Android -- Vold机制简要分析

Android -- Vold机制简要分析

Vold是用于管理和控制Android外部存储介质的后台进程，这里说的管控，主要包括SD卡的插拔、挂载/卸载和格式化等；它是Android平台外部存储系统的管控枢纽。
Vold的整个控制模块主要由三个类模块构成：NetlinkManager、VolumeManager和CommandListener，它们的功能划分大概是：

NetlinkManager：用于从kernel中获取SD卡插拔的Uevnet消息
VolumeManager：管理模块，对NetlinkManager转发的消息做一些处理，并通过CommandListener发送给framework（MountService.java）；接着framework会通过套接字下发命令，指引VolumeManager对存储设备做下一步的操作，如挂载/卸载等
CommandListener：通过socket，实现MountService.java与Vold之间的消息交换

NetLink是Linux下用户进程和kernel进行信息交互的一种机制，借助这种机制，用户进程（如Vold/Netd）可以接收来自kernel的一些消息，同时也可以向kernel发送一些控制命令。NetlinkManager就是基于此设计的。Uevent也跟Linux系统有关，它与Linux 的设备文件系统有一定关系；这里，我们可以简单的认为，Uevent就是一个字符串，它描述了外部存储设备插入/拔出、挂载/卸载的状态信息。Vold通过Netlink机制，可以得到这些信息，并进行外部存储设备的管理、控制。

由上述介绍，我们可以得到如下的Vold框架图描述：

有了Vold的架构描述，接下来就开始分析Vold进程的整体流程及实现了。

一、Vold进程的声明与创建

Vold进程的声明与创建过程跟zygote一样，在init.rc中声明，在init进程创建：

service vold /system/bin/vold \--blkid_context=u:r:blkid:s0 --blkid_untrusted_context=u:r:blkid_untrusted:s0 \--fsck_context=u:r:fsck:s0 --fsck_untrusted_context=u:r:fsck_untrusted:s0class coresocket vold stream 0660 root mountsocket cryptd stream 0660 root mountioprio be 2

在创建Vold进程时，会为它创建两个socket，用于与framework进行信息交互。其他的细节，参考之前zygote进程创建的介绍。

二、进入Vold主程序

Vold的主程序在/system/vold目录中，直接看main.cpp::main()函数：

int main(int argc, char** argv) {setenv("ANDROID_LOG_TAGS", "*:v", 1);android::base::InitLogging(argv, android::base::LogdLogger(android::base::SYSTEM));LOG(INFO) << "Vold 3.0 (the awakening) firing up";LOG(VERBOSE) << "Detected support for:"<< (android::vold::IsFilesystemSupported("ext4") ? " ext4" : "")<< (android::vold::IsFilesystemSupported("f2fs") ? " f2fs" : "")<< (android::vold::IsFilesystemSupported("vfat") ? " vfat" : "");VolumeManager *vm;CommandListener *cl;CryptCommandListener *ccl;NetlinkManager *nm;parse_args(argc, argv);sehandle = selinux_android_file_context_handle();if (sehandle) {selinux_android_set_sehandle(sehandle);}// Quickly throw a CLOEXEC on the socket we just inherited from initfcntl(android_get_control_socket("vold"), F_SETFD, FD_CLOEXEC);//拿到init进程创建的名为vold的socket句柄，并为它设置FD_CLOEXEC标志位fcntl(android_get_control_socket("cryptd"), F_SETFD, FD_CLOEXEC);//同上mkdir("/dev/block/vold", 0755);//创建/dev/block/vold目录，存放所有subdisk和sdcard的挂载点信息/* For when cryptfs checks and mounts an encrypted filesystem */klog_set_level(6);/* Create our singleton managers *///1、创建VolumeManagerif (!(vm = VolumeManager::Instance())) {LOG(ERROR) << "Unable to create VolumeManager";exit(1);}//2、创建NetlinkManagerif (!(nm = NetlinkManager::Instance())) {LOG(ERROR) << "Unable to create NetlinkManager";exit(1);}if (property_get_bool("vold.debug", false)) {vm->setDebug(true);}//3、创建CommandListener、CryptCommandListenercl = new CommandListener();ccl = new CryptCommandListener();vm->setBroadcaster((SocketListener *) cl);nm->setBroadcaster((SocketListener *) cl);//4、启动VolumeManagerif (vm->start()) {PLOG(ERROR) << "Unable to start VolumeManager";exit(1);}if (process_config(vm)) {PLOG(ERROR) << "Error reading configuration... continuing anyways";}//6、启动NetlinkManager，处理来自kernel的usb/sdcard插拔消息if (nm->start()) {PLOG(ERROR) << "Unable to start NetlinkManager";exit(1);}//7、应用层往/sys/block目录下的uevent文件写"add\n"指令，触发kernel向上发送Uevent消息，获取设备的当前信息coldboot("/sys/block");
//    coldboot("/sys/class/switch");/** Now that we're up, we can respond to commands*///8、启动CommandListenerif (cl->startListener()) {PLOG(ERROR) << "Unable to start CommandListener";exit(1);}//9、启动CryptCommandListenerif (ccl->startListener()) {PLOG(ERROR) << "Unable to start CryptCommandListener";exit(1);}// Eventually we'll become the monitoring threadwhile(1) {sleep(1000);}LOG(ERROR) << "Vold exiting";exit(0);
}

