linux内核mount系统调用源码分析 http://blog.csdn.net/wugj03/article/details/41958029

@Author:  Gordon Wu

@Time : 2014/12 

0.摘要

mount是Linux很常见的命令，本文将从用户空间的命令行开始，一步一步切入到内核的源代码，解释一个文件系统是如果挂载的。本文基于linux 2.6.32

1.SYSCALL_DEFINE5, 系统调用

Linux kernel通过系统调用的方式为用户提供陷入到内核，mount的系统调用是SYSCALL_DEFINE5，位于fs/namespace.c:
主要完成把一些安装信息从用户空间拷贝到内核空间, 见代码的中文注释。

 SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
         char __user *, type, unsigned long, flags, void __user *, data)
 {
     int ret;
     char *kernel_type;
     char *kernel_dir;
     char *kernel_dev;
     unsigned long data_page;

     ret = copy_mount_string(type, &kernel_type);       // 把用户空间的挂载类型复制到内核
     if (ret < 0)
         goto out_type;

     kernel_dir = getname(dir_name);                    //通过kmem_cache_alloc从names_cachep一个PATH_MAX大小的内核空间，把用户空间的dir_name复制过去
     if (IS_ERR(kernel_dir)) {
         ret = PTR_ERR(kernel_dir);
         goto out_dir;
     }

     ret = copy_mount_string(dev_name, &kernel_dev);     //复制挂载的设备过去
     if (ret < 0)
         goto out_dev;

     ret = copy_mount_options(data, &data_page);
     if (ret < 0)
         goto out_data;

     ret = do_mount(kernel_dev, kernel_dir, kernel_type, flags,
         (void *) data_page);                           //数据准备好，开始do_mount

     free_page(data_page);
 out_data:
     kfree(kernel_dev);
 out_dev:
     putname(kernel_dir);
 out_dir:
     kfree(kernel_type);
 out_type:
     return ret;
 }

2. do_mount

验证挂载的选项Flags，以及获取并填充相应挂载目录dir_name的路径Path结构。 再根据挂载选项Flags来判断，挂载的动作。

 /*
  * Flags is a 32-bit value that allows up to 31 non-fs dependent flags to
  * be given to the mount() call (ie: read-only, no-dev, no-suid etc).
  *
  * data is a (void *) that can point to any structure up to
  * PAGE_SIZE-1 bytes, which can contain arbitrary fs-dependent
  * information (or be NULL).
  *
  * Pre-0.97 versions of mount() didn't have a flags word.
  * When the flags word was introduced its top half was required
  * to have the magic value 0xC0ED, and this remained so until 2.4.0-test9.
  * Therefore, if this magic number is present, it carries no information
  * and must be discarded.
  */
 long do_mount(char *dev_name, char *dir_name, char *type_page,
           unsigned long flags, void *data_page)
 {
     struct path path;
     int retval = 0;
     int mnt_flags = 0;

     /* Discard magic */
     if ((flags & MS_MGC_MSK) == MS_MGC_VAL)
         flags &= ~MS_MGC_MSK;

     /* Basic sanity checks */

     if (!dir_name || !*dir_name || !memchr(dir_name, 0, PAGE_SIZE))           //挂载目录的有效性， memchr是判断dir_name是否在0~PAGE_SIZE（用户空间）中
         return -EINVAL;

     if (data_page)
         ((char *)data_page)[PAGE_SIZE - 1] = 0;

     /* Default to relatime unless overriden */
     if (!(flags & MS_NOATIME))
         mnt_flags |= MNT_RELATIME;

