Re: [PATCH v2 mptcp-net 2/2] mptcp: do not reset MP_CAPABLE subflow on mapping errors

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Mon, 17 May 2021, Paolo Abeni wrote:

> When some mapping related errors occours we close the main
> MPC subflow with a RST. We should instead fallback gracefully
> to TCP, and do the reset only for MPJ subflows.
>
> Fixes: d22f4988ffec ("mptcp: process MP_CAPABLE data option")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/192
> Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> ---
> net/mptcp/subflow.c | 39 +++++++++++++++++++--------------------
> 1 file changed, 19 insertions(+), 20 deletions(-)
>
> diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
> index 278986585088..9befe9fe7bca 100644
> --- a/net/mptcp/subflow.c
> +++ b/net/mptcp/subflow.c
> @@ -1110,10 +1110,9 @@ static bool subflow_check_data_avail(struct sock *ssk)
>
> 		status = get_mapping_status(ssk, msk);
> 		trace_subflow_check_data_avail(status, skb_peek(&ssk->sk_receive_queue));
> -		if (unlikely(status == MAPPING_INVALID)) {
> -			ssk->sk_err = EBADMSG;
> -			goto fatal;
> -		}
> +		if (unlikely(status == MAPPING_INVALID))
> +			goto fallback;
> +

There are a bunch of other ways to get MAPPING_INVALID during the life of 
a connection, including when there are multiple subflows active and 
fallback is not a valid option. Can the new fallback condition be more 
targeted to this "out of order / packet loss at connection time" issue so 
truly fatal MAPPING_INVALID cases still reset the connection?

-Mat


> 		if (unlikely(status == MAPPING_DUMMY))
> 			goto fallback;
>
> @@ -1128,10 +1127,8 @@ static bool subflow_check_data_avail(struct sock *ssk)
> 		 * MP_CAPABLE-based mapping
> 		 */
> 		if (unlikely(!READ_ONCE(msk->can_ack))) {
> -			if (!subflow->mpc_map) {
> -				ssk->sk_err = EBADMSG;
> -				goto fatal;
> -			}
> +			if (!subflow->mpc_map)
> +				goto fallback;
> 			WRITE_ONCE(msk->remote_key, subflow->remote_key);
> 			WRITE_ONCE(msk->ack_seq, subflow->map_seq);
> 			WRITE_ONCE(msk->can_ack, true);
> @@ -1160,19 +1157,21 @@ static bool subflow_check_data_avail(struct sock *ssk)
> 	subflow_sched_work_if_closed(msk, ssk);
> 	return false;
>
> -fatal:
> -	/* fatal protocol error, close the socket */
> -	/* This barrier is coupled with smp_rmb() in tcp_poll() */
> -	smp_wmb();
> -	ssk->sk_error_report(ssk);
> -	tcp_set_state(ssk, TCP_CLOSE);
> -	subflow->reset_transient = 0;
> -	subflow->reset_reason = MPTCP_RST_EMPTCP;
> -	tcp_send_active_reset(ssk, GFP_ATOMIC);
> -	subflow->data_avail = 0;
> -	return false;
> -
> fallback:
> +	if (subflow->mp_join) {
> +		/* fatal protocol error, close the socket */
> +		/* This barrier is coupled with smp_rmb() in tcp_poll() */
> +		smp_wmb();
> +		ssk->sk_err = EBADMSG;
> +		ssk->sk_error_report(ssk);
> +		tcp_set_state(ssk, TCP_CLOSE);
> +		subflow->reset_transient = 0;
> +		subflow->reset_reason = MPTCP_RST_EMPTCP;
> +		tcp_send_active_reset(ssk, GFP_ATOMIC);
> +		subflow->data_avail = 0;
> +		return false;
> +	}
> +
> 	__mptcp_do_fallback(msk);
> 	skb = skb_peek(&ssk->sk_receive_queue);
> 	subflow->map_valid = 1;
> -- 
> 2.26.3
>
>
>

--
Mat Martineau
Intel