < end && thread->return_error == BR_OK) {
    // 从用户空间获取操作命令
    if (get_user(cmd, (uint32_t __user *)ptr))
      return -EFAULT;
    ptr += sizeof(uint32_t);
    if (_IOC_NR(cmd) < ARRAY_SIZE(binder_stats.bc)) {
      // 增加命令计数器
      binder_stats.bc[_IOC_NR(cmd)]++;
      proc->stats.bc[_IOC_NR(cmd)]++;
      thread->stats.bc[_IOC_NR(cmd)]++;
    }
    switch (cmd) {
    // 这四个命令用来增加或减少对象的引用计数， 操作目标 binder_ref
    case BC_INCREFS:
    case BC_ACQUIRE:
    case BC_RELEASE:
    case BC_DECREFS: {
      uint32_t target;
      struct binder_ref *ref;
      const char *debug_string;
      
      // 获取目标进程节点描述 desc
      if (get_user(target, (uint32_t __user *)ptr))
        return -EFAULT;
      ptr += sizeof(uint32_t);
      // 索描述为 0 表示 context manager 进程
      if (target == 0 && binder_context_mgr_node &&
        (cmd == BC_INCREFS || cmd == BC_ACQUIRE)) {
        // 在 proc->refs_by_node.rb_node 红黑树中查找引用
        ref = binder_get_ref_for_node(proc,
               binder_context_mgr_node);
      } else
        // 在 proc->refs_by_desc.rb_node 红黑树中查找引用
        ref = binder_get_ref(proc, target);
      switch (cmd) {
      case BC_INCREFS:
        debug_string = "IncRefs";
        // 增加弱引用计数
        binder_inc_ref(ref, 0, NULL);
        break;
      case BC_ACQUIRE:
        debug_string = "Acquire";
        // 增加强引用计数
        binder_inc_ref(ref, 1, NULL);
        break;
      case BC_RELEASE:
        debug_string = "Release";
        // 减少强引用计数
        binder_dec_ref(ref, 1);
        break;
      case BC_DECREFS:
      default:
        debug_string = "DecRefs";
        // 减少弱引用计数
        binder_dec_ref(ref, 0);
        break;
      }
      break;
    }
    case BC_INCREFS_DONE:
    case BC_ACQUIRE_DONE: {
      void __user *node_ptr;
      void *cookie;
      struct binder_node *node;
      
      // 从用户空间读取 node_ptr
      if (get_user(node_ptr, (void * __user *)ptr))
        return -EFAULT;
      ptr += sizeof(void *);
      // 从用户空间读取 cookie
      if (get_user(cookie, (void * __user *)ptr))
        return -EFAULT;
      ptr += sizeof(void *);
      // 获得节点
      node = binder_get_node(proc, node_ptr);
      // 没有找到则返回
      if (node == NULL) {
        binder_user_error("binder: %d:%d "
          "%s u%p no match\n",
          proc->pid, thread->pid,
          cmd == BC_INCREFS_DONE ?
          "BC_INCREFS_DONE" :
          "BC_ACQUIRE_DONE",
          node_ptr);
        break;
      }
      // cookie 不匹配则返回
      if (cookie != node->cookie) {
        binder_user_error("binder: %d:%d %s u%p node %d"
          " cookie mismatch %p != %p\n",
          proc->pid, thread->pid,
          cmd == BC_INCREFS_DONE ?
          "BC_INCREFS_DONE" : "BC_ACQUIRE_DONE",
          node_ptr, node->debug_id,
          cookie, node->cookie);
        break;
      }
      
      if (cmd == BC_ACQUIRE_DONE) {
        node->pending_strong_ref = 0;
      } else {
        node->pending_weak_ref = 0;
      }
      // 减少节点使用计数
      binder_dec_node(node, cmd == BC_ACQUIRE_DONE, 0);
      break;
    }

