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

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.

Signed-off-by: default avatar"Eric W. Biederman" <>
parent ceeb0e5d
......@@ -1093,3 +1093,99 @@ simple_nosetlease(struct file *filp, long arg, struct file_lock **flp,
return -EINVAL;
* 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)
static ssize_t empty_dir_getxattr(struct dentry *dentry, const char *name,
void *value, size_t size)
static int empty_dir_removexattr(struct dentry *dentry, const char *name)
static ssize_t empty_dir_listxattr(struct dentry *dentry, char *list, size_t size)
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);
......@@ -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 *);
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