netdev - Re: [PATCH net] can: j1939: j1939_xtp_rx_dat_one(): cancel session if receive TP.DT with error length

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Date:   Fri, 8 Oct 2021 13:00:07 +0200
From:   Oleksij Rempel <o.rempel@...gutronix.de>
To:     Zhang Changzhong <zhangchangzhong@...wei.com>
Cc:     Robin van der Gracht <robin@...tonic.nl>,
        Oleksij Rempel <linux@...pel-privat.de>, kernel@...gutronix.de,
        Oliver Hartkopp <socketcan@...tkopp.net>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Maxime Jayat <maxime.jayat@...ile-devices.fr>,
        linux-can@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] can: j1939: j1939_xtp_rx_dat_one(): cancel session
 if receive TP.DT with error length

On Fri, Oct 08, 2021 at 05:22:12PM +0800, Zhang Changzhong wrote:
> Hi Kurt,
> Sorry for the late reply.
> 
> On 2021/9/30 15:42, Kurt Van Dijck wrote:
> > On Thu, 30 Sep 2021 11:33:20 +0800, Zhang Changzhong wrote:
> >> According to SAE-J1939-21, the data length of TP.DT must be 8 bytes, so
> >> cancel session when receive unexpected TP.DT message.
> > 
> > SAE-j1939-21 indeed says that all TP.DT must be 8 bytes.
> > However, the last TP.DT may contain up to 6 stuff bytes, which have no meaning.
> > If I remember well, they are even not 'reserved'.
> 
> Agree, these bytes are meaningless for last TP.DT.
> 
> >
> >>
> >> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
> >> Signed-off-by: Zhang Changzhong <zhangchangzhong@...wei.com>
> >> ---
> >>  net/can/j1939/transport.c | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
> >> index bb5c4b8..eedaeaf 100644
> >> --- a/net/can/j1939/transport.c
> >> +++ b/net/can/j1939/transport.c
> >> @@ -1789,6 +1789,7 @@ static void j1939_xtp_rx_dpo(struct j1939_priv *priv, struct sk_buff *skb,
> >>  static void j1939_xtp_rx_dat_one(struct j1939_session *session,
> >>  				 struct sk_buff *skb)
> >>  {
> >> +	enum j1939_xtp_abort abort = J1939_XTP_ABORT_FAULT;
> >>  	struct j1939_priv *priv = session->priv;
> >>  	struct j1939_sk_buff_cb *skcb, *se_skcb;
> >>  	struct sk_buff *se_skb = NULL;
> >> @@ -1803,9 +1804,11 @@ static void j1939_xtp_rx_dat_one(struct j1939_session *session,
> >>  
> >>  	skcb = j1939_skb_to_cb(skb);
> >>  	dat = skb->data;
> >> -	if (skb->len <= 1)
> >> +	if (skb->len != 8) {
> >>  		/* makes no sense */
> >> +		abort = J1939_XTP_ABORT_UNEXPECTED_DATA;
> >>  		goto out_session_cancel;
> > 
> > I think this is a situation of
> > "be strict on what you send, be tolerant on what you receive".
> > 
> > Did you find a technical reason to abort a session because the last frame didn't
> > bring overhead that you don't use?
> 
> No technical reason. The only reason is that SAE-J1939-82 requires responder
> to abort session if any TP.DT less than 8 bytes (section A.3.4, Row 7).

Do you mean: "BAM Transport: Ensure DUT discards BAM transport when
TP.DT data packets are not correct size" ... "Verify DUT discards the
BAM transport if any TP.DT data packet has less than 8 bytes"?

Regards,
Oleksij
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