Re: [PATCH v7 bpf-next 05/11] tcp: Migrate TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.

[Kuniyuki Iwashima]

From:   Eric Dumazet <erdnetdev@xxxxxxxxx>
Date:   Thu, 10 Jun 2021 20:20:11 +0200
> On 5/21/21 8:20 PM, Kuniyuki Iwashima wrote:
> > When we call close() or shutdown() for listening sockets, each child socket
> > in the accept queue are freed at inet_csk_listen_stop(). If we can get a
> > new listener by reuseport_migrate_sock() and clone the request by
> > inet_reqsk_clone(), we try to add it into the new listener's accept queue
> > by inet_csk_reqsk_queue_add(). If it fails, we have to call __reqsk_free()
> > to call sock_put() for its listener and free the cloned request.
> > 
> > After putting the full socket into ehash, tcp_v[46]_syn_recv_sock() sets
> > NULL to ireq_opt/pktopts in struct inet_request_sock, but ipv6_opt can be
> > non-NULL. So, we have to set NULL to ipv6_opt of the old request to avoid
> > double free.
> > 
> > Note that we do not update req->rsk_listener and instead clone the req to
> > migrate because another path may reference the original request. If we
> > protected it by RCU, we would need to add rcu_read_lock() in many places.
> > 
> > Link: https://lore.kernel.org/netdev/20201209030903.hhow5r53l6fmozjn@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > Suggested-by: Martin KaFai Lau <kafai@xxxxxx>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx>
> > Acked-by: Martin KaFai Lau <kafai@xxxxxx>
> > ---
> >  net/ipv4/inet_connection_sock.c | 71 ++++++++++++++++++++++++++++++++-
> >  1 file changed, 70 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > index fa806e9167ec..07e97b2f3635 100644
> > --- a/net/ipv4/inet_connection_sock.c
> > +++ b/net/ipv4/inet_connection_sock.c
> > @@ -695,6 +695,53 @@ int inet_rtx_syn_ack(const struct sock *parent, struct request_sock *req)
> >  }
> >  EXPORT_SYMBOL(inet_rtx_syn_ack);
> >  
> > +static struct request_sock *inet_reqsk_clone(struct request_sock *req,
> > +					     struct sock *sk)
> > +{
> > +	struct sock *req_sk, *nreq_sk;
> > +	struct request_sock *nreq;
> > +
> > +	nreq = kmem_cache_alloc(req->rsk_ops->slab, GFP_ATOMIC | __GFP_NOWARN);
> > +	if (!nreq) {
> > +		/* paired with refcount_inc_not_zero() in reuseport_migrate_sock() */
> > +		sock_put(sk);
> > +		return NULL;
> > +	}
> > +
> > +	req_sk = req_to_sk(req);
> > +	nreq_sk = req_to_sk(nreq);
> > +
> > +	memcpy(nreq_sk, req_sk,
> > +	       offsetof(struct sock, sk_dontcopy_begin));
> > +	memcpy(&nreq_sk->sk_dontcopy_end, &req_sk->sk_dontcopy_end,
> > +	       req->rsk_ops->obj_size - offsetof(struct sock, sk_dontcopy_end));
> > +
> > +	sk_node_init(&nreq_sk->sk_node);
> > +	nreq_sk->sk_tx_queue_mapping = req_sk->sk_tx_queue_mapping;
> > +#ifdef CONFIG_XPS
> > +	nreq_sk->sk_rx_queue_mapping = req_sk->sk_rx_queue_mapping;
> > +#endif
> > +	nreq_sk->sk_incoming_cpu = req_sk->sk_incoming_cpu;
> > +	refcount_set(&nreq_sk->sk_refcnt, 0);
> 
> Not sure why you clear sk_refcnt here (it is set to 1 later)

I thought it was safer, but I'm fine to remove the line.


> 
> > +
> > +	nreq->rsk_listener = sk;
> > +
> > +	/* We need not acquire fastopenq->lock
> > +	 * because the child socket is locked in inet_csk_listen_stop().
> > +	 */
> > +	if (sk->sk_protocol == IPPROTO_TCP && tcp_rsk(nreq)->tfo_listener)
> > +		rcu_assign_pointer(tcp_sk(nreq->sk)->fastopen_rsk, nreq);
> > +
> > +	return nreq;
> > +}
> 
> Ouch, this is going to be hard to maintain...

How could I make it less hard ... ?


> 
> 
> 
> 
> > +
> > +static void reqsk_migrate_reset(struct request_sock *req)
> > +{
> > +#if IS_ENABLED(CONFIG_IPV6)
> > +	inet_rsk(req)->ipv6_opt = NULL;
> > +#endif
> > +}
> > +
> >  /* return true if req was found in the ehash table */
> >  static bool reqsk_queue_unlink(struct request_sock *req)
> >  {
> > @@ -1036,14 +1083,36 @@ void inet_csk_listen_stop(struct sock *sk)
> >  	 * of the variants now.			--ANK
> >  	 */
> >  	while ((req = reqsk_queue_remove(queue, sk)) != NULL) {
> > -		struct sock *child = req->sk;
> > +		struct sock *child = req->sk, *nsk;
> > +		struct request_sock *nreq;
> >  
> >  		local_bh_disable();
> >  		bh_lock_sock(child);
> >  		WARN_ON(sock_owned_by_user(child));
> >  		sock_hold(child);
> >  
> > +		nsk = reuseport_migrate_sock(sk, child, NULL);
> > +		if (nsk) {
> > +			nreq = inet_reqsk_clone(req, nsk);
> > +			if (nreq) {
> > +				refcount_set(&nreq->rsk_refcnt, 1);
> > +
> > +				if (inet_csk_reqsk_queue_add(nsk, nreq, child)) {
> > +					reqsk_migrate_reset(req);
> > +				} else {
> > +					reqsk_migrate_reset(nreq);
> > +					__reqsk_free(nreq);
> > +				}
> > +
> > +				/* inet_csk_reqsk_queue_add() has already
> > +				 * called inet_child_forget() on failure case.
> > +				 */
> > +				goto skip_child_forget;
> > +			}
> > +		}
> > +
> >  		inet_child_forget(sk, req, child);
> > +skip_child_forget:
> >  		reqsk_put(req);
> >  		bh_unlock_sock(child);
> >  		local_bh_enable();
> >