Linux虚拟文件系统之文件系统安装（sys_mount()）
来源：岁月联盟
时间：2012-02-10
/*sys_mount系统调用*/
/*dev_name为待安装设备的路径名；
dir_name为安装点的路径名；
type是表示文件系统类型的字符串；
*/
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;
/*从用户空间复制到系统空间*/
ret = copy_mount_string(type, &kernel_type);
if (ret < 0)
goto out_type;
kernel_dir = getname(dir_name);
if (IS_ERR(kernel_dir)) {
ret = PTR_ERR(kernel_dir);
goto out_dir;
}
ret = copy_mount_string(dev_name, &kernel_dev);
if (ret < 0)
goto out_dev;
/*用户空间复制到系统空间，拷贝整个页面*/
ret = copy_mount_options(data, &data_page);
if (ret < 0)
goto out_data;
/*操作主体*/
ret = do_mount(kernel_dev, kernel_dir, kernel_type, flags,
(void *) data_page);
free_page(data_page);
out_data:
kfree(kernel_dev);
out_dev:
putname(kernel_dir);
out_dir:
kfree(kernel_type);
out_type:
return ret;
}
下面是主体实现
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))
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;
if (flags & MS_NODEV)
mnt_flags |= MNT_NODEV;
if (flags & MS_NOEXEC)
mnt_flags |= MNT_NOEXEC;
if (flags & MS_NOATIME)
mnt_flags |= MNT_NOATIME;
if (flags & MS_NODIRATIME)
mnt_flags |= MNT_NODIRATIME;
if (flags & MS_STRICTATIME)
mnt_flags &= ~(MNT_RELATIME | MNT_NOATIME);
if (flags & MS_RDONLY)
mnt_flags |= MNT_READONLY;
flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
MS_STRICTATIME);
/* ... and get the mountpoint */
/*获得安装点path结构*/
retval = kern_path(dir_name, LOOKUP_FOLLOW, &path);
if (retval)
return retval;
retval = security_sb_mount(dev_name, &path,
type_page, flags, data_page);
if (retval)
goto dput_out;
if (flags & MS_REMOUNT)
/*修改已经存在的文件系统参数,即改变超级块对象s_flags
字段的安装标志*/
retval = do_remount(&path, flags & ~MS_REMOUNT, mnt_flags,
data_page);
else if (flags & MS_BIND)
/*要求在系统目录树的另一个安装点上得文件或目录能够可见*/
retval = do_loopback(&path, dev_name, flags & MS_REC);
else if (flags & (MS_SHARED | MS_PRIVATE | MS_SLAVE | MS_UNBINDABLE))
/*is responsible for handling shared, slave, and unbindable mounts by changing
the mount flags or building up the required data structure connections
between the vfsmount
instances involved.*/
retval = do_change_type(&path, flags);
else if (flags & MS_MOVE)
/*改变已安装文件的安装点*/
/*used to move a mounted filesystem*/
retval = do_move_mount(&path, dev_name);
else
/*handles normal mount operations. This is the default situation, so no special
flags
are required*/
/*当用户要求安装一个特殊文件系统或存放在磁盘分区
中的普通文件系统时，调用*/
retval = do_new_mount(&path, type_page, flags, mnt_flags,
dev_name, data_page);
dput_out:
path_put(&path);
return retval;
}
/*
* create a new mount for userspace and request it to be added into the
* namespace's tree
*/
static int do_new_mount(struct path *path, char *type, int flags,
int mnt_flags, char *name, void *data)
{
struct vfsmount *mnt;
if (!type)
return -EINVAL;
/* we need capabilities... */
if (!capable(CAP_SYS_ADMIN))
return -EPERM;
lock_kernel();
/*处理实际的安装操作并返回一个新的安装文件系统
描述符地址,使用get_fs_type扫描已经注册文件系统链表
找到匹配的file_system_type实例。然后分配或获取sb结构
并与mnt关联，初始化mnt并返回*/
*/
mnt = do_kern_mount(type, flags, name, data);
unlock_kernel();
