Android-vold源码分析之挂载SD卡(8)

浅浅的花香味﹌ 2022-05-14 01:15 211阅读 0赞

这里续上一篇文章，讲解挂载磁盘的操作：

3.挂载磁盘。这里都有一个const char \*类型的参数，这参数保存着每个磁盘的标签信息，比如sd卡的label是sdcard。

int VolumeManager::mountVolume(const char \*label) \{ Volume \*v = lookupVolume(label); if (!v) \{ errno = ENOENT; return -1; \} return v->mountVol(); \}  
lookupVolume函数寻找与label匹配的对象：

Volume \*VolumeManager::lookupVolume(const char \*label) \{ VolumeCollection::iterator i; for (i = mVolumes->begin(); i != mVolumes->end(); ++i) \{ if (label\[0\] == '/') \{ if (!strcmp(label, (\*i)->getMountpoint())) return (\*i); \} else \{ if (!strcmp(label, (\*i)->getLabel())) return (\*i); \} \} return NULL; \}  
如果找到，直接返回磁盘对象Volume\*，挂载操作在mountVol函数里面，该函数内容有点多，贴源码:

int Volume::mountVol() \{ dev\_t deviceNodes\[4\]; int n, i, rc = 0; char errmsg\[255\]; if (getState() == Volume::State\_NoMedia) \{ snprintf(errmsg, sizeof(errmsg), "Volume %s %s mount failed - no media", getLabel(), getMountpoint()); mVm->getBroadcaster()->sendBroadcast( ResponseCode::VolumeMountFailedNoMedia, errmsg, false); errno = ENODEV; return -1; \} else if (getState() != Volume::State\_Idle) \{ errno = EBUSY; return -1; \} /\*判断该挂载点是否已经挂载，若已经挂载，则直接返回。\*/ if (isMountpointMounted(getMountpoint())) \{ SLOGW("Volume is idle but appears to be mounted - fixing"); /\*这里的setState函数用得很频繁，这函数就是将状态通知给framework\*/ setState(Volume::State\_Mounted); // mCurrentlyMountedKdev = XXX return 0; \} /\*获取磁盘的设备号与分区数量,在下面说明\*/ n = getDeviceNodes((dev\_t \*) &deviceNodes, 4); if (!n) \{ SLOGE("Failed to get device nodes (%s)\\n", strerror(errno)); return -1; \} /\*将循环挂载n个分区，但从代码上看，只适用于挂载一个分区\*/ for (i = 0; i < n; i++) \{ char devicePath\[255\]; /\*这里看到了吧，用这种方式来使用磁盘的设备节点很方便，直接用主次设备号来命令\*/ sprintf(devicePath, "/dev/block/vold/%d:%d", MAJOR(deviceNodes\[i\]), MINOR(deviceNodes\[i\])); SLOGI("%s being considered for volume %s\\n", devicePath, getLabel()); errno = 0; setState(Volume::State\_Checking); /\*挂载之前先检测一下该分区是否是fat分区，如果不是fat格式，返回-1\*/ if (Fat::check(devicePath)) \{ if (errno == ENODATA) \{ SLOGW("%s does not contain a FAT filesystem\\n", devicePath); continue; \} errno = EIO; /\* Badness - abort the mount \*/ SLOGE("%s failed FS checks (%s)", devicePath, strerror(errno)); setState(Volume::State\_Idle); return -1; \} /\* \* Mount the device on our internal staging mountpoint so we can \* muck with it before exposing it to non priviledged users. \*/ errno = 0; /\*将设备节点挂载在/mnt/secure/staging目录\*/ if (Fat::doMount(devicePath, "/mnt/secure/staging", false, false, 1000, 1015, 0702, true)) \{ SLOGE("%s failed to mount via VFAT (%s)\\n", devicePath, strerror(errno)); continue; \} SLOGI("Device %s, target %s mounted @ /mnt/secure/staging", devicePath, getMountpoint()); protectFromAutorunStupidity(); /\*挂载一个只有root用户能够访问的目录，这函数挂载了两次 将/mnt/secure/staging/.android\_secure 挂载在 /mnt/secure/asec， 将tmpfs 挂载在 /mnt/secure/staging/.android\_secure\*/ if (createBindMounts()) \{ SLOGE("Failed to create bindmounts (%s)", strerror(errno)); umount("/mnt/secure/staging"); setState(Volume::State\_Idle); return -1; \} /\* \* Now that the bindmount trickery is done, atomically move the \* whole subtree to expose it to non priviledged users. \*/ /\*将挂载点挂载的目录再挂载到最终的目录/mnt/sdcard\*/ if (doMoveMount("/mnt/secure/staging", getMountpoint(), false)) \{ SLOGE("Failed to move mount (%s)", strerror(errno)); umount("/mnt/secure/staging"); setState(Volume::State\_Idle); return -1; \} setState(Volume::State\_Mounted); mLastMountedKdev = mCurrentlyMountedKdev = deviceNodes\[i\]; return 0; \} SLOGE("Volume %s found no suitable devices for mounting :(\\n", getLabel()); setState(Volume::State\_Idle); return -1; \}  
这个挂载函数看起来，会发现很繁琐，好几个目录的挂载关系，有以下挂载目录：  
/dev/block/vold/8:1 挂载在-> /mnt/secure/staging  
/mnt/secure/staging/.android\_secure 挂载在-> /mnt/secure/asec  
tmpfs 挂载在-> /mnt/secure/staging/.android\_secure  
/mnt/secure/staging 挂载在-> /mnt/sdcard  
从程序的注释看，这样的目的是挂载一个只有root用户能查看的目录，具体还是没搞清楚谷歌为什么要这样挂载，  
还是有疑问，希望有清楚的高手指点一下。  
sd卡的挂载比较清楚，中间多了一个中介，将设备节点8:1挂载在/mnt/secure/staging,最后又将该目录挂载在/mnt/sdcard，  
这目录就是最终用户能够看到文件的目录。  
函数里面涉及到几个函数：  
getDeviceNodes函数获取挂载设备的设备号与分区数量，是Volume类的一个纯虚函数，在子类DirectVolume中实现，源码：

