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

On Tue, Feb 17, 2026 at 8:48 PM Jeff Layton <[email protected]> wrote:
>
> 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.

As long as the "connection" is initiated from the userspace, I think
it should be aware of all namespaces (not just net namespace).
As you said, this will also mean that there's a need for a daemon to
watch this fd.

>
> 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.

I see a few other users of UMH: coredump, initrd, kmod and kobject.
https://elixir.bootlin.com/linux/v6.19.3/C/ident/call_usermodehelper_setup
Based on the descriptions, most of these are used in early boot or are
done in subsystems that don't need namespace awareness.
To me, request_key is the odd one out here. I think we should have
upcall handlers in the different namespaces to watch for upcalls.

> --
> Jeff Layton <[email protected]>



-- 
Regards,
Shyam