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

[Christian Brauner]

On Wed, Jan 14, 2026 at 10:42:48PM -0800, Christoph Hellwig wrote:
> On Wed, Jan 14, 2026 at 04:20:13PM +0100, Christian Brauner wrote:
> > > You're still think of it the wrong way.  If we do have file systems
> > > that break the original exportfs semantics we need to fix that, and
> > > something like a "stable handles" flag will work well for that.  But
> > > a totally arbitrary "is exportable" flag is total nonsense.
> > 
> > File handles can legitimately be conceptualized independently of
> > exporting a filesystem. If we wanted to tear those concepts apart
> > implementation wise we could.
> > 
> > It is complete nonsense to expect the kernel to support exporting any
> > arbitrary internal filesystem or to not support file handles at all.
> 
> You are going even further down the path of entirely missing the point
> (or the two points by now).

You're arguing for the sake of arguing imho. You're getting exactly what
we're all saying as evidenced by the last paragraph in your mail: it is
entirely what this whole thing is about.

> If a file systems meets all technical requirements of being nfsd
> exportable and the users asks for it, it is not our job to make an
> arbitrary policy decision to say no.

This is an entirely irrelevant point because we're talking about
cgroupfs, nsfs, and pidfs. And they don't meet this criteria. cgroupfs
is a _local resource management filesystem_ why would we ever want to
support exporting it over the network. It allows to break the local
delegation model as I've explained. cgroupfs shows _local processes_. So
a server will see completely nonsensical PID identifiers listed in
cgroup files and it can fsck around with processes in a remote system.
Hard NAK. Entirely irrelevant if that filesystem meets the theoretical
standards.

> If it does not meet the technical requirements it obviously should
> not be exportable.  And it seems like the spread of file handles
> beyond nfs exporting created some ambiguity here, which we need to
> fix.

We are all in agreement here.