     /* Separate the per-mountpoint flags */
     if (flags & MS_NOSUID)
         mnt_flags |= MNT_NOSUID;                                           //忽略suid和sgid位的影响
     if (flags & MS_NODEV)
         mnt_flags |= MNT_NODEV;                                         //不允许访问设备专用文件
     if (flags & MS_NOEXEC)
         mnt_flags |= MNT_NOEXEC;                                    //不允许执行程序
     if (flags & MS_NOATIME)
         mnt_flags |= MNT_NOATIME;                                  //下面三个标志是关于是否更新文件或目录的atime
     if (flags & MS_NODIRATIME)
         mnt_flags |= MNT_NODIRATIME;
     if (flags & MS_STRICTATIME)
         mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
     if (flags & MS_RDONLY)                                            //只读标志
         mnt_flags |= MNT_READONLY;
                                                                             //相应的一些标志已经备份到mnt_flags了，flags去除相应位
     flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
            MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
            MS_STRICTATIME);

     /* ... and get the mountpoint */
     retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);                  //！！根据dir_name，获取挂载目录的路径信息path
     if (retval)
         return retval;

     retval = security_sb_mount(dev_name, &path,
                    type_page, flags, data_page);
     if (retval)
         goto dput_out;
         <span style="color:#FF0000;">/*根据不同选项，进行下面五种不同的挂载*/</span>
     if (flags & MS_REMOUNT)
         retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
                     data_page);
     else if (flags & MS_BIND)
         retval = do_loopback(&path, dev_name, flags & MS_REC);
     else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
         retval = do_change_type(&path, flags);
     else if (flags & MS_MOVE)
         retval = do_move_mount(&path, dev_name);
     else
         retval = do_new_mount(&path, type_page, flags, mnt_flags,                //do_new_mount是最常用的挂载,path: 挂载目录，dev_name：挂载设备
                       dev_name, data_page);
 dput_out:
     path_put(&path);
     return retval;
 }

3. do_new_mount

利用do_kern_mount为用户空间生成一个新的挂载，并do_add_mount把新安装加入到命名空间树上

 /*
  * create a new mount for userspace and request it to be added into the
  * namespace's tree
  */  // *path: 挂载目录， *name:挂载设备，*type:挂载文件系统类型
 static int do_new_mount(struct path *path, char *type, int flags,
             int mnt_flags, char *name, void *data)
 {
     struct vfsmount *mnt;                  //包含已挂载文件系统的信息。

     if (!type)
         return -EINVAL;

     /* we need capabilities... */           // root权限，才能挂载
     if (!capable(CAP_SYS_ADMIN))
         return -EPERM;

         //内核锁
     lock_kernel();
     mnt = do_kern_mount(type, flags, name, data);        //完成mnt信息的填充。
     unlock_kernel();
     if (IS_ERR(mnt))
         return PTR_ERR(mnt);

     return do_add_mount(mnt, path, mnt_flags, NULL);     //添加到命名空间树上
 }

4. do_kern_mount

4.1 do_kern_mount函数首先会调用get_fs_type来查看内核是否注册了参数type所指的文件系统，对于内核源码下fs目录下的所有文件系统都会通过调用register_filesystem来注册这个文件系统，其实就是添加到内核文件系统链表中，get_fs_type会将参数type字符串跟内核链表中所有已经注册的文件系统结构体file_system_type的name成员向比较，如果找到，则说明内核已经注册了相应文件系统，并且返回相应文件系统注册的file_system_type结构体。后面的挂载过程需要使用到这个结构体中的成员。[1]

 struct vfsmount *
 do_kern_mount(const char *fstype, int flags, const char *name, void *data)
 {
     struct file_system_type *type = get_fs_type(fstype);    //获取挂载文件系统的类型结构，见4.2,4.3简介
     struct vfsmount *mnt;
     if (!type)
         return ERR_PTR(-ENODEV);
     mnt = vfs_kern_mount(type, flags, name, data);
     if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
         !mnt->mnt_sb->s_subtype)
         mnt = fs_set_subtype(mnt, fstype);
     put_filesystem(type);
     return mnt;
 }

