FUSE 用戶空間文件系統 （Filesystem in Userspace）

FUSE 倉庫 

Wiki

FUSE 性能評測

 關於Fuse文件系統：

FUSE (Filesystem in Userspace) is an interface for userspace programs to export a filesystem to the Linux kernel. The FUSE project consists of two components: the fuse kernel module (maintained in the regular kernel repositories) and the libfuseuserspace library (maintained in this repository). libfuse provides the reference implementation for communicating with the FUSE kernel module.

A FUSE file system is typically implemented as a standalone application that links with libfuse. libfuse provides functions to mount the file system, unmount it, read requests from the kernel, and send responses back. libfuse offers two APIs: a "high-level", synchronous API, and a "low-level" asynchronous API. In both cases, incoming requests from the kernel are passed to the main program using callbacks. When using the high-level API, the callbacks may work with file names and paths instead of inodes, and processing of a request finishes when the callback function returns. When using the low-level API, the callbacks must work with inodes and responses must be sent explicitly using a separate set of API functions.

 FUSE 巨集觀調用：

以open為例，整個調用的過程如下：

1-      用戶態app調用glibc open介面，觸發sys_open系統調用。

2-      sys_open 調用fuse中inode節點定義的open方法。

3-      inode中open生成一個request消息，並通過/dev/fuse發送request消息到用戶態libfuse。

4-      Libfuse調用fuse_application用戶自定義的open的方法，並將返回值通過/dev/fuse通知給內核。

5-      內核收到request消息的處理完成的喚醒，並將結果放回給VFS系統調用結果。

6- 用戶態app收到open的返回結果。

  FUSE 包含兩個大的發行版本：fuse2 和 fuse3 ,那麼fuse2 和 fuse3之間有什麼區別呢？（What exactly is the difference between fuse2 and fuse3?）

FUSE 安裝

FUSE 安裝分為兩種方式，一種是 編譯安裝，另外一張是通過包管理軟體（YUM）進行安裝。編譯安裝按照官方教程安裝即可。

我使用的操作系統是 Fedora 29, 所以嘗試使用yum進行安裝：

首先使用rpm 命令查一下當前系統是否已經安裝了fuse:

[root@docker 4.19.13-300.fc29.x86_64]# rpm -aq | grep fuse
fuse-common-3.2.3-14.fc29.x86_64
fuse-2.9.7-14.fc29.x86_64
fuse-libs-2.9.7-14.fc29.x86_64
glusterfs-fuse-5.2-1.fc29.x86_64
gvfs-fuse-1.38.1-1.fc29.x86_64
zfs-fuse-0.7.2.2-6.fc27.x86_64

可見已經安裝好了，當前系統中使用的是fuse的2.9.7版本，並且是有相應的庫。所以算是已經安裝好了。如果系統中沒有上述安裝包，可以使用yum進行安裝。

[root@docker 4.19.13-300.fc29.x86_64]# yum search fuse
Last metadata expiration check: 1:10:16 ago on Sat 12 Jan 2019 10:13:49 PM CST.
============================================================================ Summary & Name Matched: fuse ============================================================================
fuse.x86_64 : File System in Userspace (FUSE) v2 utilities
fuse.x86_64 : File System in Userspace (FUSE) v2 utilities
fuse-libs.x86_64 : File System in Userspace (FUSE) v2 libraries
fuse-libs.i686 : File System in Userspace (FUSE) v2 libraries
fuse-libs.x86_64 : File System in Userspace (FUSE) v2 libraries
fuse3-libs.i686 : File System in Userspace (FUSE) v3 libraries
fuse3-libs.x86_64 : File System in Userspace (FUSE) v3 libraries
fuse-devel.i686 : File System in Userspace (FUSE) v2 devel files
fuse-devel.x86_64 : File System in Userspace (FUSE) v2 devel files
fuse3-devel.i686 : File System in Userspace (FUSE) v3 devel files
fuse3-devel.x86_64 : File System in Userspace (FUSE) v3 devel files
fuse-common.x86_64 : Common files for File System in Userspace (FUSE) v2 and v3
fuse-common.x86_64 : Common files for File System in Userspace (FUSE) v2 and v3