    // 释放 binder_bffer
    case BC_FREE_BUFFER: {
      void __user *data_ptr;
      struct binder_buffer *buffer;

      // 从用户空间获取 data_ptr
      if (get_user(data_ptr, (void * __user *)ptr))
        return -EFAULT;
      ptr += sizeof(void *);

      // 查找 binder_buffer
      buffer = binder_buffer_lookup(proc, data_ptr);
      // 没有找到则返回
      if (buffer == NULL) {
        binder_user_error("binder: %d:%d "
          "BC_FREE_BUFFER u%p no match\n",
          proc->pid, thread->pid, data_ptr);
        break;
      }
      // 不允许用户释放则返回
      if (!buffer->allow_user_free) {
        binder_user_error("binder: %d:%d "
          "BC_FREE_BUFFER u%p matched "
          "unreturned buffer\n",
          proc->pid, thread->pid, data_ptr);
        break;
      }

      // 将 buffer->transaction 置空
      if (buffer->transaction) {
        buffer->transaction->buffer = NULL;
        buffer->transaction = NULL;
      }
      if (buffer->async_transaction && buffer->target_node) {
        if (list_empty(&buffer->target_node->async_todo))
          buffer->target_node->has_async_transaction = 0;
        else
          list_move_tail(buffer->target_node->async_todo.next, &thread->todo);
      }
      // 释放 binder_buffer 对象
      trace_binder_transaction_buffer_release(buffer);
      binder_transaction_buffer_release(proc, buffer, NULL);
      binder_free_buf(proc, buffer);
      break;
    }

    // binder 数据传递处理
    case BC_TRANSACTION:
    case BC_REPLY: {
      struct binder_transaction_data tr;

      // 从用户空间拷贝 binder_transaction_data 对象
      if (copy_from_user(&tr, ptr, sizeof(tr)))
        return -EFAULT;
      ptr += sizeof(tr);
      // 实际的传输函数，在下文讲解
      binder_transaction(proc, thread, &tr, cmd == BC_REPLY);
      break;
    }

    // 设置 looper 为 BINDER_LOOPER_STATE_REGISTERED 状态
    case BC_REGISTER_LOOPER:
      if (thread->looper & BINDER_LOOPER_STATE_ENTERED) {
        thread->looper |= BINDER_LOOPER_STATE_INVALID;
        binder_user_error("binder: %d:%d ERROR:"
          " BC_REGISTER_LOOPER called "
          "after BC_ENTER_LOOPER\n",
          proc->pid, thread->pid);
      } else if (proc->requested_threads == 0) {
        thread->looper |= BINDER_LOOPER_STATE_INVALID;
        binder_user_error("binder: %d:%d ERROR:"
          " BC_REGISTER_LOOPER called "
          "without request\n",
          proc->pid, thread->pid);
      } else {
        proc->requested_threads--;
        proc->requested_threads_started++;
      }
      thread->looper |= BINDER_LOOPER_STATE_REGISTERED;
      break;
    // 设置 looper 为 BINDER_LOOPER_STATE_ENTERED 状态
    case BC_ENTER_LOOPER:
      if (thread->looper & BINDER_LOOPER_STATE_REGISTERED) {
        thread->looper |= BINDER_LOOPER_STATE_INVALID;
        binder_user_error("binder: %d:%d ERROR:"
          " BC_ENTER_LOOPER called after "
          "BC_REGISTER_LOOPER\n",
          proc->pid, thread->pid);
      }
      thread->looper |= BINDER_LOOPER_STATE_ENTERED;
      break;
    // 设置 looper 为 BINDER_LOOPER_STATE_EXITED 状态
    case BC_EXIT_LOOPER:
      thread->looper |= BINDER_LOOPER_STATE_EXITED;
      break;

    // 发送 REQUEST_DEATH 或 CLEAR_DEATH 通知
    case BC_REQUEST_DEATH_NOTIFICATION:
    case BC_CLEAR_DEATH_NOTIFICATION: {
      uint32_t target;
      void __user *cookie;
      struct binder_ref *ref;
      struct binder_ref_death *death;
      
