Android fuse文件系统

一、fuse文件系统挂载
　　从android4.4 以来，第三方应用程序是不能再随便的访问sdcard，sdcard的权限管理是fuse 即用户空间文件系统（Filesystem in Userspace）来实现的，android 7.0版本中的直接在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。
　　在android源码system/core/sdcard/sdcard.c这么个文件。它最终被编译成为system/bin/sdcard可执行程序，这就是所谓的sdcard的守护进程。在sdcard守护进程中存在for (;;)的循环，一直从/dev/fuse这个设备中读取请求，然后交给handle_fuse_request去处理。
　　sdcard守护进程的的启动实在init.project.rc文件中：

on init# Refer to http://source.android.com/devices/tech/storage/index.html# It said, "Starting in Android 4.4, multiple external storage devices are surfaced to developers through#           Context.getExternalFilesDirs(), Context.getExternalCacheDirs(), and Context.getObbDirs().#           External storage devices surfaced through these APIs must be a semi-permanent part of the device (such as an SD card slot in a battery compartment).#           Developers expect data stored in these locations to be available over long periods of time."# Therefore, if the target doesn't support sd hot-plugging (Ex: the SD card slot in a battery compartment), we need to export SECONDARY_STORAGE in 'boot' section## export SECONDARY_STORAGE /storage/sdcard1service fuse_usbotg /system/bin/sdcard -u 1023 -g 1023 -w 1023 -d /mnt/media_rw/usbotg /storage/usbotgclass late_startdisabled

　　上面代码中/system/bin/sdcard 启动时候加了很多参数，其中：
　　　　-u 1023 表示是uid；
　　　　-g 1023 表示是gid；
　　下面分析/system/core/sdcard/sdcard.c文件，先从mian函数介绍：

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;char load_buf[20]={0};   /*Add the default val buffer*/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();}}rlim.rlim_cur = 8192;rlim.rlim_max = 8192;if (setrlimit(RLIMIT_NOFILE, &rlim)) {ERROR("Error setting RLIMIT_NOFILE, errno = %d\n", errno);}if(get_boot_mode() == NORMAL_BOOT) {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);}}
#ifdef LIMIT_SDCARD_SIZEchar *tmp=malloc(30);if (!tmp) {ERROR("malloc value fail !\n");     } else {sprintf(load_buf,"%ld", DATA_FREE_SIZE_TH_DEFAULT);internal_sdcard_free_size_threshold =strtoll(load_buf,NULL,10);//设置预留空间大小if(set_env_value(DATA_FREE_SIZE_TH_NAME,load_buf,30)) {ERROR("set %s to lk_env fail\n",DATA_FREE_SIZE_TH_NAME);}}free(tmp);
#endif//调用run函数，run函数中进行一些初始化run(source_path, label, uid, gid, userid, multi_user, full_write);return 1;
}

　　在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;//分配空间并进行初始化工作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));//初始化多线程互斥量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;//赋值给fuse.dest_path用于fuse_setup中挂载fuse文件系统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);//根据是否是multi_user,创建不同权限的fuse文件系统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);}//创建handler_default、thread_read、thread_write线程//并调用start_handler来执行相关处理工作if (pthread_create(&thread_default, NULL, start_handler, &handler_default)|| pthread_create(&thread_read, NULL, start_handler, &handler_default)|| pthread_create(&thread_write, NULL, start_handler, &handler_default)) {ERROR("failed to pthread_create\n");exit(1);}watch_package_list(&global);ERROR("terminated prematurely\n");exit(1);
}

　　在run函数中，前面执行了fuse相关结构的初始化工作，真正挂载fuse文件系统的函数是fuse_setup。

//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);//通过结构体fuse(fuse_read,fuse_write,fuse_default),来挂载不同的fuse系统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;
}

　　上面fuse_setup中先open了/dev/fuse节点，然后将创建的fuse文件系统mount挂载在dev/fuse节点上，然后分别创建线程handler_default、thread_read、thread_write并调用start_handler函数来处理相关的数据。

static void* start_handler(void* data)
{struct fuse_handler* handler = data;handle_fuse_requests(handler);return NULL;
}

　　在handle_fuse_requests中现将获取的数据进行前期处理，然后调用handle_fuse_request来对数据进行判断：

static void handle_fuse_requests(struct fuse_handler* handler)
{struct fuse* fuse = handler->fuse;for (;;) {//调用read函数从/dev/fuse中读取数据包并存放到handler->request_buffer中ssize_t len = TEMP_FAILURE_RETRY(read(fuse->fd,handler->request_buffer, sizeof(handler->request_buffer)));if (len < 0) {if (errno == ENODEV) {ERROR("[%d] someone stole our marbles!\n", handler->token);exit(2);}ERROR("[%d] handle_fuse_requests: errno=%d\n", handler->token, errno);continue;}if ((size_t)len < sizeof(struct fuse_in_header)) {ERROR("[%d] request too short: len=%zu\n", handler->token, (size_t)len);continue;}//强制转换handler->request_buffer的数据类型const struct fuse_in_header *hdr = (void*)handler->request_buffer;if (hdr->len != (size_t)len) {ERROR("[%d] malformed header: len=%zu, hdr->len=%u\n",handler->token, (size_t)len, hdr->len);continue;}const void *data = handler->request_buffer + sizeof(struct fuse_in_header);size_t data_len = len - sizeof(struct fuse_in_header);__u64 unique = hdr->unique;//调用handle_fuse_request处理kernel fuse requst请求包int res = handle_fuse_request(fuse, handler, hdr, data, data_len);/* We do not access the request again after this point because the underlying* buffer storage may have been reused while processing the request. */if (res != NO_STATUS) {if (res) {TRACE("[%d] ERROR %d\n", handler->token, res);}fuse_status(fuse, unique, res); //返回fuse request请求的处理结果}}
}

　　下面看handle_fuse_request：

static int handle_fuse_request(struct fuse *fuse, struct fuse_handler* handler,const struct fuse_in_header *hdr, const void *data, size_t data_len)
{...//打开要操作的文件获取记录文件描述符fdcase FUSE_OPEN: {  const struct fuse_open_in *req = data;return handle_open(fuse, handler, hdr, req);}//read和write都是sdcard usrspace与//kernel fuse kernel spcace之间的读、写数据的内存copy交互。case FUSE_READ: { /* read_in -> byte[] */const struct fuse_read_in *req = data;return handle_read(fuse, handler, hdr, req);}case FUSE_WRITE: { /* write_in, byte[write_in.size] -> write_out */const struct fuse_write_in *req = data;const void* buffer = (const __u8*)data + sizeof(*req);return handle_write(fuse, handler, hdr, req, buffer);}...
}

作者：frank_zyp
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