android4.4 的時候vold，也是利用fuse文件系統達到，將sd卡的目錄（storage目錄）獲取sd實際掛載目錄（mnt/media_rw）的權限。但是android4.4的時候vold只是寫屬性而已，然后init監測這個屬性，屬性改變時，才會去啟動sdcard進程。
然后android6.0直接在vold中，fork一個進程直接開啟sdcard進程掛載fuse文件系統。并且在卸載sd的時候，在vold中卸載fuse文件系統。
一、掛載sd卡
下面是解析android6.0vold，掛載sd卡是的一段代碼，我們來看下。顯示掛載sd卡，然后進行fuse操作。
if (vfat::Mount(mDevPath, mRawPath, false, false, false,// 掛載sd卡 AID_MEDIA_RW, AID_MEDIA_RW, 0007, true)) { PLOG(ERROR) << getId() << " failed to mount " << mDevPath; return -EIO; } if (getMountFlags() & MountFlags::kPrimary) { initAsecStage(); } if (!(getMountFlags() & MountFlags::kVisible)) { // Not visible to apps, so no need to spin up FUSE return OK; } //creat dir by fuse before fuse action. if (fs_prepare_dir(mFuseDefault.c_str(), 0700, AID_ROOT, AID_ROOT) || fs_prepare_dir(mFuseRead.c_str(), 0700, AID_ROOT, AID_ROOT) || fs_prepare_dir(mFuseWrite.c_str(), 0700, AID_ROOT, AID_ROOT)) { PLOG(ERROR) << getId() << " failed to create fuse points"; return -errno; } dev_t before = GetDevice(mFuseWrite); if (!(mFusePid = fork())) { if (getMountFlags() & MountFlags::kPrimary) { if (execl(kFusePath, kFusePath,//fuse操作 "-u", "1023", // AID_MEDIA_RW "-g", "1023", // AID_MEDIA_RW "-U", std::to_string(getMountUserId()).c_str(), "-w", mRawPath.c_str(), stableName.c_str(), NULL)) { PLOG(ERROR) << "Failed to exec"; } } else { if (execl(kFusePath, kFusePath, "-u", "1023", // AID_MEDIA_RW "-g", "1023", // AID_MEDIA_RW "-U", std::to_string(getMountUserId()).c_str(), mRawPath.c_str(), stableName.c_str(), NULL)) { PLOG(ERROR) << "Failed to exec"; } }我們再來看看fuse的代碼，也就是在sdcard中。先在main函數中獲取數據，
int main(int argc, char **argv) { const char *source_path = NULL; const char *label = NULL; uid_t uid = 0; gid_t gid = 0; userid_t userid = 0; bool multi_user = false; bool full_write = false; int i; struct rlimit rlim; int fs_version; int opt; while ((opt = getopt(argc, argv, "u:g:U:mw")) != -1) { switch (opt) { case 'u': uid = strtoul(optarg, NULL, 10); break; case 'g': gid = strtoul(optarg, NULL, 10); break; case 'U': userid = strtoul(optarg, NULL, 10); break; case 'm': multi_user = true; break; case 'w': full_write = true; break; case '?': default: return usage(); } } for (i = optind; i < argc; i++) { char* arg = argv[i]; if (!source_path) { source_path = arg; } else if (!label) { label = arg; } else { ERROR("too many arguments/n"); return usage(); } } if (!source_path) { ERROR("no source path specified/n"); return usage(); } if (!label) { ERROR("no label specified/n"); return usage(); } if (!uid || !gid) { ERROR("uid and gid must be nonzero/n"); return usage(); } rlim.rlim_cur = 8192; rlim.rlim_max = 8192; if (setrlimit(RLIMIT_NOFILE, &rlim)) { ERROR("Error setting RLIMIT_NOFILE, errno = %d/n", errno); } while ((fs_read_atomic_int("/data/.layout_version", &fs_version) == -1) || (fs_version < 3)) { ERROR("installd fs upgrade not yet complete. Waiting.../n"); sleep(1); } ERROR("kangchen sdcard path:%s label:%s/n", source_path, label); run(source_path, label, uid, gid, userid, multi_user, full_write); return 1;}其中source_path就是sd卡實際掛載的地址，然后調用run函數，run函數中進行了一些初始化，然后掛載了default，read，write 3個fuse文件系統，后面又開啟3個線程處理這3個文件系統的read，write，open等處理。
