Android -- Vold机制简要分析.doc

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SDK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：java view plain copy 在CODE上查看代码片派生到我的代码片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:s0 class core socket vold stream 0660 root mount socket cryptd stream 0660 root mount ioprio 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：cpp view plain copy 在CODE上查看代码片派生到我的代码片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 init fcntl(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VolumeManager if (!(vm = VolumeManager:Instance() LOG(ERROR) Unable to create VolumeManager; exit(1); /2、创建NetlinkManager if (!(nm = NetlinkManager:Instance() LOG(ERROR) setDebug(true); /3、创建CommandListener、CryptCommandListener cl = new CommandListener(); ccl = new CryptCommandListener(); vm-setBroadcaster(SocketListener *) cl); nm-setBroadcaster(SocketListener *) cl); /4、启动VolumeManager if (vm-start() PLOG(ERROR) Unable to start VolumeManager; exit(1); if (process_config(vm) PLOG(ERROR) start() PLOG(ERROR) startListener() PLOG(ERROR) startListener() PLOG(ERROR) Unable to start CryptCommandListener; exit(1); / Eventually well become the monitoring thread while(1) sleep(1000); LOG(ERROR) setBroadcaster(SocketListener *) cl)nm-start()现按步骤进行分析。1、NetlinkManager:Instance()：cpp view plain copy 在CODE上查看代码片派生到我的代码片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()：cpp view plain copy 在CODE上查看代码片派生到我的代码片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()：cpp view plain copy 在CODE上查看代码片派生到我的代码片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) SLOGE(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) 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：cpp view plain copy 在CODE上查看代码片派生到我的代码片mHandler = new NetlinkHandler(mSock); NetlinkHandler:NetlinkHandler(int listenerSocket) : NetlinkListener(listenerSocket) cpp view plain copy 在CODE上查看代码片派生到我的代码片/* 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; cpp view plain copy 在CODE上查看代码片派生到我的代码片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false mSocketName = 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：cpp view plain copy 在CODE上查看代码片派生到我的代码片int NetlinkHandler:start() return this-startListener(); 实际调用的是SocketListener:startListener()：cpp view plain copy 在CODE上查看代码片派生到我的代码片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) 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()：cpp view plain copy 在CODE上查看代码片派生到我的代码片void *SocketListener:threadStart(void *obj) SocketListener *me = reinterpret_cast(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) max = mSock; FD_SET(mSock, &read_fds); FD_SET(mCtrlPipe0, &read_fds); if (mCtrlPipe0 max) max = mCtrlPipe0; 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) 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(mCtrlPipe0, &read_fds) char c = CtrlPipe_Shutdown; TEMP_FAILURE_RETRY(read(mCtrlPipe0, &c, 1); if (c = CtrlPipe_Shutdown) break; continue; if (mListen & FD_ISSET(mSock, &read_fds) /mListener实际为false；监听端，有客户端连接请求 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 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；有数据可读，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()：cpp view plain copy 在CODE上查看代码片派生到我的代码片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) 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) / Dont complain if parseBinaryNetlinkMessage returns false. That can / just mean that the buffer contained no messages were 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：cpp view plain copy 在CODE上查看代码片派生到我的代码片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）、VolumeManagerVold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：cpp view plain copy 在CODE上查看代码片派生到我的代码片/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 active mUmsDirtyRatio = 0; void setBroadcaster(SocketListener *sl) mBroadcaster = sl; 直接看VolumeManager:start()的处理：cpp view plain copy 在CODE上查看代码片派生到我的代码片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( 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Volume return 0; 再直接看NetlinkManager和VolumeManager直接信息处理的调用：cpp vie