从代码中的注释可知，Vold主要创了三个对象：NetlinkManager、VolumeManager和CommandListener。根据Vold的架构图，现分别对它们的创建及启动过程进行分析。

（1）、NetlinkManager

主要的处理过程：

nm = NetlinkManager::Instance()
nm->setBroadcaster((SocketListener *) cl)
nm->start()

现按步骤进行分析。

1、NetlinkManager::Instance()：

NetlinkManager *NetlinkManager::Instance() {if (!sInstance)sInstance = new NetlinkManager();return sInstance;
}NetlinkManager::NetlinkManager() {mBroadcaster = NULL; //type:SocketListener*，用来进行socket通信
}

这里使用了单例模式来构建NetlinkManager对象，构造函数中只是简单地初始化了成员变量。
2、NetlinkManager::setBroadcaster()：

    cl = new CommandListener();nm->setBroadcaster((SocketListener *) cl);void setBroadcaster(SocketListener *sl) { mBroadcaster = sl; }

setBroadcaster()函数也很简单，为mBroadcast进行赋值。

3、NetlinkManager::start()：

int NetlinkManager::start() {struct sockaddr_nl nladdr;int sz = 64 * 1024;int on = 1;memset(&nladdr, 0, sizeof(nladdr));nladdr.nl_family = AF_NETLINK;nladdr.nl_pid = getpid();nladdr.nl_groups = 0xffffffff;//创建地址族为PF_NETLINK的socket，与Kernel进行通信；也可以为AF_NETLINK.参照Linux Netlink机制资料if ((mSock = socket(PF_NETLINK, SOCK_DGRAM | SOCK_CLOEXEC,NETLINK_KOBJECT_UEVENT)) < 0) {//NETLINK_KOBJECT_UEVENT协议：Kernel messages to userspaceSLOGE("Unable to create uevent socket: %s", strerror(errno));return -1;}//设置套接字if (setsockopt(mSock, SOL_SOCKET, SO_RCVBUFFORCE, &sz, sizeof(sz)) < 0) {SLOGE("Unable to set uevent socket SO_RCVBUFFORCE option: %s", strerror(errno));goto out;}if (setsockopt(mSock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on)) < 0) {SLOGE("Unable to set uevent socket SO_PASSCRED option: %s", strerror(errno));goto out;}//为套接字绑定地址if (bind(mSock, (struct sockaddr *) &nladdr, sizeof(nladdr)) < 0) {SLOGE("Unable to bind uevent socket: %s", strerror(errno));goto out;}mHandler = new NetlinkHandler(mSock);//mHandler、mSock都是成员变量.mHandler对象主要保存了套接字的文件描述符，供后续使用if (mHandler->start()) {//startListener通过父类方法，在mSock上监听连接请求SLOGE("Unable to start NetlinkHandler: %s", strerror(errno));goto out;}return 0;out:close(mSock);return -1;
}

start()方法中创建了一个句柄值为mSock的套接字，用来和kernel通信；而实际具体的socket信息交互是由NetlinkHandler处理的。

NetlinkHandler的实现有一套继承机制，其实际继承关系如图所示：

按照继承关系，分析它的构建过程：

mHandler = new NetlinkHandler(mSock);
NetlinkHandler::NetlinkHandler(int listenerSocket) :NetlinkListener(listenerSocket) {
}

/* temporary version until we can get Motorola to update their* ril.so.  Their prebuilt ril.so is using this private class* so changing the NetlinkListener() constructor breaks their ril.*/
NetlinkListener::NetlinkListener(int socket) :SocketListener(socket, false) {mFormat = NETLINK_FORMAT_ASCII;
}

SocketListener::SocketListener(const char *socketName, bool listen) {init(socketName, -1, listen, false);
}
void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {mListen = listen;//是否是监听端，这里为falsemSocketName = socketName;//保存socket的名字mSock = socketFd;//保存socket的句柄值，与Kernel通信mUseCmdNum = useCmdNum;pthread_mutex_init(&mClientsLock, NULL);mClients = new SocketClientCollection();//集合对象，保存类型SocketClient为的变量；保存了socket通信中的客户端对象
}