int DirectVolume::getDeviceNodes(dev\_t \*devs, int max) \{ if (mPartIdx == -1) \{ // If the disk has no partitions, try the disk itself if (!mDiskNumParts) \{ devs\[0\] = MKDEV(mDiskMajor, mDiskMinor); return 1; \} int i; for (i = 0; i < mDiskNumParts; i++) \{ if (i == max) break; devs\[i\] = MKDEV(mDiskMajor, mPartMinors\[i\]); \} return mDiskNumParts; \} devs\[0\] = MKDEV(mDiskMajor, mPartMinors\[mPartIdx -1\]); return 1; \}  
下面贴一些mountVol里面挂载的源码：

int Fat::doMount(const char \*fsPath, const char \*mountPoint, bool ro, bool remount, int ownerUid, int ownerGid, int permMask, bool createLost) \{ int rc; unsigned long flags; char mountData\[255\]; flags = MS\_NODEV | MS\_NOEXEC | MS\_NOSUID | MS\_DIRSYNC; flags |= (ro ? MS\_RDONLY : 0); flags |= (remount ? MS\_REMOUNT : 0); /\* \* Note: This is a temporary hack. If the sampling profiler is enabled, \* we make the SD card world-writable so any process can write snapshots. \* \* TODO: Remove this code once we have a drop box in system\_server. \*/ char value\[PROPERTY\_VALUE\_MAX\]; property\_get("persist.sampling\_profiler", value, ""); if (value\[0\] == '1') \{ SLOGW("The SD card is world-writable because the" " 'persist.sampling\_profiler' system property is set to '1'."); permMask = 0; \} sprintf(mountData, "utf8,uid=%d,gid=%d,fmask=%o,dmask=%o,shortname=mixed", ownerUid, ownerGid, permMask, permMask); rc = mount(fsPath, mountPoint, "vfat", flags, mountData); if (rc && errno == EROFS) \{ SLOGE("%s appears to be a read only filesystem - retrying mount RO", fsPath); flags |= MS\_RDONLY; rc = mount(fsPath, mountPoint, "vfat", flags, mountData); \} if (rc == 0 && createLost) \{ char \*lost\_path; asprintf(&lost\_path, "%s/LOST.DIR", mountPoint); if (access(lost\_path, F\_OK)) \{ /\* \* Create a LOST.DIR in the root so we have somewhere to put \* lost cluster chains (fsck\_msdos doesn't currently do this) \*/ if (mkdir(lost\_path, 0755)) \{ SLOGE("Unable to create LOST.DIR (%s)", strerror(errno)); \} \} free(lost\_path); \} return rc; \} int Volume::createBindMounts() \{ unsigned long flags; /\* \* Rename old /android\_secure -> /.android\_secure \*/ if (!access("/mnt/secure/staging/android\_secure", R\_OK | X\_OK) && access(SEC\_STG\_SECIMGDIR, R\_OK | X\_OK)) \{ if (rename("/mnt/secure/staging/android\_secure", SEC\_STG\_SECIMGDIR)) \{ SLOGE("Failed to rename legacy asec dir (%s)", strerror(errno)); \} \} /\* \* Ensure that /android\_secure exists and is a directory \*/ if (access(SEC\_STG\_SECIMGDIR, R\_OK | X\_OK)) \{ if (errno == ENOENT) \{ if (mkdir(SEC\_STG\_SECIMGDIR, 0777)) \{ SLOGE("Failed to create %s (%s)", SEC\_STG\_SECIMGDIR, strerror(errno)); return -1; \} \} else \{ SLOGE("Failed to access %s (%s)", SEC\_STG\_SECIMGDIR, strerror(errno)); return -1; \} \} else \{ struct stat sbuf; if (stat(SEC\_STG\_SECIMGDIR, &sbuf)) \{ SLOGE("Failed to stat %s (%s)", SEC\_STG\_SECIMGDIR, strerror(errno)); return -1; \} if (!S\_ISDIR(sbuf.st\_mode)) \{ SLOGE("%s is not a directory", SEC\_STG\_SECIMGDIR); errno = ENOTDIR; return -1; \} \} /\* \* Bind mount /mnt/secure/staging/android\_secure -> /mnt/secure/asec so we'll \* have a root only accessable mountpoint for it. \*/ if (mount(SEC\_STG\_SECIMGDIR, SEC\_ASECDIR, "", MS\_BIND, NULL)) \{ SLOGE("Failed to bind mount points %s -> %s (%s)", SEC\_STG\_SECIMGDIR, SEC\_ASECDIR, strerror(errno)); return -1; \} /\* \* Mount a read-only, zero-sized tmpfs on <mountpoint>/android\_secure to \* obscure the underlying directory from everybody - sneaky eh? ;) \*/ if (mount("tmpfs", SEC\_STG\_SECIMGDIR, "tmpfs", MS\_RDONLY, "size=0,mode=000,uid=0,gid=0")) \{ SLOGE("Failed to obscure %s (%s)", SEC\_STG\_SECIMGDIR, strerror(errno)); umount("/mnt/asec\_secure"); return -1; \} return 0; \} int Volume::doMoveMount(const char \*src, const char \*dst, bool force) \{ unsigned int flags = MS\_MOVE; int retries = 5; while(retries--) \{ if (!mount(src, dst, "", flags, NULL)) \{ if (mDebug) \{ SLOGD("Moved mount %s -> %s sucessfully", src, dst); \} return 0; \} else if (errno != EBUSY) \{ SLOGE("Failed to move mount %s -> %s (%s)", src, dst, strerror(errno)); return -1; \} int action = 0; if (force) \{ if (retries == 1) \{ action = 2; // SIGKILL \} else if (retries == 2) \{ action = 1; // SIGHUP \} \} SLOGW("Failed to move %s -> %s (%s, retries %d, action %d)", src, dst, strerror(errno), retries, action); Process::killProcessesWithOpenFiles(src, action); usleep(1000\*250); \} errno = EBUSY; SLOGE("Giving up on move %s -> %s (%s)", src, dst, strerror(errno)); return -1; \}  
篇幅有点长了，挂载点也挂载了好几个，mountVol处理的事情最多的，也最繁琐，但都是简单的，主要是有点摸不着头脑，  
谷歌为啥要这样挂载，也许自己太菜了。。  
下一篇文章继续。。