      // 从用户空间获取 binder_ref 描述 desc
      if (get_user(target, (uint32_t __user *)ptr))
        return -EFAULT;
      ptr += sizeof(uint32_t);
      // 从用户空间获取 cookie
      if (get_user(cookie, (void __user * __user *)ptr))
        return -EFAULT;
      ptr += sizeof(void *);
      // 获取 binder_ref 引用
      ref = binder_get_ref(proc, target);
      if (ref == NULL) {
        binder_user_error("binder: %d:%d %s "
          "invalid ref %d\n",
          proc->pid, thread->pid,
          cmd == BC_REQUEST_DEATH_NOTIFICATION ?
          "BC_REQUEST_DEATH_NOTIFICATION" :
          "BC_CLEAR_DEATH_NOTIFICATION",
          target);
        break;
      }

      if (cmd == BC_REQUEST_DEATH_NOTIFICATION) {
        if (ref->death) {
          binder_user_error("binder: %d:%"
            "d BC_REQUEST_DEATH_NOTI"
            "FICATION death notific"
            "ation already set\n",
            proc->pid, thread->pid);
          break;
        }
        // 为 binder_ref_death 对象分配内存空间
        death = kzalloc(sizeof(*death), GFP_KERNEL);
        if (death == NULL) {
          thread->return_error = BR_ERROR;
          break;
        }
        // 初始化 binder_ref_death 对象
        binder_stats_created(BINDER_STAT_DEATH);
        INIT_LIST_HEAD(&death->work.entry);
        death->cookie = cookie;
        ref->death = death;
        if (ref->node->proc == NULL) {
          ref->death->work.type = BINDER_WORK_DEAD_BINDER;
          if (thread->looper & (BINDER_LOOPER_STATE_REGISTERED | BINDER_LOOPER_STATE_ENTERED)) {
            list_add_tail(&ref->death->work.entry, &thread->todo);
          } else {
            list_add_tail(&ref->death->work.entry, &proc->todo);
            // 唤醒目标进程
            wake_up_interruptible(&proc->wait);
          }
        }
      } else {
        if (ref->death == NULL) {
          binder_user_error("binder: %d:%"
            "d BC_CLEAR_DEATH_NOTIFI"
            "CATION death notificat"
            "ion not active\n",
            proc->pid, thread->pid);
          break;
        }
        death = ref->death;
        if (death->cookie != cookie) {
          binder_user_error("binder: %d:%"
            "d BC_CLEAR_DEATH_NOTIFI"
            "CATION death notificat"
            "ion cookie mismatch "
            "%p != %p\n",
            proc->pid, thread->pid,
            death->cookie, cookie);
          break;
        }
        // 将 ref->death 置空
        ref->death = NULL;
        if (list_empty(&death->work.entry)) {
          death->work.type = BINDER_WORK_CLEAR_DEATH_NOTIFICATION;
          if (thread->looper & (BINDER_LOOPER_STATE_REGISTERED | BINDER_LOOPER_STATE_ENTERED)) {
            list_add_tail(&death->work.entry, &thread->todo);
          } else {
            list_add_tail(&death->work.entry, &proc->todo);
            // 唤醒目标进程
            wake_up_interruptible(&proc->wait);
          }
        } else {
          BUG_ON(death->work.type != BINDER_WORK_DEAD_BINDER);
          death->work.type = BINDER_WORK_DEAD_BINDER_AND_CLEAR;
        }
      }
    } break;
    case BC_DEAD_BINDER_DONE: {
      struct binder_work *w;
      void __user *cookie;
      struct binder_ref_death *death = NULL;
      // 从用户空间获取 cookie
      if (get_user(cookie, (void __user * __user *)ptr))
        return -EFAULT;

