Re: [PATCH v6 3/3] NFSD: add write_ports to netlink command

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2024-01-24 at 13:55 +0000, Chuck Lever III wrote:
> 
> > On Jan 24, 2024, at 6:24 AM, Jeff Layton <jlayton@kernel.org> wrote:
> > 
> > On Wed, 2024-01-24 at 10:52 +0100, Lorenzo Bianconi wrote:
> > > [...]
> > > > 
> > > > That's a great question. We do need to properly support the -H option to
> > > > rpc.nfsd. What we do today is look up the hostname or address using
> > > > getaddrinfo, and then open a listening socket for that address and then
> > > > pass that fd down to the kernel, which I think then takes the socket and
> > > > sticks it on sv_permsocks.
> > > > 
> > > > All of that seems a bit klunky. Ideally, I'd say the best thing would be
> > > > to allow userland to pass the sockaddr we look up directly via netlink,
> > > > and then let the kernel open the socket. That will probably mean
> > > > refactoring some of the svc_xprt_create machinery to take a sockaddr,
> > > > but I don't think it looks too hard to do.
> > > 
> > > Do we already have a specific use case for it? I think we can even add it
> > > later when we have a defined use case for it on top of the current series.
> > > 
> > 
> > Yes:
> > 
> > rpc.nfsd -H makes nfsd listen on a particular address and port. By
> > passing down the sockaddr instead of an already-opened socket
> > descriptor, we can achieve the goal without having to open sockets in
> > userland.
> 
> Tearing down a listener that was created that way would be a
> use case for:
> 
> > Do we ever want/need to remove listening sockets?
> > Normal practice when making any changes is to stop and restart where
> > "stop" removes all sockets, unexports all filesystems, disables all
> > versions.
> 

Good point. Even if we don't want to allow it today, passing down a
sockaddr over netlink gives us an interface to properly match a
listening socket for shutting them down without taking down the server. 

It would be a little silly to make userland open a socket in order to
close the one that nfsd is listening on.
-- 
Jeff Layton <jlayton@kernel.org>