Re: [LSF/MM/BPF TOPIC] Namespace-aware upcalls from kernel filesystems

On Tue, 2026-02-17 at 09:21 -0500, Chuck Lever wrote:
> 
> On Mon, Feb 16, 2026, at 11:14 PM, Shyam Prasad N wrote:
> > On Sat, Feb 14, 2026 at 9:10 PM Chuck Lever <[email protected]> wrote:
> > > 
> > > 
> > > On Sat, Feb 14, 2026, at 5:06 AM, Shyam Prasad N wrote:
> > > > Kernel filesystems sometimes need to upcall to userspace to get some
> > > > work done, which cannot be achieved in kernel code (or rather it is
> > > > better to be done in userspace). Some examples are DNS resolutions,
> > > > user authentication, ID mapping etc.
> > > > 
> > > > Filesystems like SMB and NFS clients use the kernel keys subsystem for
> > > > some of these, which has an upcall facility that can exec a binary in
> > > > userspace. However, this upcall mechanism is not namespace aware and
> > > > upcalls to the host namespaces (namespaces of the init process).
> > > 
> > > Hello Shyam, we've been introducing netlink control interfaces, which
> > > are namespace-aware. The kernel TLS handshake mechanism now uses
> > > this approach, as does the new NFSD netlink protocol.
> > > 
> > > 
> > > --
> > > Chuck Lever
> > 
> > Hi Chuck,
> > 
> > Interesting. Let me explore this a bit more.
> > I'm assuming that this is the file that I should be looking into:
> > fs/nfsd/nfsctl.c
> 
> Yes, clustered towards the end of the file. NFSD's use of netlink
> is as a downcall-style administrative control plane.
> 
> net/handshake/netlink.c uses netlink as an upcall for driving
> kernel-initiated TLS handshake requests up to a user daemon. This
> mechanism has been adopted by NFSD, the NFS client, and the NVMe
> over TCP drivers. An in-kernel QUIC implementation is planned and
> will also be using this.
> 
> 
> > And that there would be a corresponding handler in nfs-utils?
> 
> For NFSD, nfs-utils has a new tool called nfsdctl.
> 
> The TLS handshake user space components are in ktls-utils. See:
> https://github.com/oracle/ktls-utils


I think the consensus at this point is to move away from usermodehelper
as an upcall mechanism. The Linux kernel lacks a container object that
allows you to associate namespaces with one another, so you need an
already-running userspace process to do that association in userland.

netlink upcalls are bound to a network namespace. That works in the
above examples because they are also bound to a network namespace.
netlink upcalls require a running daemon in that namespace, which is
what ties that network namespace to other sorts of namespaces.

So, a related discussion we should have is whether and how we should
deprecate the old usermodehelper upcalls, given that they are
problematic in this way. 
-- 
Jeff Layton <[email protected]>