      ptr += sizeof(void *);
      list_for_each_entry(w, &proc->delivered_death, entry) {
        struct binder_ref_death *tmp_death = container_of(w, struct binder_ref_death, work);
        if (tmp_death->cookie == cookie) {
          death = tmp_death;
          break;
        }
      }
      if (death == NULL) {
        binder_user_error("binder: %d:%d BC_DEAD"
          "_BINDER_DONE %p not found\n",
          proc->pid, thread->pid, cookie);
        break;
      }

      list_del_init(&death->work.entry);
      // 如果 death->work.t 为 BINDER_WORK_DEAD_BINDER_AND_CLEAR 则修改为 BINDER_WORK_CLEAR_DEATH_NOTIFICATION
      if (death->work.t == BINDER_WORK_DEAD_BINDER_AND_CLEAR ) {
        death->work.type = BINDER_WORK_CLEAR_DEATH_NOTIFICATION;
        if (thread->looper & (BINDER_LOOPER_STATE_REGISTERED | BINDER_LOOPER_STATE_ENTERED)) {
          list_add_tail(&death->work.entry, &thread->todo);
        } else {
          list_add_tail(&death->work.entry, &proc->todo);
          // 唤醒目标进程
          wake_up_interruptible(&proc->wait);
        }
      }
    } break;

    default:
      return -EINVAL;
    }
    *consumed = ptr - buffer;
  }
  return 0;
}

binder_transaction() 函数

在上文处理 BC_TRANSACTION 和 BC_REPLY 时，调用了 binder_transaction() 函数。我们继续追踪

static void binder_transaction(struct binder_proc *proc,
           struct binder_thread *thread,
           struct binder_transaction_data *tr, int reply)
{
  struct binder_transaction *t;
  struct binder_work *tcomplete;
  size_t *offp, *off_end;
  struct binder_proc *target_proc;
  struct binder_thread *target_thread = NULL;
  struct binder_node *target_node = NULL;
  struct list_head *target_list;
  wait_queue_head_t *target_wait;
  struct binder_transaction *in_reply_to = NULL;
  
  if (reply) {
    // BC_REPLY 处理流程
    // 得到 binder_transaction 对象
    in_reply_to = thread->transaction_stack;
    if (in_reply_to == NULL) {
      return_error = BR_FAILED_REPLY;
      goto err_empty_call_stack;
    }
    binder_set_nice(in_reply_to->saved_priority);
    thread->transaction_stack = in_reply_to->to_parent;
    // 获取目标线程
    target_thread = in_reply_to->from;
    target_proc = target_thread->proc;
  } else {
    // BC_TRANSACTION 处理流程
    // 查找目标节点
    if (tr->target.handle) {
      struct binder_ref *ref;
      // 获取 binder_ref 对象
      ref = binder_get_ref(proc, tr->target.handle);
      target_node = ref->node;
    } else {
      // 索引为 0 则返回 context manager
      target_node = binder_context_mgr_node;
    }
    // 得到目标进程
    target_proc = target_node->proc;
    if (!(tr->flags & TF_ONE_WAY) && thread->transaction_stack) {
      struct binder_transaction *tmp;
      tmp = thread->transaction_stack;
      while (tmp) {
        if (tmp->from && tmp->from->proc == target_proc)
          // 获得目标线程
          target_thread = tmp->from;
        tmp = tmp->from_parent;
      }
    }
  }
  // 设置要处理的目标进程或目标线程任务
  if (target_thread) {
    target_list = &target_thread->todo;
    target_wait = &target_thread->wait;
  } else {
    target_list = &target_proc->todo;
    target_wait = &target_proc->wait;
  }

  // 为 binder_transaction 对象分配内存空间
  t = kzalloc(sizeof(*t), GFP_KERNEL);
  binder_stats_created(BINDER_STAT_TRANSACTION);

  tcomplete = kzalloc(sizeof(*tcomplete), GFP_KERNEL);
  binder_stats_created(BINDER_STAT_TRANSACTION_COMPLETE);