再看NetlinkHandler::start()方法：

int NetlinkHandler::start() {return this->startListener();
}

实际调用的是SocketListener::startListener()：

int SocketListener::startListener() {return startListener(4);
}int SocketListener::startListener(int backlog) {if (!mSocketName && mSock == -1) {SLOGE("Failed to start unbound listener");errno = EINVAL;return -1;} else if (mSocketName) {if ((mSock = android_get_control_socket(mSocketName)) < 0) {SLOGE("Obtaining file descriptor socket '%s' failed: %s",mSocketName, strerror(errno));return -1;}SLOGV("got mSock = %d for %s", mSock, mSocketName);fcntl(mSock, F_SETFD, FD_CLOEXEC);}if (mListen && listen(mSock, backlog) < 0) {//如果mListen为true，则监听该socket；表明此socket应该是服务端SLOGE("Unable to listen on socket (%s)", strerror(errno));return -1;} else if (!mListen)//实际传入的mListen值为false，走此分支mClients->push_back(new SocketClient(mSock, false, mUseCmdNum));//创建一个SocketClient对象，并保存到集合中if (pipe(mCtrlPipe)) {SLOGE("pipe failed (%s)", strerror(errno));return -1;}if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {//创建一个线程，在其中调用threadStart()，根据mListen值，等待接收来自Kernel的Uevent消息SLOGE("pthread_create (%s)", strerror(errno));return -1;}return 0;
}

创建一个SocketListener对象，并加入mClients中；随后，创建一个线程，并调用SocketListener::threadStart()：

void *SocketListener::threadStart(void *obj) {SocketListener *me = reinterpret_cast<SocketListener *>(obj);me->runListener();pthread_exit(NULL);return NULL;
}void SocketListener::runListener() {SocketClientCollection pendingList;while(1) {SocketClientCollection::iterator it;fd_set read_fds;int rc = 0;int max = -1;FD_ZERO(&read_fds);if (mListen) {//如果我们是服务端，则将该socket的套接字加入到可读监控队列中max = mSock;FD_SET(mSock, &read_fds);}FD_SET(mCtrlPipe[0], &read_fds);if (mCtrlPipe[0] > max)max = mCtrlPipe[0];pthread_mutex_lock(&mClientsLock);for (it = mClients->begin(); it != mClients->end(); ++it) {// NB: calling out to an other object with mClientsLock held (safe)int fd = (*it)->getSocket();FD_SET(fd, &read_fds);if (fd > max) {max = fd;}}pthread_mutex_unlock(&mClientsLock);SLOGV("mListen=%d, max=%d, mSocketName=%s", mListen, max, mSocketName);if ((rc = select(max + 1, &read_fds, NULL, NULL, NULL)) < 0) {//如果监测到read_fds集合中有socket可读if (errno == EINTR)continue;SLOGE("select failed (%s) mListen=%d, max=%d", strerror(errno), mListen, max);sleep(1);continue;} else if (!rc)continue;if (FD_ISSET(mCtrlPipe[0], &read_fds)) {char c = CtrlPipe_Shutdown;TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));if (c == CtrlPipe_Shutdown) {break;}continue;}if (mListen && FD_ISSET(mSock, &read_fds)) {//监听端客户端连接请求struct sockaddr addr;socklen_t alen;int c;do {alen = sizeof(addr);c = accept(mSock, &addr, &alen);//接受client的连接请求，c是代表client套接字的文件描述符SLOGV("%s got %d from accept", mSocketName, c);} while (c < 0 && errno == EINTR);if (c < 0) {SLOGE("accept failed (%s)", strerror(errno));sleep(1);continue;}fcntl(c, F_SETFD, FD_CLOEXEC);pthread_mutex_lock(&mClientsLock);mClients->push_back(new SocketClient(c, true, mUseCmdNum));//根据c,创建一个SocketListener对象，并加入到队列中pthread_mutex_unlock(&mClientsLock);}/* Add all active clients to the pending list first */pendingList.clear();pthread_mutex_lock(&mClientsLock);for (it = mClients->begin(); it != mClients->end(); ++it) {SocketClient* c = *it;// NB: calling out to an other object with mClientsLock held (safe)int fd = c->getSocket();if (FD_ISSET(fd, &read_fds)) {//遍历所有保存的客户端；如果该socket可读，将该套接字加入到队列中pendingList.push_back(c);c->incRef();}}pthread_mutex_unlock(&mClientsLock);/* Process the pending list, since it is owned by the thread,* there is no need to lock it */while (!pendingList.empty()) {/* Pop the first item from the list */it = pendingList.begin();SocketClient* c = *it;pendingList.erase(it);/* Process it, if false is returned, remove from list */if (!onDataAvailable(c)) {//客户端收到数据，调用NetlinkListener::onDataAvailable()处理release(c, false);//数据处理失败，则释放socket资源}c->decRef();}}
}

