Re: [PATCH 17/21] nullfs: refuse file locks

[NeilBrown]

On Fri, 02 Oct 2026, Christian Brauner wrote:
> Refuse flock() and POSIX locks on nullfs. Its one inode is the root of
> every kernel thread and the following patches make it the directory
> that stands in for an unmounted mount, so a lock taken through one such
> directory would block the locks of every other holder and F_GETLK would
> tell them the pid of the holder. Give the directory file operations of
> its own: what libfs gives an empty directory plus ->lock and ->flock
> that fail with ENOLCK, the way a filesystem without lock support does.
> 
> Signed-off-by: Christian Brauner (Amutable) <brauner@xxxxxxxxxx>
> ---
>  fs/nullfs.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/fs/nullfs.c b/fs/nullfs.c
> index 55a04f2d7761..d90c71f5eece 100644
> --- a/fs/nullfs.c
> +++ b/fs/nullfs.c
> @@ -10,6 +10,34 @@ static const struct super_operations nullfs_super_operations = {
>  	.statfs	= simple_statfs,
>  };
>  
> +static loff_t nullfs_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 nullfs_dir_readdir(struct file *file, struct dir_context *ctx)
> +{
> +	dir_emit_dots(file, ctx);
> +	return 0;
> +}
> +
> +/* the one inode of nullfs is shared by every holder, so no locks on it */
> +static int nullfs_nolock(struct file *file, int cmd, struct file_lock *fl)
> +{
> +	return -ENOLCK;
> +}
> +
> +/* what libfs gives an empty directory, plus the refusal of file locks */

It's a pity that you need separate copies for readdir and llseek -
identical to what is in libfs.

I would probably put all this code in libfs.c with make_null_dir_inode()
or similar.  But I cannot argue strongly in that direction, and the code
looks sensible as-is.

Reviewed-by: NeilBrown <neil@xxxxxxxxxx>

Thanks,
NeilBrown


> +static const struct file_operations nullfs_dir_operations = {
> +	.llseek		= nullfs_dir_llseek,
> +	.read		= generic_read_dir,
> +	.iterate_shared	= nullfs_dir_readdir,
> +	.fsync		= noop_fsync,
> +	.lock		= nullfs_nolock,
> +	.flock		= nullfs_nolock,
> +};
> +
>  static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc)
>  {
>  	struct inode *inode;
> @@ -30,6 +58,7 @@ static int nullfs_fs_fill_super(struct super_block *s, struct fs_context *fc)
>  
>  	/* nullfs is permanently empty... */
>  	make_empty_dir_inode(inode);
> +	inode->i_fop = &nullfs_dir_operations;
>  	simple_inode_init_ts(inode);
>  	inode->i_ino	= 1;
>  	/* ... and immutable, reading it leaves no trace either. */
> 
> -- 
> 2.53.0
> 
>