if (IS_ERR(mnt))
return PTR_ERR(mnt);
/*处理必要的锁定操作，确保一个文件系统不会重复装载到
同一个位置，将文件系统整合到系统中*/
return do_add_mount(mnt, path, mnt_flags, NULL);
}
do_kern_mount函数在前面初始化中介绍过了，下面看do_add_mount函数用于将文件系统整合到系统中。
/*
* 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 */
while (d_mountpoint(path->dentry) &&
follow_down(path))
;
err = -EINVAL;
/*验证再改安装点上最近安装的文件系统是否任然指向
当前命名空间*/
if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))
goto unlock;
/* Refuse the same filesystem on the same mount point */
err = -EBUSY;
/*如果要安装的文件系统已经被安装在由系统调用的参数
所指定的安装点上*/
if (path->mnt->mnt_sb == newmnt->mnt_sb &&
path->mnt->mnt_root == path->dentry)
goto unlock;
err = -EINVAL;
/*该安装点是一个符号链接*/
if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
goto unlock;
newmnt->mnt_flags = mnt_flags;
/*把新安装的文件系统对象插入到namespace链表、
散列表以及父文件系统的子链表中*/
if ((err = graft_tree(newmnt, path)))
goto unlock;
if (fslist) /* add to the specified expiration list */
list_add_tail(&newmnt->mnt_expire, fslist);
up_write(&namespace_sem);
return 0;
unlock:
up_write(&namespace_sem);
mntput(newmnt);
return err;
}
新安装的文件系统对象链入系统树。
static int graft_tree(struct vfsmount *mnt, struct path *path)
{
int err;
if (mnt->mnt_sb->s_flags & MS_NOUSER)
return -EINVAL;
if (S_ISDIR(path->dentry->d_inode->i_mode) !=
S_ISDIR(mnt->mnt_root->d_inode->i_mode))
return -ENOTDIR;
err = -ENOENT;
mutex_lock(&path->dentry->d_inode->i_mutex);
if (IS_DEADDIR(path->dentry->d_inode))
goto out_unlock;
err = security_sb_check_sb(mnt, path);
if (err)
goto out_unlock;
err = -ENOENT;
if (!d_unlinked(path->dentry))/*加入树*/
err = attach_recursive_mnt(mnt, path, NULL);
out_unlock:
mutex_unlock(&path->dentry->d_inode->i_mutex);
if (!err)
security_sb_post_addmount(mnt, path);
return err;
}
/*将文件系统添加到父文件系统的命名空间中*/
static int attach_recursive_mnt(struct vfsmount *source_mnt,
struct path *path, struct path *parent_path)
{
LIST_HEAD(tree_list);
struct vfsmount *dest_mnt = path->mnt;
struct dentry *dest_dentry = path->dentry;
struct vfsmount *child, *p;
int err;
if (IS_MNT_SHARED(dest_mnt)) {
err = invent_group_ids(source_mnt, true);
if (err)
goto out;
}
err = propagate_mnt(dest_mnt, dest_dentry, source_mnt, &tree_list);
if (err)
goto out_cleanup_ids;
if (IS_MNT_SHARED(dest_mnt)) {
for (p = source_mnt; p; p = next_mnt(p, source_mnt))
set_mnt_shared(p);
}
spin_lock(&vfsmount_lock);
if (parent_path) {
detach_mnt(source_mnt, parent_path);
attach_mnt(source_mnt, path);
touch_mnt_namespace(parent_path->mnt->mnt_ns);
} else {
/*确保新的vfsmount实例的mnt_parent成员指向父文件系统
的vfsmount实例，而mnt_mountpoint成员指向装载点在父文件
系统中的dentry实例*/
mnt_set_mountpoint(dest_mnt, dest_dentry, source_mnt);
commit_tree(source_mnt);
}
list_for_each_entry_safe(child, p, &tree_list, mnt_hash) {
list_del_init(&child->mnt_hash);
/*将新的mnt添加到全局散列表以及父文件系统mnt实例中
的子文件系统链表*/
commit_tree(child);
}
spin_unlock(&vfsmount_lock);
return 0;
out_cleanup_ids:
if (IS_MNT_SHARED(dest_mnt))
cleanup_group_ids(source_mnt, NULL);
out:
return err;
}
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