我们初始化NetlinkListener时传入的mListener值为false；上述代码中，会遍历所有保存的客户端socket，如果收到数据，则进行处理。

调用NetlinkListener::onDataAvailable()：

bool NetlinkListener::onDataAvailable(SocketClient *cli)
{int socket = cli->getSocket();ssize_t count;uid_t uid = -1;bool require_group = true;if (mFormat == NETLINK_FORMAT_BINARY_UNICAST) {require_group = false;}count = TEMP_FAILURE_RETRY(uevent_kernel_recv(socket,mBuffer, sizeof(mBuffer), require_group, &uid));//从kernel获取Uevent消息，保存到mBuffer中if (count < 0) {if (uid > 0)LOG_EVENT_INT(65537, uid);SLOGE("recvmsg failed (%s)", strerror(errno));return false;//读取失败，则返回false，上层调用则会关闭socket资源}NetlinkEvent *evt = new NetlinkEvent();//事件的代码封装if (evt->decode(mBuffer, count, mFormat)) {解析Uevent数据，填充到NetlinkEvent对象中onEvent(evt);//NetlinkHandler::onEvent()} else if (mFormat != NETLINK_FORMAT_BINARY) {// Don't complain if parseBinaryNetlinkMessage returns false. That can// just mean that the buffer contained no messages we're interested in.SLOGE("Error decoding NetlinkEvent");}delete evt;return true;
}

先通过socket获取到Uevent数据，再解析并将信息封装到NetlinkEvent对象中。NetlinkHandler::onEvent()分发处理该对象：

void NetlinkHandler::onEvent(NetlinkEvent *evt) {VolumeManager *vm = VolumeManager::Instance();const char *subsys = evt->getSubsystem();if (!subsys) {//如果事件和外部存储设备无关，则不处理SLOGW("No subsystem found in netlink event");return;}if (!strcmp(subsys, "block")) {//如果Uevent是block子系统vm->handleBlockEvent(evt);//进入VolumeManager中处理；此处和VolumeManager进行交互}
}

如果事件是和外部存储有关，则调用VolumeManager::handleBlockEvent()处理该事件；这里，就看到了NetlinkManager和VolumeManager之间进行数据流动的处理了。

（2）、VolumeManager

Vold使用VolumeManager的过程和NetlinkManager类似，也是三步：

vm= VolumeManager::Instance()
vm->setBroadcaster((SocketListener *) cl)
vm->start()

1、2步与NetlinkManager的处理类似：

//单例模式
VolumeManager *VolumeManager::Instance() {if (!sInstance)sInstance = new VolumeManager();return sInstance;
}VolumeManager::VolumeManager() {mDebug = false;mActiveContainers = new AsecIdCollection();mBroadcaster = NULL;mUmsSharingCount = 0;mSavedDirtyRatio = -1;// set dirty ratio to 0 when UMS is activemUmsDirtyRatio = 0;
}void setBroadcaster(SocketListener *sl) { mBroadcaster = sl; }

直接看VolumeManager::start()的处理：

int VolumeManager::start() {// Always start from a clean slate by unmounting everything in// directories that we own, in case we crashed.unmountAll();//在处理外部设备事件之前，先重置所有状态// Assume that we always have an emulated volume on internal// storage; the framework will decide if it should be mounted.CHECK(mInternalEmulated == nullptr);mInternalEmulated = std::shared_ptr<android::vold::VolumeBase>(new android::vold::EmulatedVolume("/data/media"));mInternalEmulated->create();//预先设定/data/media，由framework决定是否mount；EmulatedVolume和VolumeBase之间是继承关系，代表不同类型的Volumereturn 0;
}

