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Commit fbabfd0f authored by Eric W. Biederman's avatar Eric W. Biederman
Browse files

fs: Add helper functions for permanently empty directories. 



To ensure it is safe to mount proc and sysfs I need to check if
filesystems that are mounted on top of them are mounted on truly empty
directories.  Given that some directories can gain entries over time,
knowing that a directory is empty right now is insufficient.

Therefore add supporting infrastructure for permantently empty
directories that proc and sysfs can use when they create mount points
for filesystems and fs_fully_visible can use to test for permanently
empty directories to ensure that nothing will be gained by mounting a
fresh copy of proc or sysfs.

Cc: stable@vger.kernel.org
Signed-off-by: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
parent ceeb0e5d

fs/libfs.c

+96 −0
Original line number Diff line number Diff line
@@ -1093,3 +1093,99 @@ simple_nosetlease(struct file *filp, long arg, struct file_lock **flp, 
	return -EINVAL;

}

EXPORT_SYMBOL(simple_nosetlease);





/*

 * Operations for a permanently empty directory.

 */

static struct dentry *empty_dir_lookup(struct inode *dir, struct dentry *dentry, unsigned int flags)

{

	return ERR_PTR(-ENOENT);

}



static int empty_dir_getattr(struct vfsmount *mnt, struct dentry *dentry,

				 struct kstat *stat)

{

	struct inode *inode = d_inode(dentry);

	generic_fillattr(inode, stat);

	return 0;

}



static int empty_dir_setattr(struct dentry *dentry, struct iattr *attr)

{

	return -EPERM;

}



static int empty_dir_setxattr(struct dentry *dentry, const char *name,

			      const void *value, size_t size, int flags)

{

	return -EOPNOTSUPP;

}



static ssize_t empty_dir_getxattr(struct dentry *dentry, const char *name,

				  void *value, size_t size)

{

	return -EOPNOTSUPP;

}



static int empty_dir_removexattr(struct dentry *dentry, const char *name)

{

	return -EOPNOTSUPP;

}



static ssize_t empty_dir_listxattr(struct dentry *dentry, char *list, size_t size)

{

	return -EOPNOTSUPP;

}



static const struct inode_operations empty_dir_inode_operations = {

	.lookup		= empty_dir_lookup,

	.permission	= generic_permission,

	.setattr	= empty_dir_setattr,

	.getattr	= empty_dir_getattr,

	.setxattr	= empty_dir_setxattr,

	.getxattr	= empty_dir_getxattr,

	.removexattr	= empty_dir_removexattr,

	.listxattr	= empty_dir_listxattr,

};



static loff_t empty_dir_llseek(struct file *file, loff_t offset, int whence)

{

	/* An empty directory has two entries . and .. at offsets 0 and 1 */

	return generic_file_llseek_size(file, offset, whence, 2, 2);

}



static int empty_dir_readdir(struct file *file, struct dir_context *ctx)

{

	dir_emit_dots(file, ctx);

	return 0;

}



static const struct file_operations empty_dir_operations = {

	.llseek		= empty_dir_llseek,

	.read		= generic_read_dir,

	.iterate	= empty_dir_readdir,

	.fsync		= noop_fsync,

};





void make_empty_dir_inode(struct inode *inode)

{

	set_nlink(inode, 2);

	inode->i_mode = S_IFDIR | S_IRUGO | S_IXUGO;

	inode->i_uid = GLOBAL_ROOT_UID;

	inode->i_gid = GLOBAL_ROOT_GID;

	inode->i_rdev = 0;

	inode->i_size = 2;

	inode->i_blkbits = PAGE_SHIFT;

	inode->i_blocks = 0;



	inode->i_op = &empty_dir_inode_operations;

	inode->i_fop = &empty_dir_operations;

}



bool is_empty_dir_inode(struct inode *inode)

{

	return (inode->i_fop == &empty_dir_operations) &&

		(inode->i_op == &empty_dir_inode_operations);

}

include/linux/fs.h

+2 −0
Original line number Diff line number Diff line
@@ -2780,6 +2780,8 @@ extern struct dentry *simple_lookup(struct inode *, struct dentry *, unsigned in 
extern ssize_t generic_read_dir(struct file *, char __user *, size_t, loff_t *);

extern const struct file_operations simple_dir_operations;

extern const struct inode_operations simple_dir_inode_operations;

extern void make_empty_dir_inode(struct inode *inode);

extern bool is_empty_dir_inode(struct inode *inode);

struct tree_descr { char *name; const struct file_operations *ops; int mode; };

struct dentry *d_alloc_name(struct dentry *, const char *);

extern int simple_fill_super(struct super_block *, unsigned long, struct tree_descr *);