Message-ID: <20260112-gemeldet-gelitten-7d48bae7ef3f@brauner>
Date: Mon, 12 Jan 2026 10:49:16 +0100
From: Christian Brauner <brauner@...nel.org>
To: Amir Goldstein <amir73il@...il.com>
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Subject: Re: [PATCH 00/24] vfs: require filesystems to explicitly opt-in to
 lease support

On Fri, Jan 09, 2026 at 07:52:57PM +0100, Amir Goldstein wrote:
> On Thu, Jan 8, 2026 at 7:57 PM Jeff Layton <jlayton@...nel.org> wrote:
> >
> > On Thu, 2026-01-08 at 18:40 +0100, Jan Kara wrote:
> > > On Thu 08-01-26 12:12:55, Jeff Layton wrote:
> > > > Yesterday, I sent patches to fix how directory delegation support is
> > > > handled on filesystems where the should be disabled [1]. That set is
> > > > appropriate for v6.19. For v7.0, I want to make lease support be more
> > > > opt-in, rather than opt-out:
> > > >
> > > > For historical reasons, when ->setlease() file_operation is set to NULL,
> > > > the default is to use the kernel-internal lease implementation. This
> > > > means that if you want to disable them, you need to explicitly set the
> > > > ->setlease() file_operation to simple_nosetlease() or the equivalent.
> > > >
> > > > This has caused a number of problems over the years as some filesystems
> > > > have inadvertantly allowed leases to be acquired simply by having left
> > > > it set to NULL. It would be better if filesystems had to opt-in to lease
> > > > support, particularly with the advent of directory delegations.
> > > >
> > > > This series has sets the ->setlease() operation in a pile of existing
> > > > local filesystems to generic_setlease() and then changes
> > > > kernel_setlease() to return -EINVAL when the setlease() operation is not
> > > > set.
> > > >
> > > > With this change, new filesystems will need to explicitly set the
> > > > ->setlease() operations in order to provide lease and delegation
> > > > support.
> > > >
> > > > I mainly focused on filesystems that are NFS exportable, since NFS and
> > > > SMB are the main users of file leases, and they tend to end up exporting
> > > > the same filesystem types. Let me know if I've missed any.
> > >
> > > So, what about kernfs and fuse? They seem to be exportable and don't have
> > > .setlease set...
> > >
> >
> > Yes, FUSE needs this too. I'll add a patch for that.
> >
> > As far as kernfs goes: AIUI, that's basically what sysfs and resctrl
> > are built on. Do we really expect people to set leases there?
> >
> > I guess it's technically a regression since you could set them on those
> > sorts of files earlier, but people don't usually export kernfs based
> > filesystems via NFS or SMB, and that seems like something that could be
> > used to make mischief.
> >
> > AFAICT, kernfs_export_ops is mostly to support open_by_handle_at(). See
> > commit aa8188253474 ("kernfs: add exportfs operations").
> >
> > One idea: we could add a wrapper around generic_setlease() for
> > filesystems like this that will do a WARN_ONCE() and then call
> > generic_setlease(). That would keep leases working on them but we might
> > get some reports that would tell us who's setting leases on these files
> > and why.
> 
> IMO, you are being too cautious, but whatever.
> 
> It is not accurate that kernfs filesystems are NFS exportable in general.
> Only cgroupfs has KERNFS_ROOT_SUPPORT_EXPORTOP.
> 
> If any application is using leases on cgroup files, it must be some
> very advanced runtime (i.e. systemd), so we should know about the
> regression sooner rather than later.
> 
> There are also the recently added nsfs and pidfs export_operations.
> 
> I have a recollection about wanting to be explicit about not allowing
> those to be exportable to NFS (nsfs specifically), but I can't see where
> and if that restriction was done.
> 
> Christian? Do you remember?

I don't think it does.

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