  // 如果是同步传输（双向)，则将当前的 binder_thread 对象保存在 binder_transaction 对象的 from 中。
  if (!reply && !(tr->flags & TF_ONE_WAY))
    t->from = thread;
  else
    t->from = NULL;
  // 设置 binder_transaction 对象
  t->sender_euid = proc->tsk->cred->euid;
  t->to_proc = target_proc;
  t->to_thread = target_thread;
  t->code = tr->code;
  t->flags = tr->flags;
  t->priority = task_nice(current);

  // 为 binder_buffer 分配内存空间
  t->buffer = binder_alloc_buf(target_proc, tr->data_size,
    tr->offsets_size, !reply && (t->flags & TF_ONE_WAY));
  // 设置 binder_buffer
  t->buffer->allow_user_free = 0;
  t->buffer->debug_id = t->debug_id;
  t->buffer->transaction = t;
  t->buffer->target_node = target_node;
  if (target_node)
    binder_inc_node(target_node, 1, 0, NULL);

  offp = (size_t *)(t->buffer->data + ALIGN(tr->data_size, sizeof(void *)));

  // 从用户空间拷贝数据到 binder_buffer
  if (copy_from_user(t->buffer->data, tr->data.ptr.buffer, tr->data_size)) {
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
  }
  if (copy_from_user(offp, tr->data.ptr.offsets, tr->offsets_size)) {
    return_error = BR_FAILED_REPLY;
    goto err_copy_data_failed;
  }

  off_end = (void *)offp + tr->offsets_size;
  for (; offp < off_end; offp++) {
    struct flat_binder_object *fp;
    // 为 flat_binder_object 赋值
    fp = (struct flat_binder_object *)(t->buffer->data + *offp);
    // 转换 binder 类型，如果是 BINDER 则转换为 HANDLE， 如果是 HANDLE 则转为 BANDLE
    switch (fp->type) {
    case BINDER_TYPE_BINDER:
    case BINDER_TYPE_WEAK_BINDER: {
      struct binder_ref *ref;
      // 获取 binder_node 节点
      struct binder_node *node = binder_get_node(proc, fp->binder);
      if (node == NULL) {
        node = binder_new_node(proc, fp->binder, fp->cookie);
        if (node == NULL) {
          return_error = BR_FAILED_REPLY;
          goto err_binder_new_node_failed;
        }
        node->min_priority = fp->flags & FLAT_BINDER_FLAG_PRIORITY_MASK;
        node->accept_fds = !!(fp->flags & FLAT_BINDER_FLAG_ACCEPTS_FDS);
      }
      if (fp->cookie != node->cookie) {
        goto err_binder_get_ref_for_node_failed;
      }
      // 获取 binder_ref 对象
      ref = binder_get_ref_for_node(target_proc, node);
      // 转换类型
      if (fp->type == BINDER_TYPE_BINDER)
        fp->type = BINDER_TYPE_HANDLE;
      else
        fp->type = BINDER_TYPE_WEAK_HANDLE;
      fp->handle = ref->desc;
      binder_inc_ref(ref, fp->type == BINDER_TYPE_HANDLE,
             &thread->todo);
    } break;
    case BINDER_TYPE_HANDLE:
    case BINDER_TYPE_WEAK_HANDLE: {
      // 获取 binder_ref 对象
      struct binder_ref*ref = binder_get_ref(proc, fp->handle);
      // 转换类型
      if (ref->node->proc == target_proc) {
        if (fp->type == BINDER_TYPE_HANDLE)
          fp->type = BINDER_TYPE_BINDER;
        else
          fp->type = BINDER_TYPE_WEAK_BINDER;
        fp->binder = ref->node->ptr;
        fp->cookie = ref->node->cookie;
        binder_inc_node(ref->node, fp->type == BINDER_TYPE_BINDER, 0, NULL);
      } else {
        struct binder_ref *new_ref;
        new_ref = binder_get_ref_for_node(target_proc, ref->node);
        fp->handle = new_ref->desc;
        binder_inc_ref(new_ref, fp->type == BINDER_TYPE_HANDLE, NULL);
      }
    } break;

