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蓝牙源码学习笔记

本文主要是介绍蓝牙源码学习笔记,对大家解决编程问题具有一定的参考价值,需要的程序猿们随着小编来一起学习吧!

术语

在阅读源码的过程中发现许多函数名称带有意义不明的缩写,下面是笔者整理的一些缩写及其对应含义:

  • BTIF: Bluetooth Interface
  • BTU : Bluetooth Upper Layer
  • BTM: Bluetooth Manager
  • BTE: Bluetooth embedded system
  • BTA :Blueetooth application layer
  • CO: call out
  • CI: call in
  • HF : Handsfree Profile
  • HH: HID Host Profile
  • HL: Health Device Profile
  • av: audio/vidio
  • ag: audio gateway
  • ar: audio/video registration
  • gattc: GATT client

Android Bluetooth Stack

安卓中蓝牙协议栈主要分为三个时期,上古时期使用的是BlueZ,后来在4.2之后自己独立出来称为BlueDroid,现在好像又改名叫Fluoride了。BlueZ时期和PC上的结构差不多,但是安卓上不使用DBus IPC,因此需要将这部分代码去除,其他部分可参考BlueZ的介绍。

7.0

在Android<=7.0时期,蓝牙协议栈的实现架构如下:

https://www.www.zyiz.net/i/ll/?i=20210711125102119.pngbd70

8.0

Android 8.0 以后对蓝牙协议栈进行了重构,主要优化是使用HIDL来取代之前的硬件抽象层,方便厂商的接口集成:

https://www.www.zyiz.net/i/ll/?i=20210711125133710.pngbd80

实现分析

Android蓝牙协议栈的实现在system/bt目录中,本节记录下其代码分析的过程,使用的是 Android 10 分支(ae35d7765)。

首先,该目录下包含1000+文件,有点无从入手。一般遇到这种情况我们都是从具体的入口函数出发,比如main函数,但这里并不是一个单纯的客户端程序。蓝牙协议栈一方面是以系统服务的方式提供接口,另一方面也以client的方式给应用程序提供SDK,不管怎样,最终都是需要经过HCI协议去与Controller进行交互。

对于BlueZ而言,蓝牙协议栈部分在内核中实现,socket系统调用提供了AF_BLUETOOTH的 family,可以支持获取HCI、L2CAP、RFCOMM类型的socket;但对于BlueDroid而言,协议栈是在用户层实现的,内核只暴露出HCI(USB/UART)的接口。因此,我们可以从HCI出发,自底向上进行分析,也可以参考上面的框架图,从用户应用程序开始,自顶向下进行分析。

用户层

首先从用户接口出发,参考Android的开发者文档是如何发现设备以及创建蓝牙连接的:

  • https://developer.android.com/guide/topics/connectivity/bluetooth
  • https://developer.android.com/guide/topics/connectivity/bluetooth-le

以BR/EDR为例,其中需要注意的是paired和connected的区别:

  • paired 表示两个设备知道彼此的存在,并且已经协商好了链路秘钥(Link Key),可用该秘钥来进行认证和创建加密链接
  • connected 表示两个已经配对的设备创建了一个RFCOMM链接,共享一个RFCOMM channel

Android使用蓝牙接口的流程大致如下:

// 获取本地适配器 BluetoothAdapter bluetoothAdapter = BluetoothAdapter.getDefaultAdapter(); // 开启蓝牙,需要权限 if (!bluetoothAdapter.isEnabled()) {    Intent enableBtIntent = new Intent(BluetoothAdapter.ACTION_REQUEST_ENABLE);    startActivityForResult(enableBtIntent, REQUEST_ENABLE_BT); } // Discover、Pair、Connect 

以设置本机蓝牙可被发现(300秒)为例,应用层代码为:

Intent discoverableIntent =        new Intent(BluetoothAdapter.ACTION_REQUEST_DISCOVERABLE); discoverableIntent.putExtra(BluetoothAdapter.EXTRA_DISCOVERABLE_DURATION, 300); startActivity(discoverableIntent); 

Settings Activity (Binder Client)