再直接看NetlinkManager和VolumeManager之间信息处理的调用过程：

void VolumeManager::handleBlockEvent(NetlinkEvent *evt) {std::lock_guard<std::mutex> lock(mLock);if (mDebug) {LOG(VERBOSE) << "----------------";LOG(VERBOSE) << "handleBlockEvent with action " << (int) evt->getAction();evt->dump();}std::string eventPath(evt->findParam("DEVPATH"));//设备路径std::string devType(evt->findParam("DEVTYPE"));//设备类型if (devType != "disk") return;//主次设备号，两者可以描述一个具体设备int major = atoi(evt->findParam("MAJOR"));int minor = atoi(evt->findParam("MINOR"));dev_t device = makedev(major, minor);//根据主次设备号创建设备switch (evt->getAction()) {case NetlinkEvent::Action::kAdd: {for (auto source : mDiskSources) {if (source->matches(eventPath)) {// For now, assume that MMC devices are SD, and that// everything else is USBint flags = source->getFlags();if (major == kMajorBlockMmc) {flags |= android::vold::Disk::Flags::kSd;} else {flags |= android::vold::Disk::Flags::kUsb;}auto disk = new android::vold::Disk(eventPath, device,source->getNickname(), flags);//将信息封装成Disk对象，表示一个检测到的物理设备disk->create();//Disk::create()mDisks.push_back(std::shared_ptr<android::vold::Disk>(disk));//加进集合break;}}break;}case NetlinkEvent::Action::kChange: {LOG(DEBUG) << "Disk at " << major << ":" << minor << " changed";for (auto disk : mDisks) {if (disk->getDevice() == device) {disk->readMetadata();disk->readPartitions();}}break;}case NetlinkEvent::Action::kRemove: {auto i = mDisks.begin();while (i != mDisks.end()) {if ((*i)->getDevice() == device) {(*i)->destroy();i = mDisks.erase(i);} else {++i;}}break;}default: {LOG(WARNING) << "Unexpected block event action " << (int) evt->getAction();break;}}
}

向上层发送各种类型的消息都是通过notifyEvent()函数处理的，其实际就是通过socket来发送的。

同时，Vold的主函数中还有一个重要的函数调用process_config()：

static int process_config(VolumeManager *vm) {std::string path(android::vold::DefaultFstabPath()); //获取到vold.fstab文件路径fstab = fs_mgr_read_fstab(path.c_str());//解析.fstab文件，并返回封装的fstab对象if (!fstab) {PLOG(ERROR) << "Failed to open default fstab " << path;return -1;}/* Loop through entries looking for ones that vold manages */bool has_adoptable = false;for (int i = 0; i < fstab->num_entries; i++) {if (fs_mgr_is_voldmanaged(&fstab->recs[i])) {if (fs_mgr_is_nonremovable(&fstab->recs[i])) {LOG(WARNING) << "nonremovable no longer supported; ignoring volume";continue;}std::string sysPattern(fstab->recs[i].blk_device);std::string nickname(fstab->recs[i].label);int flags = 0;if (fs_mgr_is_encryptable(&fstab->recs[i])) {flags |= android::vold::Disk::Flags::kAdoptable;has_adoptable = true;}if (fs_mgr_is_noemulatedsd(&fstab->recs[i])|| property_get_bool("vold.debug.default_primary", false)) {flags |= android::vold::Disk::Flags::kDefaultPrimary;//is Primary?}vm->addDiskSource(std::shared_ptr<VolumeManager::DiskSource>(new VolumeManager::DiskSource(sysPattern, nickname, flags)));//添加一个VolumeManager::DiskSource对象，保存了一些信息}}property_set("vold.has_adoptable", has_adoptable ? "1" : "0");return 0;
}