    // 文件类型
    case BINDER_TYPE_FD: {
      int target_fd;
      struct file *file;
      // 获得文件对象
      file = fget(fp->handle);
      // 分配一个新的文件描述符
      target_fd = task_get_unused_fd_flags(target_proc, O_CLOEXEC);
      task_fd_install(target_proc, target_fd, file);
      fp->handle = target_fd;
    } break;

    default:
      return_error = BR_FAILED_REPLY;
      goto err_bad_object_type;
    }
  }
  if (reply) {
    // BC_REPLY 处理流程, binder_transaction 中释放 binder_transaction 对象
    binder_pop_transaction(target_thread, in_reply_to);
  } else if (!(t->flags & TF_ONE_WAY)) {
    // 同步状态（双向) 需要设置回复
    t->need_reply = 1;
    t->from_parent = thread->transaction_stack;
    thread->transaction_stack = t;
  } else {
    // 异步传输不需要设置回复
    if (target_node->has_async_transaction) {
      target_list = &target_node->async_todo;
      target_wait = NULL;
    } else
      target_node->has_async_transaction = 1;
  }
  t->work.type = BINDER_WORK_TRANSACTION;
  list_add_tail(&t->work.entry, target_list);
  tcomplete->type = BINDER_WORK_TRANSACTION_COMPLETE;
  list_add_tail(&tcomplete->entry, &thread->todo);
  if (target_wait)
    // 唤醒目标线程
    wake_up_interruptible(target_wait);
  return;
}

数据读取 - binder_thread_read() 函数

用户空间从 binder 驱动读取数据，从驱动角度来看是写出的操作。

static int binder_thread_read(struct binder_proc *proc,
          struct binder_thread *thread,
          void  __user *buffer, int size,
          signed long *consumed, int non_block)
{
  void __user *ptr = buffer + *consumed;
  void __user *end = buffer + size;

  int ret = 0;
  int wait_for_proc_work;

  if (*consumed == 0) {
    // 第一次操作时向用户空间返回 BR_NOOP 命令
    if (put_user(BR_NOOP, (uint32_t __user *)ptr))
      return -EFAULT;
    ptr += sizeof(uint32_t);
  }

retry:
  // 获取将要处理的任务
  wait_for_proc_work = thread->transaction_stack == NULL &&
        list_empty(&thread->todo);
  if (wait_for_proc_work) {
    if (!(thread->looper & (BINDER_LOOPER_STATE_REGISTERED |
          BINDER_LOOPER_STATE_ENTERED))) {
      binder_user_error("binder: %d:%d ERROR: Thread waiting "
        "for process work before calling BC_REGISTER_"
        "LOOPER or BC_ENTER_LOOPER (state %x)\n",
        proc->pid, thread->pid, thread->looper);
      wait_event_interruptible(binder_user_error_wait,
             binder_stop_on_user_error < 2);
    }
    binder_set_nice(proc->default_priority);
    if (non_block) {
      // 非阻塞且没有数据则返回 EAGAIN
      if (!binder_has_proc_work(proc, thread))
        ret = -EAGAIN;
    } else
      // 阻塞则进入睡眠状态，等待可操作的任务
      ret = wait_event_freezable_exclusive(proc->wait, binder_has_proc_work(proc, thread));
  } else {
    if (non_block) {
      if (!binder_has_thread_work(thread))
        ret = -EAGAIN;
    } else
      ret = wait_event_freezable(thread->wait, binder_has_thread_work(thread));
  }

  binder_lock(__func__);

  if (wait_for_proc_work)
    proc->ready_threads--;
  thread->looper &= ~BINDER_LOOPER_STATE_WAITING;

  if (ret)
    return ret;

  while (1) {
    uint32_t cmd;
    struct binder_transaction_data tr;
    struct binder_work *w;
    struct binder_transaction *t = NULL;