根据intents-filters的介绍,我们知道这是由于其他App去处理的请求,即com.android.settings/.bluetooth.RequestPermissionActivity,该页面调用弹窗询问(startActivityForResult)用户是否允许本设备被发现,并且在回调(onActivityResult)中注册蓝牙的回调中调用:

mBluetoothAdapter.setScanMode(  BluetoothAdapter.SCAN_MODE_CONNECTABLE_DISCOVERABLE, mTimeout); 

frameworks/base/core/java/android/bluetooth/BluetoothAdapter.java

@UnsupportedAppUsage public boolean setScanMode(@ScanMode int mode, int duration) {    if (getState() != STATE_ON) {        return false;    }    try {        mServiceLock.readLock().lock();        if (mService != null) {            return mService.setScanMode(mode, duration);        }    } catch (RemoteException e) {        Log.e(TAG, "", e);    } finally {        mServiceLock.readLock().unlock();    }    return false; } 

题外话: 上面的annotation表示该接口不是SDK的一部分,在9.0之前APP是可以通过反射进行调用的,9.0之后安卓更新了限制方法,不过也有其他的绕过方式,见: https://stackoverflow.com/questions/55970137/bypass-androids-hidden-api-restrictions

其中mServiceIBluetooth类型,直指蓝牙服务system/bt/binder/android/bluetooth/IBluetooth.aidl,值得一提的是,该目录下还包含了数十个AIDL文件,用于描述进程所提供的服务。

AIDL Server

该AIDL的实现在packages/apps/Bluetooth/src/com/android/bluetooth/btservice/AdapterService.java,该Server的JNI实现在packages/apps/Bluetooth/jni/com_android_bluetooth_btservice_AdapterService.cpp,内部主要使用sBluetoothInterface接口来实现功能,该接口的定义为:

static const bt_interface_t* sBluetoothInterface 

该接口的实现在btif/src/bluetooth.cc中。

回到SetScanMode,其实现在system/bt/service/adapter.cc:

bool SetScanMode(int scan_mode) override {  switch (scan_mode) {    case BT_SCAN_MODE_NONE:    case BT_SCAN_MODE_CONNECTABLE:    case BT_SCAN_MODE_CONNECTABLE_DISCOVERABLE:      break;    default:      LOG(ERROR) << "Unknown scan mode: " << scan_mode;      return false;  }   auto bd_scanmode = static_cast<bt_scan_mode_t>(scan_mode);   if (!SetAdapterProperty(BT_PROPERTY_ADAPTER_SCAN_MODE, &bd_scanmode,                          sizeof(bd_scanmode))) {    LOG(ERROR) << "Failed to set scan mode to : " << scan_mode;    return false;  }   return true; } 

核心是SetAdapterProperty:

// Sends a request to set the given HAL adapter property type and value. bool SetAdapterProperty(bt_property_type_t type, void* value, int length) {  CHECK(length > 0);  CHECK(value);   bt_property_t property;  property.len = length;  property.val = value;  property.type = type;   int status =      hal::BluetoothInterface::Get()->GetHALInterface()->set_adapter_property(          &property);  if (status != BT_STATUS_SUCCESS) {    VLOG(1) << "Failed to set property";    return false;  }   return true; } 

其中GetHALInterface是蓝牙的核心接口bt_interface_t,定义在接口子系统中system/bt/btif/src/bluetooth.cc,随后依次调用:

  • set_adapter_property
  • btif_set_adapter_property (btif/src/btif_core.c)
  • BTA_DmSetVisibility (bta/dm/bta_dm_api.cc)
  /** This function sets the Bluetooth connectable, discoverable, pairable and   * conn paired only modes of local device   */   void BTA_DmSetVisibility(tBTA_DM_DISC disc_mode, tBTA_DM_CONN conn_mode,                           uint8_t pairable_mode, uint8_t conn_paired_only) {    do_in_main_thread(FROM_HERE,                      base::Bind(bta_dm_set_visibility, disc_mode, conn_mode,                                 pairable_mode, conn_paired_only));  } 

do_in_main_thread是将任务push到对应的线程任务池中执行,所执行的函数是bta_dm_set_visibility(bta/dm/bta_dm_act.cc),主要功能是根据参数的逻辑分别设置对应的属性:

  • BTM_SetDiscoverability
  • BTM_SetConnectability
  • BTM_SetPairableMode
stack/btm/btm_inq.cc

这其中涉及了几个API:

  • btm_ble_set_discoverability
  • btsnd_hcic_write_cur_iac_lap
  • btsnd_hcic_write_inqscan_cfg
  • btsnd_hcic_write_scan_enable

