Re: [PATCH] ovl: add ioctls to retrieve layer file descriptors

On Wed, 8 Jul 2026 at 12:00, Giuseppe Scrivano <[email protected]> wrote:
>
> Add two ioctls to overlay filesystem to allow userspace to retrieve
> information about the overlay layers:
>
> OVL_IOC_OPEN_LAYER:     return an O_PATH fd to the root of a layer.
>                         arg == 0 returns the upper layer (-ENOENT if
>                         no upper is configured), arg >= 1 returns
>                         lower layers (-ENOENT if index is out of
>                         range).

We could do this with a plain open() call.  Something like the magic
symlinks we have under /proc/PID/fd/.   Question is where could these
live...

> OVL_IOC_GET_LAYERS_INFO: copy a struct ovl_layers_info to userspace
>                         with numlower, numlowerdata, and has_upper.

Isn't this info obtainable via statmount(2) already?  If not, it
should be there, instead of a specialized ioctl.

> --- a/fs/overlayfs/ovl_entry.h
> +++ b/fs/overlayfs/ovl_entry.h
> @@ -35,6 +35,8 @@ struct ovl_layer {
>         struct vfsmount *mnt;
>         /* Trap in ovl inode cache */
>         struct inode *trap;
> +       /* Keeps the original fsmount file alive for OVL_IOC_OPEN_LAYER */
> +       struct file *origin;

Don't need to keep the file open: the only info missing is the
original vfsmount, everything else is already there to reconstruct the
file.

Thanks,
Miklos