Re: [PATCH v2 1/3] ovl: disable nfs_export for same-sb lower layers with different idmaps

[Christian Brauner <brauner@kernel.org>]

On Mon, Sep 21, 2026 at 10:10:28AM +0200, Amir Goldstein wrote:
> On Sun, Sep 20, 2026 at 10:33 PM Jérémy Jean
> <Jeremy.Jean@oss.cyber.gouv.fr> wrote:
> >
> > Overlay file handles identify a backing filesystem, but not the lower
> > mount used to decode it. If lower layers share a superblock and use
> > different idmaps, a file handle can decode through the wrong layer and
> > initialize the overlay inode with the wrong owner.
> >
> > Track the first lower idmap seen for each ovl_sb and disable nfs_export
> > when another lower layer on the same backing superblock uses a different
> > idmap. Keep index enabled: copy up starts from the overlay dentry selected
> > by lookup and does not need to decode a lower file handle through another
> > layer.
> >
> > Fixes: bc70682a497c ("ovl: support idmapped layers")
> > Assisted-by: Codex:gpt-5
> > Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
> > ---
> >  fs/overlayfs/ovl_entry.h |  2 ++
> >  fs/overlayfs/super.c     | 28 +++++++++++++++++++++++++++-
> >  2 files changed, 29 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h
> > index 80cad4ea96a3..2a6e12a0d4dd 100644
> > --- a/fs/overlayfs/ovl_entry.h
> > +++ b/fs/overlayfs/ovl_entry.h
> > @@ -24,6 +24,8 @@ struct ovl_config {
> >  struct ovl_sb {
> >         struct super_block *sb;
> >         dev_t pseudo_dev;
> > +       /* Idmap of the first lower layer on this fs */
> > +       struct mnt_idmap *lower_idmap;
> >         /* Unusable (conflicting) uuid */
> >         bool bad_uuid;
> >         /* Used as a lower layer (but maybe also as upper) */
> > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> > index bd0a3f9039d2..8e78da420b75 100644
> > --- a/fs/overlayfs/super.c
> > +++ b/fs/overlayfs/super.c
> > @@ -944,6 +944,24 @@ static bool ovl_lower_uuid_ok(struct ovl_fs *ofs, const uuid_t *uuid)
> >         return true;
> >  }
> >
> > +/*
> > + * Lower file handles identify a lower fs, but not the mount whose idmap was
> > + * used to decode them.  Remember the first lower idmap seen for each fs so
> > + * decodable export handles can be disabled if another one appears.

I find this behavior counterintuitive. Shouldn't we just fail the mount then?