4.2 struct file_system_type结构体，下面的get_sb函数会在下面的挂载过程中用到，用来完成具体文件系统挂载的操作。

 struct file_system_type {
     const char *name;
     int fs_flags;
     int (*get_sb) (struct file_system_type *, int,
                const char *, void *, struct vfsmount *);
     void (*kill_sb) (struct super_block *);
     struct module *owner;
     struct file_system_type * next;
     struct list_head fs_supers;

     struct lock_class_key s_lock_key;
     struct lock_class_key s_umount_key;

     struct lock_class_key i_lock_key;
     struct lock_class_key i_mutex_key;
     struct lock_class_key i_mutex_dir_key;
     struct lock_class_key i_alloc_sem_key;
 };

4.3 get_fs_type用到的最主要的函数: 就是从已挂载文件系统队列file_systems中查找，返回相应name类型的file_system_type.

 static struct file_system_type **find_filesystem(const char *name, unsigned len)
 {
     struct file_system_type **p;
     for (p=&file_systems; *p; p=&(*p)->next)
         if (strlen((*p)->name) == len &&
             strncmp((*p)->name, name, len) == 0)
             break;
     return p;
 }

5. vfs_kern_mount

具体文件系统file_system_type的get_sb成员函数是关键，它填充vfsmnt结构体的super_block结构体，是写文件系统的第一步。

 struct vfsmount *
 vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void *data)
 {
     struct vfsmount *mnt;
     char *secdata = NULL;
     int error;

     if (!type)
         return ERR_PTR(-ENODEV);

     error = -ENOMEM;
     mnt = alloc_vfsmnt(name);      //alloc_vfsmnt以设备名为参数，为mnt函数分配一个空间，初始化mnt的基本信息，包括mnt->mnt_devname，以及一些list
     if (!mnt)
         goto out;

     if (data && !(type->fs_flags & FS_BINARY_MOUNTDATA)) {
         secdata = alloc_secdata();
         if (!secdata)
             goto out_mnt;

         error = security_sb_copy_data(data, secdata);
         if (error)
             goto out_free_secdata;
     }

     error = type->get_sb(type, flags, name, data, mnt);      //调用具体文件系统file_system_type的get_sb函数，填充vfsmnt结构体的super_block结构体
     if (error < 0)
         goto out_free_secdata;
     BUG_ON(!mnt->mnt_sb);

     error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
     if (error)
         goto out_sb;

     /*
      * filesystems should never set s_maxbytes larger than MAX_LFS_FILESIZE
      * but s_maxbytes was an unsigned long long for many releases. Throw
      * this warning for a little while to try and catch filesystems that
      * violate this rule. This warning should be either removed or
      * converted to a BUG() in 2.6.34.
      */
     WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
         "negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);

     mnt->mnt_mountpoint = mnt->mnt_root;                   //
     mnt->mnt_parent = mnt;
     up_write(&mnt->mnt_sb->s_umount);
     free_secdata(secdata);
     return mnt;
 out_sb:
     dput(mnt->mnt_root);
     deactivate_locked_super(mnt->mnt_sb);
 out_free_secdata:
     free_secdata(secdata);
 out_mnt:
     free_vfsmnt(mnt);
 out:
     return ERR_PTR(error);
 }

5.2 ext2_get_sb, 以ext2文件系统为例，通过ext2_fill_super来填充super_block的信息。

 static int ext2_get_sb(struct file_system_type *fs_type,
     int flags, const char *dev_name, void *data, struct vfsmount *mnt)
 {
     return get_sb_bdev(fs_type, flags, dev_name, data, ext2_fill_super, mnt);
 }

5.3  get_sb_bdev

获取相应块设备的super_block，通过ext2_fill_super来完成super_block信息的填充。

 int get_sb_bdev(struct file_system_type *fs_type,
     int flags, const char *dev_name, void *data,
     int (*fill_super)(struct super_block *, void *, int),
     struct vfsmount *mnt)
 {
     struct block_device *bdev;          //块设备信息
     struct super_block *s;
     fmode_t mode = FMODE_READ;
     int error = 0;

     if (!(flags & MS_RDONLY))
         mode |= FMODE_WRITE;

     bdev = open_bdev_exclusive(dev_name, mode, fs_type);    //根据dev_name，去读取块设备信息。
     if (IS_ERR(bdev))
         return PTR_ERR(bdev);