//解析.fstab配置文件，并返回一个fstab结构对象
struct fstab *fs_mgr_read_fstab(const char *fstab_path)
{FILE *fstab_file;int cnt, entries;ssize_t len;size_t alloc_len = 0;char *line = NULL;const char *delim = " \t";char *save_ptr, *p;struct fstab *fstab = NULL;struct fs_mgr_flag_values flag_vals;
#define FS_OPTIONS_LEN 1024char tmp_fs_options[FS_OPTIONS_LEN];fstab_file = fopen(fstab_path, "r");if (!fstab_file) {ERROR("Cannot open file %s\n", fstab_path);return 0;}entries = 0;while ((len = getline(&line, &alloc_len, fstab_file)) != -1) {/* if the last character is a newline, shorten the string by 1 byte */if (line[len - 1] == '\n') {line[len - 1] = '\0';}/* Skip any leading whitespace */p = line;while (isspace(*p)) {p++;}/* ignore comments or empty lines */if (*p == '#' || *p == '\0')continue;entries++;}if (!entries) {ERROR("No entries found in fstab\n");goto err;}/* Allocate and init the fstab structure */fstab = calloc(1, sizeof(struct fstab));fstab->num_entries = entries;fstab->fstab_filename = strdup(fstab_path);fstab->recs = calloc(fstab->num_entries, sizeof(struct fstab_rec));fseek(fstab_file, 0, SEEK_SET);cnt = 0;//解析fstab中每行的内容，并进行封装while ((len = getline(&line, &alloc_len, fstab_file)) != -1) {/* if the last character is a newline, shorten the string by 1 byte */if (line[len - 1] == '\n') {line[len - 1] = '\0';}/* Skip any leading whitespace */p = line;while (isspace(*p)) {p++;}/* ignore comments or empty lines */if (*p == '#' || *p == '\0')continue;/* If a non-comment entry is greater than the size we allocated, give an* error and quit.  This can happen in the unlikely case the file changes* between the two reads.*/if (cnt >= entries) {ERROR("Tried to process more entries than counted\n");break;}if (!(p = strtok_r(line, delim, &save_ptr))) {ERROR("Error parsing mount source\n");goto err;}fstab->recs[cnt].blk_device = strdup(p);if (!(p = strtok_r(NULL, delim, &save_ptr))) {ERROR("Error parsing mount_point\n");goto err;}fstab->recs[cnt].mount_point = strdup(p);//mount的位置if (!(p = strtok_r(NULL, delim, &save_ptr))) {ERROR("Error parsing fs_type\n");goto err;}fstab->recs[cnt].fs_type = strdup(p);if (!(p = strtok_r(NULL, delim, &save_ptr))) {ERROR("Error parsing mount_flags\n");goto err;}tmp_fs_options[0] = '\0';fstab->recs[cnt].flags = parse_flags(p, mount_flags, NULL,tmp_fs_options, FS_OPTIONS_LEN);/* fs_options are optional */if (tmp_fs_options[0]) {fstab->recs[cnt].fs_options = strdup(tmp_fs_options);} else {fstab->recs[cnt].fs_options = NULL;}if (!(p = strtok_r(NULL, delim, &save_ptr))) {ERROR("Error parsing fs_mgr_options\n");goto err;}fstab->recs[cnt].fs_mgr_flags = parse_flags(p, fs_mgr_flags,&flag_vals, NULL, 0);fstab->recs[cnt].key_loc = flag_vals.key_loc;fstab->recs[cnt].verity_loc = flag_vals.verity_loc;fstab->recs[cnt].length = flag_vals.part_length;fstab->recs[cnt].label = flag_vals.label;fstab->recs[cnt].partnum = flag_vals.partnum;fstab->recs[cnt].swap_prio = flag_vals.swap_prio;fstab->recs[cnt].zram_size = flag_vals.zram_size;cnt++;}fclose(fstab_file);free(line);return fstab;err:fclose(fstab_file);free(line);if (fstab)fs_mgr_free_fstab(fstab);return NULL;
}

fstab文件是Linux下配置分区的一个文件，这部分后续补充......总之，就是解析fstab文件后，会根据配置信息创建DiskSource对象，加入到VolumeManager::mDiskSource中。

（3 ）、CommandListener

VolumeManager要想向MountService发送消息，就要借助CommandListener。CommandListener有一个较为复杂的继承关系：

CommandListener的创建过程跟NetlinkManager类似：

CommandListener::CommandListener() :FrameworkListener("vold", true) {//vold是socket名称，init.rc文件中声明的一个socket资源，用于和framework通信registerCmd(new DumpCmd());//注册不同的命令对象，保存到mCommands成员中;同时，创建Cmd对象时，会保存一个字符串标识(一般是上层下发命令中的第一个字符串)，用于后续区分不同的命令registerCmd(new VolumeCmd());//标识：volumeregisterCmd(new AsecCmd());//标识：asecregisterCmd(new ObbCmd());//标识：obbregisterCmd(new StorageCmd());//标识：storageregisterCmd(new FstrimCmd());//标识：fstrim
}

FrameworkListener::FrameworkListener(const char *socketName, bool withSeq) :SocketListener(socketName, true, withSeq) {init(socketName, withSeq);
}

SocketListener::SocketListener(const char *socketName, bool listen, bool useCmdNum) {init(socketName, -1, listen, useCmdNum);
}void SocketListener::init(const char *socketName, int socketFd, bool listen, bool useCmdNum) {mListen = listen;//是否是监听端，与前面不同，这里为truemSocketName = socketName;//保存socket的名字"vold",与MountService通信mSock = socketFd;//保存socket的句柄值mUseCmdNum = useCmdNum;pthread_mutex_init(&mClientsLock, NULL);mClients = new SocketClientCollection();//集合对象
}