通過自己的需求，安裝對應版本的安裝包：fuse-common.x86_64 這個軟體包在v2和v3兩個版本中都能進行使用。當然你也可以查看包里的內容，進一步瞭解fuse的組成。

fuse 的內核模塊也已經內置到操作系統內核之中了：

[root@docker fuse]# pwd
/lib/modules/4.19.10-300.fc29.x86_64/kernel/fs/fuse
[root@docker fuse]# ls
fuse.ko.xz

為了能夠開發屬於自己的文件系統，還要安裝fuse開發包：

fuse-devel.x86_64 : File System in Userspace (FUSE) v2 devel files

Name         : fuse-devel
Version      : 2.9.7
Release      : 14.fc29
Arch         : x86_64
Size         : 36 k
Source       : fuse-2.9.7-14.fc29.src.rpm
Repo         : fedora
Summary      : File System in Userspace (FUSE) v2 devel files
URL          : http://fuse.sf.net
License      : LGPLv2+
Description  : With FUSE it is possible to implement a fully functional filesystem in a
             : userspace program. This package contains development files (headers,
             : pgk-config) to develop FUSE v2 based applications/filesystems.

我們從包描述可以看到，包中包括了一些開發所需要一些頭文件, 動態鏈接庫（.so）,pgk-config 文件等。

 FUSE 簡單使用（DEMO）

 high-level API:

/*
  FUSE: Filesystem in Userspace
  Copyright (C) 2001-2007  Miklos Szeredi <[email protected]>

  
  This program can be distributed under the terms of the GNU GPL.
  See the file COPYING.

  gcc -Wall hello.c `pkg-config fuse --cflags --libs` -o hello
*/

#define FUSE_USE_VERSION 26 //先定義, fuse.h中有判斷

#include <fuse.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>

static const char *hello_str = "Hello World!\n";
static const char *hello_path = "/hello";

// 與函數stat()類似，用於得到文件屬性，並將其存入到結構體struct stat當中 struct stat *stbuf
static int hello_getattr(const char *path, struct stat *stbuf)
{
    int res = 0;

    memset(stbuf, 0, sizeof(struct stat));  // 使用memset進行初始化結構體
    if (strcmp(path, "/") == 0) {
        stbuf->st_mode = S_IFDIR | 0755; // S_IFDIR 用於說明 / 為目錄
        stbuf->st_nlink = 2;
    } else if (strcmp(path, hello_path) == 0) {
        stbuf->st_mode = S_IFREG | 0444; // S_IFREG 用於說明/hello 為常規文件
        stbuf->st_nlink = 1;
        stbuf->st_size = strlen(hello_str); // 設置文件長度為hello_str的長度
    } else
        res = -ENOENT; // 返回錯誤信息，沒有該文件或者目錄

    return res; // 成功執行的時候，此函數返回值為 0
}

// 該函數用於讀取目錄中的內容，併在/目錄下增加了. .. hello 三個目錄項
static int hello_readdir(const char *path, void *buf, fuse_fill_dir_t filler,
             off_t offset, struct fuse_file_info *fi)
{
    (void) offset;
    (void) fi;

    if (strcmp(path, "/") != 0)
        return -ENOENT;

    /* fill, 其作用是在readdir函數中增加一個目錄項
    typedef int (*fuse_fill_dir_t) (void *buf, const char *name,
        const struct stat *stbuf, off_t off);
    */

    filler(buf, ".", NULL, 0);
    filler(buf, "..", NULL, 0);
    filler(buf, hello_path + 1, NULL, 0); //指針+1(/hello), 即增加 hello 目錄項，去掉前面的'/'

    return 0;
}

// 打開文件函數
static int hello_open(const char *path, struct fuse_file_info *fi)
{
    if (strcmp(path, hello_path) != 0)
        return -ENOENT;

    if ((fi->flags & 3) != O_RDONLY)
        return -EACCES;

    return 0;
}

// 讀文件函數
static int hello_read(const char *path, char *buf, size_t size, off_t offset,
              struct fuse_file_info *fi)
{
    size_t len;
    (void) fi;
    if(strcmp(path, hello_path) != 0)
        return -ENOENT;

    len = strlen(hello_str);
    if (offset < len) {
        if (offset + size > len)
            size = len - offset;
        memcpy(buf, hello_str + offset, size);
    } else
        size = 0;

    return size;
}

// 註冊自定義函數
static struct fuse_operations hello_oper = {
    .getattr    = hello_getattr,
    .readdir    = hello_readdir,
    .open        = hello_open,
    .read        = hello_read, // 讀文件函數
};