     /*
      * once the super is inserted into the list by sget, s_umount
      * will protect the lockfs code from trying to start a snapshot
      * while we are mounting
      */
     mutex_lock(&bdev->bd_fsfreeze_mutex);
     if (bdev->bd_fsfreeze_count > 0) {
         mutex_unlock(&bdev->bd_fsfreeze_mutex);
         error = -EBUSY;
         goto error_bdev;
     }
     s = sget(fs_type, test_bdev_super, set_bdev_super, bdev);     //根据bdev，去读取相应的super_block
     mutex_unlock(&bdev->bd_fsfreeze_mutex);
     if (IS_ERR(s))
         goto error_s;

     if (s->s_root) {
         if ((flags ^ s->s_flags) & MS_RDONLY) {
             deactivate_locked_super(s);
             error = -EBUSY;
             goto error_bdev;
         }

         close_bdev_exclusive(bdev, mode);
     } else {
         char b[BDEVNAME_SIZE];

         s->s_flags = flags;
         s->s_mode = mode;
         strlcpy(s->s_id, bdevname(bdev, b), sizeof(s->s_id));
         sb_set_blocksize(s, block_size(bdev));
         error = fill_super(s, data, flags & MS_SILENT ? 1 : 0);        //具体文件系统的fill_super，这里是ext2_fill_super，完成对super_block各个域的初始化
         if (error) {
             deactivate_locked_super(s);
             goto error;
         }

         s->s_flags |= MS_ACTIVE;
         bdev->bd_super = s;
     }

     simple_set_mnt(mnt, s);
     return 0;

 error_s:
     error = PTR_ERR(s);
 error_bdev:
     close_bdev_exclusive(bdev, mode);
 error:
     return error;
 }

6.do_new_mount->do_add_mount 

回到do_new_mount的do_add_mount函数，把mnt挂载到命名空间上。do_add_mount将新挂载的文件系统（由vfsmnt表示）添加到系统的命名空间结构体的已挂载文件系统链表中，命名空间是指系统中以挂载文件系统树，每个进程的PCB中都有namespace成员来表示该进程的命名空间，大多数的进程共享同一个命名空间，所以如果在一个进程中将磁盘挂载到系统中，在另一个进程也是可以看到的，这就是由命名空间来实现的[1]。

 *
  * add a mount into a namespace's mount tree
  * - provide the option of adding the new mount to an expiration list
  */
 int do_add_mount(struct vfsmount *newmnt, struct path *path,
          int mnt_flags, struct list_head *fslist)
 {
     int err;

     down_write(&namespace_sem);
     /* Something was mounted here while we slept */             /*挂载目录可能是已挂载的，follow_down可以把path->mnt和path->dentry指向上一层挂载*/
     while (d_mountpoint(path->dentry) &&
            follow_down(path))
         ;
     err = -EINVAL;
     if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))
         goto unlock;

     /* Refuse the same filesystem on the same mount point */
     err = -EBUSY;
     if (path->mnt->mnt_sb == newmnt->mnt_sb &&
         path->mnt->mnt_root == path->dentry)
         goto unlock;

     err = -EINVAL;
     if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
         goto unlock;

     newmnt->mnt_flags = mnt_flags;
     if ((err = graft_tree(newmnt, path)))
         goto unlock;

     if (fslist) /* add to the specified expiration list */                  /*添加*/
         list_add_tail(&newmnt->mnt_expire, fslist);

     up_write(&namespace_sem);
     return 0;

 unlock:
     up_write(&namespace_sem);
     mntput(newmnt);
     return err;
 }

参考博文: [1]:  http://blog.csdn.net/skyflying2012/article/details/9748133  

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