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

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Message-ID: <20260114-blamabel-hanfernte-ea1345885b46@brauner>
Date: Wed, 14 Jan 2026 16:26:03 +0100
From: Christian Brauner <brauner@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
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Subject: Re: [PATCH 00/24] vfs: require filesystems to explicitly opt-in to
 lease support

On Wed, Jan 14, 2026 at 08:41:16AM -0500, Jeff Layton wrote:
> On Wed, 2026-01-14 at 05:06 -0800, Christoph Hellwig wrote:
> > On Wed, Jan 14, 2026 at 10:34:04AM +0100, Amir Goldstein wrote:
> > > On Wed, Jan 14, 2026 at 7:28 AM Christoph Hellwig <hch@...radead.org> wrote:
> > > > 
> > > > On Tue, Jan 13, 2026 at 12:06:42PM -0500, Jeff Layton wrote:
> > > > > Fair point, but it's not that hard to conceive of a situation where
> > > > > someone inadvertantly exports cgroupfs or some similar filesystem:
> > > > 
> > > > Sure.  But how is this worse than accidentally exporting private data
> > > > or any other misconfiguration?
> > > > 
> > > 
> > > My POV is that it is less about security (as your question implies), and
> > > more about correctness.
> > 
> > I was just replying to Jeff.
> > 
> > > The special thing about NFS export, as opposed to, say, ksmbd, is
> > > open by file handle, IOW, the export_operations.
> > > 
> > > I perceive this as a very strange and undesired situation when NFS
> > > file handles do not behave as persistent file handles.
> > 
> > That is not just very strange, but actually broken (discounting the
> > obscure volatile file handles features not implemented in Linux NFS
> > and NFSD).  And the export ops always worked under the assumption
> > that these file handles are indeed persistent.  If they're not we
> > do have a problem.
> > 
> > > 
> > > cgroupfs, pidfs, nsfs, all gained open_by_handle_at() capability for
> > > a known reason, which was NOT NFS export.
> > > 
> > > If the author of open_by_handle_at() support (i.e. brauner) does not
> > > wish to imply that those fs should be exported to NFS, why object?
> > 
> > Because "want to export" is a stupid category.
> > 
> > OTOH "NFS exporting doesn't actually properly work because someone
> > overloaded export_ops with different semantics" is a valid category.
> > 
> 
> cgroupfs definitely doesn't behave as expected when exported via NFS.
> The files aren't readable, at least. I'd also be surprised if the
> filehandles were stable across a reboot, which is sort of necessary for

They aren't and it's not desirable.