下面直接看CommandListener->startListener()：

int SocketListener::startListener() {return startListener(4);
}int SocketListener::startListener(int backlog) {if (!mSocketName && mSock == -1) {SLOGE("Failed to start unbound listener");errno = EINVAL;return -1;} else if (mSocketName) {if ((mSock = android_get_control_socket(mSocketName)) < 0) {//名为"vold"的socket的句柄值SLOGE("Obtaining file descriptor socket '%s' failed: %s",mSocketName, strerror(errno));return -1;}SLOGV("got mSock = %d for %s", mSock, mSocketName);fcntl(mSock, F_SETFD, FD_CLOEXEC);}if (mListen && listen(mSock, backlog) < 0) {//mListener为true，则监听该socketSLOGE("Unable to listen on socket (%s)", strerror(errno));return -1;} else if (!mListen)//mListener为false，走此分支mClients->push_back(new SocketClient(mSock, false, mUseCmdNum));if (pipe(mCtrlPipe)) {SLOGE("pipe failed (%s)", strerror(errno));return -1;}if (pthread_create(&mThread, NULL, SocketListener::threadStart, this)) {//创建一个线程，在其中调用threadStart()，并在mSock代表的套接字上等待客户端的连接请求SLOGE("pthread_create (%s)", strerror(errno));return -1;}return 0;
}

在CommandListener监听流程中，mListene为true；表示这一端是监听侧，等待Client的连接请求。这种场景下，MountService就是这里描述的客户端。MountService在创建过程，会通过创建NativeDaemonConnector对象，去连接名为"vold"的socket，这样两者就可以通信了。

SocketListener::threadStart()：

void *SocketListener::threadStart(void *obj) {SocketListener *me = reinterpret_cast<SocketListener *>(obj);me->runListener();pthread_exit(NULL);return NULL;
}void SocketListener::runListener() {SocketClientCollection pendingList;while(1) {SocketClientCollection::iterator it;fd_set read_fds;int rc = 0;int max = -1;FD_ZERO(&read_fds);if (mListen) {//如果我们是服务端，则将该socket的套接字加入到可读监控队列中max = mSock;FD_SET(mSock, &read_fds);}FD_SET(mCtrlPipe[0], &read_fds);if (mCtrlPipe[0] > max)max = mCtrlPipe[0];pthread_mutex_lock(&mClientsLock);for (it = mClients->begin(); it != mClients->end(); ++it) {// NB: calling out to an other object with mClientsLock held (safe)int fd = (*it)->getSocket();FD_SET(fd, &read_fds);if (fd > max) {max = fd;}}pthread_mutex_unlock(&mClientsLock);SLOGV("mListen=%d, max=%d, mSocketName=%s", mListen, max, mSocketName);if ((rc = select(max + 1, &read_fds, NULL, NULL, NULL)) < 0) {//如果集合read_fds中有socket可读if (errno == EINTR)监测continue;SLOGE("select failed (%s) mListen=%d, max=%d", strerror(errno), mListen, max);sleep(1);continue;} else if (!rc)continue;if (FD_ISSET(mCtrlPipe[0], &read_fds)) {char c = CtrlPipe_Shutdown;TEMP_FAILURE_RETRY(read(mCtrlPipe[0], &c, 1));if (c == CtrlPipe_Shutdown) {break;}continue;}if (mListen && FD_ISSET(mSock, &read_fds)) {//mListener值实际为true；服务端，等待客户端连接请求struct sockaddr addr;socklen_t alen;int c;do {alen = sizeof(addr);c = accept(mSock, &addr, &alen);//接受MountService发起的socket连接请求，SLOGV("%s got %d from accept", mSocketName, c);} while (c < 0 && errno == EINTR);if (c < 0) {SLOGE("accept failed (%s)", strerror(errno));sleep(1);continue;}fcntl(c, F_SETFD, FD_CLOEXEC);pthread_mutex_lock(&mClientsLock);mClients->push_back(new SocketClient(c, true, mUseCmdNum));//根据c,创建一个SocketListener对象，并加入到集合中pthread_mutex_unlock(&mClientsLock);}/* Add all active clients to the pending list first */pendingList.clear();pthread_mutex_lock(&mClientsLock);for (it = mClients->begin(); it != mClients->end(); ++it) {SocketClient* c = *it;// NB: calling out to an other object with mClientsLock held (safe)int fd = c->getSocket();if (FD_ISSET(fd, &read_fds)) {//遍历保存的所有客户端socket，如果对应的socket可读，则将该套接字加入到队列中pendingList.push_back(c);c->incRef();}}pthread_mutex_unlock(&mClientsLock);/* Process the pending list, since it is owned by the thread,* there is no need to lock it */while (!pendingList.empty()) {/* Pop the first item from the list */it = pendingList.begin();SocketClient* c = *it;pendingList.erase(it);/* Process it, if false is returned, remove from list */if (!onDataAvailable(c)) {//有数据来，调用FrameworkListener::onDataAvailable()处理release(c, false);}c->decRef();}}
}