    // 获取 binder_work 对象
    if (!list_empty(&thread->todo))
      w = list_first_entry(&thread->todo, struct binder_work, entry);
    else if (!list_empty(&proc->todo) && wait_for_proc_work)
      w = list_first_entry(&proc->todo, struct binder_work, entry);
    else {
      if (ptr - buffer == 4 && !(thread->looper & BINDER_LOOPER_STATE_NEED_RETURN)) /* no data added */
        goto retry;
      break;
    }

    if (end - ptr < sizeof(tr) + 4)
      break;

    switch (w->type) {
    case BINDER_WORK_TRANSACTION: {
      // 获取 binder_transaction 对象
      t = container_of(w, struct binder_transaction, work);
    } break;
    case BINDER_WORK_TRANSACTION_COMPLETE: {
      cmd = BR_TRANSACTION_COMPLETE;
      // 返回 BR_TRANSACTION_COMPLETE 命令
      if (put_user(cmd, (uint32_t __user *)ptr))
        return -EFAULT;
      ptr += sizeof(uint32_t);

      binder_stat_br(proc, thread, cmd);

      // 从 work 链表中删除并释放内存
      list_del(&w->entry);
      kfree(w);
      binder_stats_deleted(BINDER_STAT_TRANSACTION_COMPLETE);
    } break;
    case BINDER_WORK_NODE: {
      // 获得 binder_node 节点
      struct binder_node *node = container_of(w, struct binder_node, work);
      uint32_t cmd = BR_NOOP;
      const char *cmd_name;
      // 根据节点类型，增加/获取、减少/释放节点索引
      int strong = node->internal_strong_refs || node->local_strong_refs;
      int weak = !hlist_empty(&node->refs) || node->local_weak_refs || strong;
      // 构造 BR_* 命令
      if (weak && !node->has_weak_ref) {
        cmd = BR_INCREFS;
        cmd_name = "BR_INCREFS";
        node->has_weak_ref = 1;
        node->pending_weak_ref = 1;
        node->local_weak_refs++;
      } else if (strong && !node->has_strong_ref) {
        cmd = BR_ACQUIRE;
        cmd_name = "BR_ACQUIRE";
        node->has_strong_ref = 1;
        node->pending_strong_ref = 1;
        node->local_strong_refs++;
      } else if (!strong && node->has_strong_ref) {
        cmd = BR_RELEASE;
        cmd_name = "BR_RELEASE";
        node->has_strong_ref = 0;
      } else if (!weak && node->has_weak_ref) {
        cmd = BR_DECREFS;
        cmd_name = "BR_DECREFS";
        node->has_weak_ref = 0;
      }
      // 向用户空间返回命令
      if (cmd != BR_NOOP) {
        if (put_user(cmd, (uint32_t __user *)ptr))
          return -EFAULT;
        ptr += sizeof(uint32_t);
        if (put_user(node->ptr, (void * __user *)ptr))
          return -EFAULT;
        ptr += sizeof(void *);
        if (put_user(node->cookie, (void * __user *)ptr))
          return -EFAULT;
        ptr += sizeof(void *);

        binder_stat_br(proc, thread, cmd);
      } else {
        list_del_init(&w->entry);
        if (!weak && !strong) {
          rb_erase(&node->rb_node, &proc->nodes);
          kfree(node);
          binder_stats_deleted(BINDER_STAT_NODE);
        }
      }
    } break;
    case BINDER_WORK_DEAD_BINDER:
    case BINDER_WORK_DEAD_BINDER_AND_CLEAR:
    case BINDER_WORK_CLEAR_DEATH_NOTIFICATION: {
      struct binder_ref_death *death;
      uint32_t cmd;

