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

["Jérémy Jean" <jeremy.jean@oss.cyber.gouv.fr>]

On 2026-09-25 16:17, Amir Goldstein wrote:
> On Fri, Sep 25, 2026 at 3:51 PM Christian Brauner <brauner@kernel.org> 
> wrote:
>> 
>> 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?
> 
> I would love to fail the mount if you confirm that setting up an ovl
> with several
> lower layers some on the same fs with different idmap, is a 
> non-feature.
> 
> I don't know can't /home/amir and /home/brauner be on the same fs with
> different idmapping?
> 
> Could it ever make sense to compose an overlay from two such 
> directories?
> If not then I would gladly remove this setup from the test matrix.
> 
> Thanks,
> Amir.

Hello,

I'm digging this up: should I investigate to prepare a new version that
simply fails the mount? Please tell me what you'd prefer.

Regards,
Jérémy