详谈android 6.0 fuse文件系统的挂载和卸载问题

android4.4 的时候vold，也是利用fuse文件系统达到，将sd卡的目录（storage目录）获取sd实际挂载目录（mnt/media_rw）的权限。但是android4.4的时候vold只是写属性而已，然后init监测这个属性，属性改变时，才会去启动sdcard进程。

然后android6.0直接在vold中，fork一个进程直接开启sdcard进程挂载fuse文件系统。并且在卸载sd的时候，在vold中卸载fuse文件系统。

一、挂载sd卡

下面是解析android6.0vold，挂载sd卡是的一段代码，我们来看下。显示挂载sd卡，然后进行fuse操作。

 if (vfat::Mount(mDevPath, mRawPath, false, false, false,// 挂载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";
   }
  }

我们再来看看fuse的代码，也就是在sdcard中。先在main函数中获取数据，

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卡实际挂载的地址，然后调用run函数，run函数中进行了一些初始化，然后挂载了default，read，write 3个fuse文件系统，后面又开启3个线程处理这3个文件系统的read，write，open等处理。

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中挂载了fuse文件系统

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;
}

但是虽然挂载了default，read，write 3个fuse文件系统。

但好像只有default有用，因为在init.rc中将default目录直接挂载到了storage，而应用也就只能拿到storage目录，所以只有default的fuse实际有用。

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卡卸载过程

然后我们再来看看android卸载sd卡的过程，卸载sd卡的时候，是先卸载了fuse文件系统，然后在卸载了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;
}

总所周知，卸载sd卡mount地址的时候，会去检查哪些进程在使用sd卡中的文件。

如何检查呢？是通过proc/pid下面各个文件的软链接，然后通过readlink找到真正的文件地址，来判定是否正在占用sd卡中的文件。

但是在卸载fuse文件系统的时候，比如你有进程在操作sd卡中的文件，这个时候操作sd卡的storage目录会fuse到sd卡真正的挂载地址上，实际上fuse文件系统是在工作的，导致不能卸载。

但是这个时候去查找谁占用fuse文件又是查不出来的，因为是进程在操作sd卡文件，会导致fuse文件系统的操作，才会卸载不掉fuse文件系统。但是能找到占用的文件只能是sd卡的。

而且实际中也碰到这样的问题，所以个人认为应该先kill正在使用sd卡的进程，然后再卸载fuse文件系统。这样就不会有进程操作sd卡中的文件的时候，导致fuse文件系统也在忙而卸载不掉了。我碰到的问题是：一个如下进程占用的sd卡文件

root@lte26007:/proc/2365/fd # ls -l
lrwx------ root  radio    2016-05-25 13:42 0 -> /dev/null
lrwx------ root  radio    2016-05-25 13:42 1 -> /dev/null
lrwx------ root  radio    2016-05-25 13:42 10 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/test_20160525_134208_00.bin_last_0
lrwx------ root  radio    2016-05-25 13:42 11 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/log_up_data.dat
lrwx------ root  radio    2016-05-25 13:42 12 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/test_20160525_134209_head.bin
lrwx------ root  radio    2016-05-25 13:42 13 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/test_20160525_134209_00.bin_last_0
lrwx------ root  radio    2016-05-25 13:42 14 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY1/log_up_data.dat
lrwx------ root  radio    2016-05-25 13:42 15 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/test_20160525_134209_head.bin
lrwx------ root  radio    2016-05-25 13:42 16 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/test_20160525_134209_00.bin_last_0
lrwx------ root  radio    2016-05-25 13:42 17 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PFM/log_up_data.dat
lrwx------ root  radio    2016-05-25 13:42 18 -> /dev/lmi10
lrwx------ root  radio    2016-05-25 13:42 19 -> /dev/TPC0
lrwx------ root  radio    2016-05-25 13:42 2 -> /dev/null
lrwx------ root  radio    2016-05-25 13:42 20 -> /dev/modem
lrwx------ root  radio    2016-05-25 13:42 21 -> /dev/TPC1
lrwx------ root  radio    2016-05-25 13:42 22 -> /dev/modem
lrwx------ root  radio    2016-05-25 13:42 23 -> /dev/lmi9
lrwx------ 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.pid
lrwx------ root  radio    2016-05-25 13:42 5 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/test_20160525_134208_head.bin
lrwx------ root  radio    2016-05-25 13:42 6 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/test_20160525_134208_00.bin_last_0
lrwx------ root  radio    2016-05-25 13:42 7 -> /storage/2C10-0CCC/elog/elog_20160525_134206/HLS/log_up_data.dat
lrwx------ root  radio    2016-05-25 13:42 8 -> /storage/2C10-0CCC/elog/elog_20160525_134206/PHY0/test_20160525_134208_head.bin
lr-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卡的进程呢：

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;
}

可以在卸载fuse文件系统之前，调用KillProcessesUsingPath，来kill那些正在使用sd卡目录的进程。而这个mPath路径，如果是sd卡的话，它是storage下的目录，而不是sd卡的mount地址。如果是otg插的sd卡的话，是sd卡的mount地址，因为otg在storage目录下没有目录，只有一个mount地址访问，也有没有fuse。这样问题就解决了。

以上这篇详谈android 6.0 fuse文件系统的挂载和卸载问题就是小编分享给大家的全部内容了，希望能给大家一个参考，也希望大家多多支持自由互联。