static void run(const char* source_path, const char* label, uid_t uid, gid_t gid, userid_t userid, bool multi_user, bool full_write) { struct fuse_global global; struct fuse fuse_default; struct fuse fuse_read; struct fuse fuse_write; struct fuse_handler handler_default; struct fuse_handler handler_read; struct fuse_handler handler_write; pthread_t thread_default; pthread_t thread_read; pthread_t thread_write; memset(&global, 0, sizeof(global)); memset(&fuse_default, 0, sizeof(fuse_default)); memset(&fuse_read, 0, sizeof(fuse_read)); memset(&fuse_write, 0, sizeof(fuse_write)); memset(&handler_default, 0, sizeof(handler_default)); memset(&handler_read, 0, sizeof(handler_read)); memset(&handler_write, 0, sizeof(handler_write)); pthread_mutex_init(&global.lock, NULL); global.package_to_appid = hashmapCreate(256, str_hash, str_icase_equals); global.uid = uid; global.gid = gid; global.multi_user = multi_user; global.next_generation = 0; global.inode_ctr = 1; memset(&global.root, 0, sizeof(global.root)); global.root.nid = FUSE_ROOT_ID; /* 1 */ global.root.refcount = 2; global.root.namelen = strlen(source_path); global.root.name = strdup(source_path); global.root.userid = userid; global.root.uid = AID_ROOT; global.root.under_android = false; strcpy(global.source_path, source_path); if (multi_user) { global.root.perm = PERM_PRE_ROOT; snprintf(global.obb_path, sizeof(global.obb_path), "%s/obb", source_path); } else { global.root.perm = PERM_ROOT; snprintf(global.obb_path, sizeof(global.obb_path), "%s/Android/obb", source_path); } fuse_default.global = &global; fuse_read.global = &global; fuse_write.global = &global; global.fuse_default = &fuse_default; global.fuse_read = &fuse_read; global.fuse_write = &fuse_write; snprintf(fuse_default.dest_path, PATH_MAX, "/mnt/runtime/default/%s", label); snprintf(fuse_read.dest_path, PATH_MAX, "/mnt/runtime/read/%s", label); snprintf(fuse_write.dest_path, PATH_MAX, "/mnt/runtime/write/%s", label); handler_default.fuse = &fuse_default; handler_read.fuse = &fuse_read; handler_write.fuse = &fuse_write; handler_default.token = 0; handler_read.token = 1; handler_write.token = 2; umask(0); if (multi_user) { /* Multi-user storage is fully isolated per user, so "other" * permissions are completely masked off. */ if (fuse_setup(&fuse_default, AID_SDCARD_RW, 0006) || fuse_setup(&fuse_read, AID_EVERYBODY, 0027) || fuse_setup(&fuse_write, AID_EVERYBODY, full_write ? 0007 : 0027)) { ERROR("failed to fuse_setup/n"); exit(1); } } else { /* Physical storage is readable by all users on device, but * the Android directories are masked off to a single user * deep inside attr_from_stat(). */ if (fuse_setup(&fuse_default, AID_SDCARD_RW, 0006) || fuse_setup(&fuse_read, AID_EVERYBODY, full_write ? 0027 : 0022) || fuse_setup(&fuse_write, AID_EVERYBODY, full_write ? 