第一个API是BLE相关,内部实际上最终也调用了btsnd_hcic_xxx的类似接口。IAC意为Inquiry Access Code,蓝牙baseband定义了几个固定IAC,分别是LIAC和GIAC(见baseband)。LAP是蓝牙地址的一部分,如下图所示:

https://www.www.zyiz.net/i/ll/?i=20210711125609757.pngBDADDR

  • NAP: Non-significant Address Part, NAP的值在跳频同步帧中会用到
  • UAP: Upper Address Part,UAP的值会参与对蓝牙协议算法的选择
  • LAP: Lower Address Part,由设备厂商分配,LAP的值作为Access Code的一部分,唯一确定某个蓝牙设备
  • SAP (significant address part) = UAP + LAP

让我们继续回到代码中,以btsnd_hcic_write_cur_iac_lap为例,其实现如下:

// stack/hcic/hcicmds.cc void btsnd_hcic_write_cur_iac_lap(uint8_t num_cur_iac, LAP* const iac_lap) {    BT_HDR* p = (BT_HDR*)osi_malloc(HCI_CMD_BUF_SIZE);    uint8_t* pp = (uint8_t*)(p + 1);     p->len = HCIC_PREAMBLE_SIZE + 1 + (LAP_LEN * num_cur_iac);    p->offset = 0;     UINT16_TO_STREAM(pp, HCI_WRITE_CURRENT_IAC_LAP);    UINT8_TO_STREAM(pp, p->len - HCIC_PREAMBLE_SIZE);     UINT8_TO_STREAM(pp, num_cur_iac);     for (int i = 0; i < num_cur_iac; i++) LAP_TO_STREAM(pp, iac_lap[i]);     btu_hcif_send_cmd(LOCAL_BR_EDR_CONTROLLER_ID, p); } 

UINTx_TO_STREAM(pp, n)的作用是将整数以小端的形式写入p->data中,最终调用btu_hcif_send_cmd函数发送数据(stack/btu/btu_hcif.cc):

/******************************************************************************* * * Function         btu_hcif_send_cmd * * Description      This function is called to send commands to the Host *                  Controller. * * Returns          void * ******************************************************************************/ void btu_hcif_send_cmd(UNUSED_ATTR uint8_t controller_id, BT_HDR* p_buf) {    if (!p_buf) return;     uint16_t opcode;    uint8_t* stream = p_buf->data + p_buf->offset;    void* vsc_callback = NULL;     STREAM_TO_UINT16(opcode, stream);     // Eww...horrible hackery here    /* If command was a VSC, then extract command_complete callback */    if ((opcode & HCI_GRP_VENDOR_SPECIFIC) == HCI_GRP_VENDOR_SPECIFIC ||        (opcode == HCI_BLE_RAND) || (opcode == HCI_BLE_ENCRYPT)) {      vsc_callback = *((void**)(p_buf + 1));    }     // Skip parameter length before logging    stream++;    btu_hcif_log_command_metrics(opcode, stream,                                 android::bluetooth::hci::STATUS_UNKNOWN, false);     hci_layer_get_interface()->transmit_command(        p_buf, btu_hcif_command_complete_evt, btu_hcif_command_status_evt,        vsc_callback); } 

可见p_buf->data中保存的就是HCI数据,前16位为opcode,其中高6字节为ogf,低10字节为ocf,也就是我们平时使用hcitool cmd时的前两个参数。

HCI 子系统

继续跟踪transmit_command,就来到了HCI子系统中(hci/src/hci_layer.cc):

static void transmit_command(BT_HDR* command,                           command_complete_cb complete_callback,                           command_status_cb status_callback, void* context) {    waiting_command_t* wait_entry = reinterpret_cast<waiting_command_t*>(        osi_calloc(sizeof(waiting_command_t)));     uint8_t* stream = command->data + command->offset;    STREAM_TO_UINT16(wait_entry->opcode, stream);    wait_entry->complete_callback = complete_callback;    wait_entry->status_callback = status_callback;    wait_entry->command = command;    wait_entry->context = context;     // Store the command message type in the event field    // in case the upper layer didn't already    command->event = MSG_STACK_TO_HC_HCI_CMD;     enqueue_command(wait_entry); } 

enqueue_command如其名字所述,就是将待执行的HCI命令放到队列command_queue的末尾中。那么这个任务队列在哪消费呢?简单搜索可以发现:

// Event/packet receiving functions void process_command_credits(int credits) {    std::lock_guard<std::mutex> command_credits_lock(command_credits_mutex);     if (!hci_thread.IsRunning()) {      // HCI Layer was shut down or not running      return;    }     // Subtract commands in flight.    command_credits = credits - get_num_waiting_commands();     while (command_credits > 0 && !command_queue.empty()) {      if (!hci_thread.DoInThread(FROM_HERE, std::move(command_queue.front()))) {        LOG(ERROR) << __func__ << ": failed to enqueue command";      }      command_queue.pop();      command_credits--;    } } 

其调用链路为:

  • BluetoothHciCallbacks::hciEventReceived (hci/src/hci_layer_android.cc)
  • hci_event_received
  • filter_incoming_event
  • process_command_credits

接收数据

BluetoothHciCallbacks::hciEventReceived这个函数回调是在HCI初始化的时候调用的BluetoothHci::initialize(vendor_libs/linux/interface/bluetooth_hci.cc):

Return<void> BluetoothHci::initialize(  const ::android::sp<IBluetoothHciCallbacks>& cb) {    ALOGI("BluetoothHci::initialize()");    if (cb == nullptr) {      ALOGE("cb == nullptr! -> Unable to call initializationComplete(ERR)");      return Void();    }     death_recipient_->setHasDied(false);    cb->linkToDeath(death_recipient_, 0);    int hci_fd = openBtHci();    auto hidl_status = cb->initializationComplete(            hci_fd > 0 ? Status::SUCCESS : Status::INITIALIZATION_ERROR);    if (!hidl_status.isOk()) {        ALOGE("VendorInterface -> Unable to call initializationComplete(ERR)");    }    hci::H4Protocol* h4_hci = new hci::H4Protocol(        hci_fd,        [cb](const hidl_vec<uint8_t>& packet) { cb->hciEventReceived(packet); },        [cb](const hidl_vec<uint8_t>& packet) { cb->aclDataReceived(packet); },        [cb](const hidl_vec<uint8_t>& packet) { cb->scoDataReceived(packet); });     fd_watcher_.WatchFdForNonBlockingReads(            hci_fd, [h4_hci](int fd) { h4_hci->OnDataReady(fd); });    hci_handle_ = h4_hci;     unlink_cb_ = [cb](sp<BluetoothDeathRecipient>& death_recipient) {      if (death_recipient->getHasDied())        ALOGI("Skipping unlink call, service died.");      else        cb->unlinkToDeath(death_recipient);    };     return Void(); } 

fd_watcher_本质上是针对hci_fd文件句柄的读端事件监控,后者由openBtHci函数产生,该函数由厂商实现,接口文件是hardware/interfaces/bluetooth/1.0/IBluetoothHci.hal。在Linux中的参考实现如下:

// system/bt/vendor_libs/linux/interface/bluetooth_hci.cc int BluetoothHci::openBtHci() {    ALOGI( "%s", __func__);     int hci_interface = 0;    rfkill_state_ = NULL;    rfKill(1);     int fd = socket(AF_BLUETOOTH, SOCK_RAW, BTPROTO_HCI);    if (fd < 0) {    ALOGE( "Bluetooth socket error: %s", strerror(errno));      return -1;    }    bt_soc_fd_ = fd;     if (waitHciDev(hci_interface)) {    ALOGE( "HCI interface (%d) not found", hci_interface);      ::close(fd);      return -1;    }    struct sockaddr_hci addr;    memset(&addr, 0, sizeof(addr));    addr.hci_family = AF_BLUETOOTH;    addr.hci_dev = hci_interface;    addr.hci_channel = HCI_CHANNEL_USER;    if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) {    ALOGE( "HCI Channel Control: %s", strerror(errno));      ::close(fd);      return -1;    }    ALOGI( "HCI device ready");    return fd; } 

发送数据

继续回头接着上节之前的内容讲,我们的任务队列是在process_command_credits中被消费的,取出来之后需要进入到hci_thread线程中执行。从接收数据一节中也能看出,hci接口本身使用的是串行总线,因此不能并发地发送数据,所有命令都是在之前的命令响应后再发送。