// 調用 fuse_main , 把控制權交給了fuse
int main(int argc, char *argv[])
{
    return fuse_main(argc, argv, &hello_oper, NULL);
}

執行：

掛載：
[root@docker example]# mkdir /tmp/fuse
[root@docker fuse-example]# ./hello /tmp/fuse/
[root@docker fuse-example]# cd /tmp/fuse/

讀取文件屬性：
[root@docker fuse]# ll
total 0
-r--r--r--. 1 root root 13 Jan  1  1970 hello
讀取目錄屬性：
[root@docker tmp]# ll /tmp/
total 4
drwxr-xr-x. 2 root root   0 Jan  1  1970 fuse

卸載：
[root@docker tmp]# fusermount -u /tmp/fuse 
[root@docker tmp]# ls /tmp/fuse/

當然你也可以修改代碼，對fuse的特性進一步嘗試。

low-level API: low-level api 相對於 high-level api 來說，有更好的自由度；high-level相對於 low-level api 則有更簡單的api使用，如果閱讀過fuse源碼過後，使用low-level api將使得可控性更強。下麵是hello.c 的low-level api 版本

/*
  FUSE: Filesystem in Userspace
  Copyright (C) 2001-2007  Miklos Szeredi <[email protected]>

  This program can be distributed under the terms of the GNU GPL.
  See the file COPYING.

  gcc -Wall hello_ll.c `pkg-config fuse --cflags --libs` -o hello_ll
*/

#define FUSE_USE_VERSION 26

#include <fuse_lowlevel.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <assert.h>

static const char *hello_str = "Hello World!\n";
static const char *hello_name = "hello";

static int hello_stat(fuse_ino_t ino, struct stat *stbuf)
{
    stbuf->st_ino = ino;
    switch (ino) {
    case 1:
        stbuf->st_mode = S_IFDIR | 0755;
        stbuf->st_nlink = 2;
        break;

    case 2:
        stbuf->st_mode = S_IFREG | 0444;
        stbuf->st_nlink = 1;
        stbuf->st_size = strlen(hello_str);
        break;

    default:
        return -1;
    }
    return 0;
}

static void hello_ll_getattr(fuse_req_t req, fuse_ino_t ino,
                 struct fuse_file_info *fi)
{
    struct stat stbuf;

    (void) fi;

    memset(&stbuf, 0, sizeof(stbuf));
    if (hello_stat(ino, &stbuf) == -1)
        fuse_reply_err(req, ENOENT);
    else
        fuse_reply_attr(req, &stbuf, 1.0);
}

static void hello_ll_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
{
    struct fuse_entry_param e;

    if (parent != 1 || strcmp(name, hello_name) != 0)
        fuse_reply_err(req, ENOENT);
    else {
        memset(&e, 0, sizeof(e));
        e.ino = 2;
        e.attr_timeout = 1.0;
        e.entry_timeout = 1.0;
        hello_stat(e.ino, &e.attr);

        fuse_reply_entry(req, &e);
    }
}

struct dirbuf {
    char *p;
    size_t size;
};

static void dirbuf_add(fuse_req_t req, struct dirbuf *b, const char *name,
               fuse_ino_t ino)
{
    struct stat stbuf;
    size_t oldsize = b->size;
    b->size += fuse_add_direntry(req, NULL, 0, name, NULL, 0);
    b->p = (char *) realloc(b->p, b->size);
    memset(&stbuf, 0, sizeof(stbuf));
    stbuf.st_ino = ino;
    fuse_add_direntry(req, b->p + oldsize, b->size - oldsize, name, &stbuf,
              b->size);
}

