[PATCH 00/24] vfs: require filesystems to explicitly opt-in to lease support

[Chuck Lever]

On 1/13/26 6:45 AM, Jeff Layton wrote:
> On Tue, 2026-01-13 at 09:54 +0100, Christian Brauner wrote:
>> On Mon, Jan 12, 2026 at 09:50:20AM -0500, Jeff Layton wrote:
>>> On Mon, 2026-01-12 at 09:31 -0500, Chuck Lever wrote:
>>>> On 1/12/26 8:34 AM, Jeff Layton wrote:
>>>>> On Fri, 2026-01-09 at 19:52 +0100, Amir Goldstein wrote:
>>>>>> On Thu, Jan 8, 2026 at 7:57 PM Jeff Layton <jlayton at kernel.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.
>>>>>>
>>>>>
>>>>> I think so too. For now, I think I'll not bother with the WARN_ONCE().
>>>>> Let's just leave kernfs out of the set until someone presents a real
>>>>> use-case.
>>>>>
>>>>>> 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?
>>>>>>
>>>>>
>>>>> (cc'ing Chuck)
>>>>>
>>>>> FWIW, you can currently export and mount /sys/fs/cgroup via NFS. The
>>>>> directory doesn't show up when you try to get to it via NFSv4, but you
>>>>> can mount it using v3 and READDIR works. The files are all empty when
>>>>> you try to read them. I didn't try to do any writes.
>>>>>
>>>>> Should we add a mechanism to prevent exporting these sorts of
>>>>> filesystems?
>>>>>
>>>>> Even better would be to make nfsd exporting explicitly opt-in. What if
>>>>> we were to add a EXPORT_OP_NFSD flag that explicitly allows filesystems
>>>>> to opt-in to NFS exporting, and check for that in __fh_verify()? We'd
>>>>> have to add it to a bunch of existing filesystems, but that's fairly
>>>>> simple to do with an LLM.
>>>>
>>>> What's the active harm in exporting /sys/fs/cgroup ? It has to be done
>>>> explicitly via /etc/exports, so this is under the NFS server admin's
>>>> control. Is it an attack surface?
>>>>
>>>
>>> Potentially?
>>>
>>> I don't see any active harm with exporting cgroupfs. It doesn't work
>>> right via nfsd, but it's not crashing the box or anything.
>>>
>>> At one time, those were only defined by filesystems that wanted to
>>> allow NFS export. Now we've grown them on filesystems that just want to
>>> provide filehandles for open_by_handle_at() and the like. nfsd doesn't
>>> care though: if the fs has export operations, it'll happily use them.
>>>
>>> Having an explicit "I want to allow nfsd" flag see ms like it might
>>> save us some headaches in the future when other filesystems add export
>>> ops for this sort of filehandle use.
>>
>> So we are re-hashing a discussion we had a few months ago (Amir was
>> involved at least).
>>
> 
> Yep, I was lurking on it, but didn't have a lot of input at the time.
> 
>> I don't think we want to expose cgroupfs via NFS that's super weird.
>> It's like remote partial resource management and it would be very
>> strange if a remote process suddenly would be able to move things around
>> in the cgroup tree. So I would prefer to not do this.
>>
>> So my preference would be to really sever file handles from the export
>> mechanism so that we can allow stuff like pidfs and nsfs and cgroupfs to
>> use file handles via name_to_handle_at() and open_by_handle_at() without
>> making them exportable.
> 
> Agreed. I think we want to make NFS export be a deliberate opt-in
> decision that filesystem developers make.

No objection, what about ksmbd, AFS, or Ceph?


> How we do that is up for
> debate, of course.
> 
> An export ops flag would be fairly simple to implement, but it sounds
> like you're thinking that we should split some export_operations into
> struct file_handle_operations and then add a pointer for that to
> super_block (and maybe to export_operations too)?
> 
> This would be a good LSF/MM topic, but I'm hoping we can come to a
> consensus before then.
> 


-- 
Chuck Lever