值得一提的是,enqueue_command实际上绑定的是函数event_command_ready,以包含我们命令内容和对应回调的类型waiting_command_t为参数:

static void enqueue_command(waiting_command_t* wait_entry) { base::Closure callback = base::Bind(&event_command_ready, wait_entry);    //... command_queue.push(std::move(callback)); } 

因此,负责执行HCI发送命令的是event_command_ready函数:

  static void event_command_ready(waiting_command_t* wait_entry) {    {      /// Move it to the list of commands awaiting response      std::lock_guard<std::recursive_timed_mutex> lock(          commands_pending_response_mutex);      wait_entry->timestamp = std::chrono::steady_clock::now();      list_append(commands_pending_response, wait_entry);    }    // Send it off    packet_fragmenter->fragment_and_dispatch(wait_entry->command);     update_command_response_timer();  } 

首先将command放到一个等待响应的队列里,然后分片发送:

static void fragment_and_dispatch(BT_HDR* packet) {    CHECK(packet != NULL);     uint16_t event = packet->event & MSG_EVT_MASK;    uint8_t* stream = packet->data + packet->offset;     // We only fragment ACL packets    if (event != MSG_STACK_TO_HC_HCI_ACL) {      callbacks->fragmented(packet, true);      return;    }    // ACL/L2CAP fragment... } 

实现中只对ACL类型的HCI数据进行分片发送,不管是不是分片,都对最后一个packet调用callbacks->fragmented(),callbacks的类型是packet_fragmenter_callbacks_t,在packet_fragmenter_t->init中初始化并设置。而packet_fragmenter的初始化发生在hci_module_start_up()中,HCI层定义的回调如下:

static const packet_fragmenter_callbacks_t packet_fragmenter_callbacks = {  transmit_fragment, dispatch_reassembled, fragmenter_transmit_finished }; 

fragmented即对应transmit_fragment,对应定义如下:

// Callback for the fragmenter to send a fragment static void transmit_fragment(BT_HDR* packet, bool send_transmit_finished) {    btsnoop->capture(packet, false);     // HCI command packets are freed on a different thread when the matching    // event is received. Check packet->event before sending to avoid a race.    bool free_after_transmit =        (packet->event & MSG_EVT_MASK) != MSG_STACK_TO_HC_HCI_CMD &&        send_transmit_finished;     hci_transmit(packet);     if (free_after_transmit) {      buffer_allocator->free(packet);    } } 

hci_transmit有不同平台的实现,分别在:

  • hci/src/hci_layer_linux.c
  • hci/src/hci_layer_android.c

前者是通过write直接向HCI socket的fd写入,后者是调用IBluetoothHci::sendHciCommand去实现,接口定义同样是在hardware/interfaces/bluetooth/1.0/IBluetoothHci.hal文件中。

因为不同手机厂商的SoC中集成蓝牙芯片的接口不同,有的是使用USB连接,有的是使用UART连接,因此需要给安卓提供一个统一的操作接口,这个接口就很适合由HAL(HIDL)来进行抽象。这部分实现通常是使用Linux中已有的UART/USB驱动进行操作,以提高代码的复用性。

小结

本文通过从从用户层的一个蓝牙接口进行跟踪,一直向下分析到HCI的硬件抽象层。在这个过程中,穿插了蓝牙中的各个子模块,比如BTA、BTM、BTU 等,并在某些回调注册的节点中分析了对应的的初始化过程。最后根据初始化以及HCI命令的任务队列实现,我们也得知了接收数据/事件时的运行流程,当然还包括ACL分片/重组的逻辑等。对整个BlueDroid系统形成大致理解,有助于为后续的代码审计和漏洞分析奠定基础。

参考链接

  • http://www.bluez.org/
  • evolution of bluetooth drivers in Linux kernel
  • programing bluetooth
  • Bluetooth on modern Linux
  • bluedroid
  • https://medium.com/@muhamed.riyas/android-bluetooth-architecture-853645eff17f
  • https://www.sciencedirect.com/topics/computer-science/bluetooth-stack
  • https://link.springer.com/content/pdf/10.1007%2F978-0-387-75462-8_20.pdf
  • https://www.cnblogs.com/blogs-of-lxl/p/7010061.html

原文链接:https://evilpan.com/2021/07/11/android-bt/

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