#define min(x, y) ((x) < (y) ? (x) : (y))

static int reply_buf_limited(fuse_req_t req, const char *buf, size_t bufsize,
                 off_t off, size_t maxsize)
{
    if (off < bufsize)
        return fuse_reply_buf(req, buf + off,
                      min(bufsize - off, maxsize));
    else
        return fuse_reply_buf(req, NULL, 0);
}

// 讀取文件夾目錄
static void hello_ll_readdir(fuse_req_t req, fuse_ino_t ino, size_t size,
                 off_t off, struct fuse_file_info *fi)
{
    (void) fi;

    if (ino != 1)
        fuse_reply_err(req, ENOTDIR);
    else {
        struct dirbuf b;

        memset(&b, 0, sizeof(b));
        dirbuf_add(req, &b, ".", 1);
        dirbuf_add(req, &b, "..", 1);
        dirbuf_add(req, &b, hello_name, 2);
        reply_buf_limited(req, b.p, b.size, off, size);
        free(b.p);
    }
}

// 處理打開操作函數，檢查許可權
static void hello_ll_open(fuse_req_t req, fuse_ino_t ino,
              struct fuse_file_info *fi)
{
    if (ino != 2)
        fuse_reply_err(req, EISDIR);
    else if ((fi->flags & 3) != O_RDONLY)
        fuse_reply_err(req, EACCES);
    else
        fuse_reply_open(req, fi);
}

// 讀取函數 返回 hello_str 中的值
static void hello_ll_read(fuse_req_t req, fuse_ino_t ino, size_t size,
              off_t off, struct fuse_file_info *fi)
{
    (void) fi;

    assert(ino == 2);
    reply_buf_limited(req, hello_str, strlen(hello_str), off, size);
}

// 註冊的自定義函數
static struct fuse_lowlevel_ops hello_ll_oper = {
    .lookup        = hello_ll_lookup,
    .getattr    = hello_ll_getattr,
    .readdir    = hello_ll_readdir,
    .open        = hello_ll_open,
    .read        = hello_ll_read,
};

int main(int argc, char *argv[])
{
    // FUSE_ARGS_INIT 本身是一個巨集函數，用於生成 struct fuse_args 結構體， 結構體在源代碼 fuse_opt.h 文件建中定義；
    struct fuse_args args = FUSE_ARGS_INIT(argc, argv); 
    
    // A communication channel, providing hooks for sending and receiving* messages
    struct fuse_chan *ch; // 與 fuse 中 session 相關, 在源代碼 fuse_session.c 中進行定義 一個channel 

    char *mountpoint; // 一個字元串指針
    int err = -1; // 錯誤返回值，預設的返回值為 -1

    // fuse_parse_cmdline 是解析命令行參數的一種工具類，他可以解析fuse本身自身的選項，第一個出現的非選項參數被認為是掛載點
    // 如果出現多個非選項參數，則認為是錯誤！
    if (fuse_parse_cmdline(&args, &mountpoint, NULL, NULL) != -1 &&
        // 掛載， 調用了底層的 fuse_mount_common 函數
        (ch = fuse_mount(mountpoint, &args)) != NULL) {
        struct fuse_session *se;

        // 創建一個session
        se = fuse_lowlevel_new(&args, &hello_ll_oper,
                       sizeof(hello_ll_oper), NULL);
        if (se != NULL) {
            if (fuse_set_signal_handlers(se) != -1) {
                fuse_session_add_chan(se, ch);
                err = fuse_session_loop(se); // 處理信息
                fuse_remove_signal_handlers(se);
                fuse_session_remove_chan(ch);
            }
            fuse_session_destroy(se);
        }
        fuse_unmount(mountpoint, ch);
    }
    // 釋放參數占用的空間
    fuse_opt_free_args(&args);

    return err ? 1 : 0;
}

執行：（同樣執行和High-level API 相同的操作進行驗證）

 上面第二個代碼，我只註釋了很少一部分，因為我還有一部分源碼沒有看懂，為此我專門寫了另外一篇文章來記錄我對fuse 示例代碼 的註釋，等看懂後陸續更新對應註釋，希望能幫助你。

FUSE 文件系統 example部分 源碼註釋 (libfuse 2.9.9)

資源來源自網路，保持更新，轉載請註明出處。https://www.cnblogs.com/xuyaowen/p/fuse.html