0007 : 0022)) { ERROR("failed to fuse_setup/n"); exit(1); } } /* Drop privs */ if (setgroups(sizeof(kGroups) / sizeof(kGroups[0]), kGroups) < 0) { ERROR("cannot setgroups: %s/n", strerror(errno)); exit(1); } if (setgid(gid) < 0) { ERROR("cannot setgid: %s/n", strerror(errno)); exit(1); } if (setuid(uid) < 0) { ERROR("cannot setuid: %s/n", strerror(errno)); exit(1); } if (multi_user) { fs_prepare_dir(global.obb_path, 0775, uid, gid); } if (pthread_create(&thread_default, NULL, start_handler, &handler_default) || pthread_create(&thread_read, NULL, start_handler, &handler_read) || pthread_create(&thread_write, NULL, start_handler, &handler_write)) { ERROR("failed to pthread_create/n"); exit(1); } watch_package_list(&global); ERROR("terminated prematurely/n"); exit(1);}其中在fuse_setup中掛載了fuse文件系統
static int fuse_setup(struct fuse* fuse, gid_t gid, mode_t mask) { char opts[256]; fuse->fd = open("/dev/fuse", O_RDWR); if (fuse->fd == -1) { ERROR("failed to open fuse device: %s/n", strerror(errno)); return -1; } umount2(fuse->dest_path, MNT_DETACH); snprintf(opts, sizeof(opts), "fd=%i,rootmode=40000,default_permissions,allow_other,user_id=%d,group_id=%d", fuse->fd, fuse->global->uid, fuse->global->gid); if (mount("/dev/fuse", fuse->dest_path, "fuse", MS_NOSUID | MS_NODEV | MS_NOEXEC | MS_NOATIME, opts) != 0) { ERROR("failed to mount fuse filesystem: %s/n", strerror(errno)); return -1; } fuse->gid = gid; fuse->mask = mask; return 0;}但是雖然掛載了default，read，write 3個fuse文件系統。
但好像只有default有用，因為在init.rc中將default目錄直接掛載到了storage，而應用也就只能拿到storage目錄，所以只有default的fuse實際有用。
on post-fs start logd #add for amt chmod 0755 /amt # once everything is setup, no need to modify / mount rootfs rootfs / ro remount # Mount shared so changes propagate into child namespaces mount rootfs rootfs / shared rec # Mount default storage into root namespace mount none /mnt/runtime/default /storage slave bind rec
二、sd卡卸載過程
然后我們再來看看android卸載sd卡的過程，卸載sd卡的時候，是先卸載了fuse文件系統，然后在卸載了sd卡的mount地址。
status_t PublicVolume::doUnmount() { if (mFusePid > 0) { kill(mFusePid, SIGTERM); TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, 0)); mFusePid = 0; } ForceUnmount(kAsecPath); ForceUnmount(mFuseDefault); ForceUnmount(mFuseRead); ForceUnmount(mFuseWrite); ForceUnmount(mRawPath); rmdir(mFuseDefault.c_str()); rmdir(mFuseRead.c_str()); rmdir(mFuseWrite.c_str()); rmdir(mRawPath.c_str()); mFuseDefault.clear(); mFuseRead.clear(); mFuseWrite.clear(); mRawPath.clear(); return OK;}總所周知，卸載sd卡mount地址的時候，會去檢查哪些進程在使用sd卡中的文件。
如何檢查呢？是通過proc/pid下面各個文件的軟鏈接，然后通過readlink找到真正的文件地址，來判定是否正在占用sd卡中的文件。
但是在卸載fuse文件系統的時候，比如你有進程在操作sd卡中的文件，這個時候操作sd卡的storage目錄會fuse到sd卡真正的掛載地址上，實際上fuse文件系統是在工作的，導致不能卸載。
但是這個時候去查找誰占用fuse文件又是查不出來的，因為是進程在操作sd卡文件，會導致fuse文件系統的操作，才會卸載不掉fuse文件系統。但是能找到占用的文件只能是sd卡的。
而且實際中也碰到這樣的問題，所以個人認為應該先kill正在使用sd卡的進程，然后再卸載fuse文件系統。這樣就不會有進程操作sd卡中的文件的時候，導致fuse文件系統也在忙而卸載不掉了。我碰到的問題是：一個如下進程占用的sd卡文件
