Re: [PATCH RFC 9/9] sunrpc: don't upgrade passive net reference in xs_create_sock

[Jeff Layton]

On Mon, 2025-03-17 at 21:37 +0000, Trond Myklebust wrote:
> On Mon, 2025-03-17 at 17:36 -0400, Jeff Layton wrote:
> > On Mon, 2025-03-17 at 21:28 +0000, Trond Myklebust wrote:
> > > On Mon, 2025-03-17 at 17:00 -0400, Jeff Layton wrote:
> > > > With the move to having sunrpc client xprts not hold active
> > > > references
> > > > to the net namespace, there is no need to upgrade the socket's
> > > > reference
> > > > in xs_create_sock. Just keep the passive reference instead.
> > > > 
> > > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > > ---
> > > >  net/sunrpc/xprtsock.c | 3 ---
> > > >  1 file changed, 3 deletions(-)
> > > > 
> > > > diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> > > > index
> > > > 83cc095846d356f24aed26e2f98525662a6cff1f..0c3d7552f772d6f8477a3ae
> > > > d8f0
> > > > c513b62cdf589 100644
> > > > --- a/net/sunrpc/xprtsock.c
> > > > +++ b/net/sunrpc/xprtsock.c
> > > > @@ -1941,9 +1941,6 @@ static struct socket *xs_create_sock(struct
> > > > rpc_xprt *xprt,
> > > >  		goto out;
> > > >  	}
> > > >  
> > > > -	if (protocol == IPPROTO_TCP)
> > > > -		sk_net_refcnt_upgrade(sock->sk);
> > > > -
> > > >  	filp = sock_alloc_file(sock, O_NONBLOCK, NULL);
> > > >  	if (IS_ERR(filp))
> > > >  		return ERR_CAST(filp);
> > > > 
> > > 
> > > Is this not going to reintroduce the bug described by
> > > https://lore.kernel.org/netdev/67b72aeb.050a0220.14d86d.0283.GAE@xxxxxxxxxx/T/#u
> > > ?
> > > 
> > > As I understand it, the problem has nothing to do with whether or
> > > not
> > > NFS or the RPC layer holds a reference to the net namespace, but
> > > rather
> > > whether there are still packets in the socket queues at the time
> > > when
> > > that net namespace is being freed.
> > > 
> > > 
> > 
> > I don't think so. That syzkaller report was closed by this patch:
> > 
> >     5c70eb5c593d net: better track kernel sockets lifetime
> > 
> > That says:
> > 
> >     "To fix this, make sure that kernel sockets own a reference on
> > net
> > passive."
> > 
> > With this, we still do keep a passive reference on the net in the
> > socket, which I think is enough.
> 
> No. You just removed that by reverting the patch that assigns the
> passive reference.
> 

That's not how I read sk_net_refcnt_upgrade(). The socket already holds
a passive reference on the netns. sk_net_refcnt_upgrade() puts that
reference and then gets an active reference to the netns.

With this patchset, we just need to keep the passive one (I think).
-- 
Jeff Layton <jlayton@xxxxxxxxxx>