      // 获取 binder_ref_death 对象
      death = container_of(w, struct binder_ref_death, work);
      // 构造返回命令
      if (w->type == BINDER_WORK_CLEAR_DEATH_NOTIFICATION)
        cmd = BR_CLEAR_DEATH_NOTIFICATION_DONE;
      else
        cmd = BR_DEAD_BINDER;
      // 向用户空间返回命令
      if (put_user(cmd, (uint32_t __user *)ptr))
        return -EFAULT;
      ptr += sizeof(uint32_t);
      // 将 cookie 返回给用户空间
      if (put_user(death->cookie, (void * __user *)ptr))
        return -EFAULT;
      ptr += sizeof(void *);
      binder_stat_br(proc, thread, cmd);

      if (w->type == BINDER_WORK_CLEAR_DEATH_NOTIFICATION) {
        list_del(&w->entry);
        kfree(death);
        binder_stats_deleted(BINDER_STAT_DEATH);
      } else
        list_move(&w->entry, &proc->delivered_death);
      if (cmd == BR_DEAD_BINDER)
        goto done; /* DEAD_BINDER notifications can cause transactions */
    } break;
    }

    if (!t)
      continue;

    if (t->buffer->target_node) {
      // 获得 binder_node 节点
      struct binder_node *target_node = t->buffer->target_node;
      // 将数据封装到 binder_transaction_data 对象
      tr.target.ptr = target_node->ptr;
      tr.cookie =  target_node->cookie;
      t->saved_priority = task_nice(current);
      if (t->priority < target_node->min_priority &&
        !(t->flags & TF_ONE_WAY))
        binder_set_nice(t->priority);
      else if (!(t->flags & TF_ONE_WAY) ||
         t->saved_priority > target_node->min_priority)
        binder_set_nice(target_node->min_priority);
      // 设置返回的命令类型
      cmd = BR_TRANSACTION;
    } else {
      tr.target.ptr = NULL;
      tr.cookie = NULL;
      cmd = BR_REPLY;
    }
    tr.code = t->code;
    tr.flags = t->flags;
    tr.sender_euid = t->sender_euid;

    if (t->from) {
      struct task_struct *sender = t->from->proc->tsk;
      tr.sender_pid = task_tgid_nr_ns(sender,
              current->nsproxy->pid_ns);
    } else {
      tr.sender_pid = 0;
    }

    tr.data_size = t->buffer->data_size;
    tr.offsets_size = t->buffer->offsets_size;
    tr.data.ptr.buffer = (void *)t->buffer->data +
          proc->user_buffer_offset;
    tr.data.ptr.offsets = tr.data.ptr.buffer +
          ALIGN(t->buffer->data_size,
            sizeof(void *));

    if (put_user(cmd, (uint32_t __user *)ptr))
      return -EFAULT;
    ptr += sizeof(uint32_t);
    // 拷贝 binder_transaction_data 对象到用户空间
    if (copy_to_user(ptr, &tr, sizeof(tr)))
      return -EFAULT;
    ptr += sizeof(tr);

    binder_stat_br(proc, thread, cmd);

    // 移除 binder_transaction 并释放空间
    list_del(&t->work.entry);
    t->buffer->allow_user_free = 1;
    // 如果是同步操作，则将 thread 对象保存在 binder_transaction 中，返回给发送方进程，否则释放 binder_transaction 对象
    if (cmd == BR_TRANSACTION && !(t->flags & TF_ONE_WAY)) {
      t->to_parent = thread->transaction_stack;
      t->to_thread = thread;
      thread->transaction_stack = t;
    } else {
      t->buffer->transaction = NULL;
      kfree(t);
      binder_stats_deleted(BINDER_STAT_TRANSACTION);
    }
    break;
  }
}

从上述代码可以看出 binder 驱动的具体实现，以及是如何发送和接收数据的。