root@lte26007:/proc/2365/fd # ls -llrwx------ root radio 2016-05-25 13:42 0 -> /dev/nulllrwx------ root radio 2016-05-25 13:42 1 -> /dev/nulllrwx------ root radio 2016-05-25 13:42 10 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/test_20160525_134208_00.bin_last_0lrwx------ root radio 2016-05-25 13:42 11 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/log_up_data.datlrwx------ root radio 2016-05-25 13:42 12 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/test_20160525_134209_head.binlrwx------ root radio 2016-05-25 13:42 13 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/test_20160525_134209_00.bin_last_0lrwx------ root radio 2016-05-25 13:42 14 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/log_up_data.datlrwx------ root radio 2016-05-25 13:42 15 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/test_20160525_134209_head.binlrwx------ root radio 2016-05-25 13:42 16 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/test_20160525_134209_00.bin_last_0lrwx------ root radio 2016-05-25 13:42 17 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/log_up_data.datlrwx------ root radio 2016-05-25 13:42 18 -> /dev/lmi10lrwx------ root radio 2016-05-25 13:42 19 -> /dev/TPC0lrwx------ root radio 2016-05-25 13:42 2 -> /dev/nulllrwx------ root radio 2016-05-25 13:42 20 -> /dev/modemlrwx------ root radio 2016-05-25 13:42 21 -> /dev/TPC1lrwx------ root radio 2016-05-25 13:42 22 -> /dev/modemlrwx------ root radio 2016-05-25 13:42 23 -> /dev/lmi9lrwx------ root radio 2016-05-25 13:42 24 -> socket:[14761]lrwx------ root radio 2016-05-25 13:42 26 -> socket:[14764]lrwx------ root radio 2016-05-25 13:42 3 -> socket:[15482]lrwx------ root radio 2016-05-25 13:42 4 -> /tmp/lc-elog.pidlrwx------ root radio 2016-05-25 13:42 5 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/test_20160525_134208_head.binlrwx------ root radio 2016-05-25 13:42 6 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/test_20160525_134208_00.bin_last_0lrwx------ root radio 2016-05-25 13:42 7 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/log_up_data.datlrwx------ root radio 2016-05-25 13:42 8 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/test_20160525_134208_head.binlr-x------ root radio 2016-05-25 13:42 9 -> /dev/__properties__root@lte26007:/proc/2365/fd
至于如何kill正在使用sd卡的進程呢：
status_t PublicVolume::doUnmount() { if (mFusePid > 0) { kill(mFusePid, SIGTERM); TEMP_FAILURE_RETRY(waitpid(mFusePid, nullptr, 0)); mFusePid = 0; } ForceUnmount(kAsecPath); LOG(VERBOSE) << "start"; KillProcessesUsingPath(getPath()); LOG(VERBOSE) << "end"; ForceUnmount(mFuseDefault); ForceUnmount(mFuseRead); ForceUnmount(mFuseWrite); ForceUnmount(mRawPath); rmdir(mFuseDefault.c_str()); rmdir(mFuseRead.c_str()); rmdir(mFuseWrite.c_str()); rmdir(mRawPath.c_str()); mFuseDefault.clear(); mFuseRead.clear(); mFuseWrite.clear(); mRawPath.clear(); return OK;}可以在卸載fuse文件系統之前，調用KillProcessesUsingPath，來kill那些正在使用sd卡目錄的進程。而這個mPath路徑，如果是sd卡的話，它是storage下的目錄，而不是sd卡的mount地址。如果是otg插的sd卡的話，是sd卡的mount地址，因為otg在storage目錄下沒有目錄，只有一個mount地址訪問，也有沒有fuse。這樣問題就解決了。
以上這篇詳談android 6.0 fuse文件系統的掛載和卸載問題就是小編分享給大家的全部內容了，希望能給大家一個參考，也希望大家多多支持VEVB武林網。
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