由于mListen值的变化（此时为true），处理流程有所不同。首先作为服务端，会等待Client的连接请求；如果有连接请求，并有数据发送过来，则通过
onDataAvailable()处理。根据继承关系，此处调用FrameworkListener::onDataAvailable()：

bool FrameworkListener::onDataAvailable(SocketClient *c) {char buffer[CMD_BUF_SIZE];int len;len = TEMP_FAILURE_RETRY(read(c->getSocket(), buffer, sizeof(buffer)));//从MountService接收指令数据，存入buffer中if (len < 0) {SLOGE("read() failed (%s)", strerror(errno));return false;} else if (!len)return false;if(buffer[len-1] != '\0')SLOGW("String is not zero-terminated");int offset = 0;int i;for (i = 0; i < len; i++) {if (buffer[i] == '\0') {/* IMPORTANT: dispatchCommand() expects a zero-terminated string */dispatchCommand(c, buffer + offset);//命令分发处理offset = i + 1;}}return true;
}

如果消息不为空，则调用FrameworkListener::dispatchCommand()进行处理：

void FrameworkListener::dispatchCommand(SocketClient *cli, char *data) {FrameworkCommandCollection::iterator i;int argc = 0;char *argv[FrameworkListener::CMD_ARGS_MAX];char tmp[CMD_BUF_SIZE];char *p = data;char *q = tmp;char *qlimit = tmp + sizeof(tmp) - 1;bool esc = false;bool quote = false;bool haveCmdNum = !mWithSeq;memset(argv, 0, sizeof(argv));memset(tmp, 0, sizeof(tmp));while(*p) {if (*p == '\\') {if (esc) {if (q >= qlimit)goto overflow;*q++ = '\\';esc = false;} elseesc = true;p++;continue;} else if (esc) {if (*p == '"') {if (q >= qlimit)goto overflow;*q++ = '"';} else if (*p == '\\') {if (q >= qlimit)goto overflow;*q++ = '\\';} else {cli->sendMsg(500, "Unsupported escape sequence", false);goto out;}p++;esc = false;continue;}if (*p == '"') {if (quote)quote = false;elsequote = true;p++;continue;}if (q >= qlimit)goto overflow;*q = *p++;if (!quote && *q == ' ') {*q = '\0';if (!haveCmdNum) {char *endptr;int cmdNum = (int)strtol(tmp, &endptr, 0);if (endptr == NULL || *endptr != '\0') {cli->sendMsg(500, "Invalid sequence number", false);goto out;}cli->setCmdNum(cmdNum);haveCmdNum = true;} else {if (argc >= CMD_ARGS_MAX)goto overflow;argv[argc++] = strdup(tmp);}memset(tmp, 0, sizeof(tmp));q = tmp;continue;}q++;}*q = '\0';if (argc >= CMD_ARGS_MAX)goto overflow;argv[argc++] = strdup(tmp);
#if 0for (int k = 0; k < argc; k++) {SLOGD("arg[%d] = '%s'", k, argv[k]);}
#endifif (quote) {cli->sendMsg(500, "Unclosed quotes error", false);goto out;}if (errorRate && (++mCommandCount % errorRate == 0)) {/* ignore this command - let the timeout handler handle it */SLOGE("Faking a timeout");goto out;}for (i = mCommands->begin(); i != mCommands->end(); ++i) {FrameworkCommand *c = *i;if (!strcmp(argv[0], c->getCommand())) {//获取命令的第一个参数（即标识），遍历mCommands，找到符合的Command对象去执行runCommand();例如，如果标识是volume，则执行VolumeCommand的runCommand()函数处理下发的指令if (c->runCommand(cli, argc, argv)) {//重要，调用不同Command类型的、我们之前注册过的对象调用runCommand()方法处理指令(定义在CommandListener中)SLOGW("Handler '%s' error (%s)", c->getCommand(), strerror(errno));}goto out;}}cli->sendMsg(500, "Command not recognized", false);
out:int j;for (j = 0; j < argc; j++)free(argv[j]);return;overflow:LOG_EVENT_INT(78001, cli->getUid());cli->sendMsg(500, "Command too long", false);goto out;

首先，根据上层下发的指令中的信息去得到一个符合要求的Command命令对象，然后执行相应的runCommand()方法来处理不同的指令。我们下发的指令有一定的规则，一般第一个字符串是标识，用以获得不同的Command对象；第二个参数一般是我们需要进行的操作命令；后续的参数一般都是下发的用以完成操作的数据。一般情况，格式类似于：