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

On Wed, Jul 8, 2026 at 2:31=E2=80=AFPM Giuseppe Scrivano <[email protected]=
om> wrote:
>
> Miklos Szeredi <[email protected]> writes:
>
> > On Wed, 8 Jul 2026 at 12:00, Giuseppe Scrivano <[email protected]> wr=
ote:
> >>
> >> 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 =3D=3D 0 returns the upper layer (-ENOENT =
if
> >>                         no upper is configured), arg >=3D 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...
>
> is there any existing user of such a mechanism?  I don't see any mount
> specific info under /proc or /sys.
>
> >
> >> 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.
>
> no that is not exposed by statmount and I don't see any way to export
> file system specific data through it.  Do you've anything in mind?
>
> >> --- 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_LAYE=
R */
> >> +       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.
>
> I didn't manage to get that to work.  As soon as the userspace process
> closes the mount fd that was passed to fsconfig, the anonymous mount
> namespace is destroyed and dissolve_on_fput sets mnt->mnt_ns to NULL.
>
> So whenever I try to use this mount again from userspace, it is not
> usable because the mount namespace is empty, causing check_mnt() to
> fail.
>
> Do you have any suggestions on how to solve this problem?

The suggestion was to store origin->f_path->mnt instead of storing origin f=
ile,
because you only end up using the origin vfsmount.

Thanks,
Amir.