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 17:22:12 +0800
From:   Zhang Changzhong <zhangchangzhong@...wei.com>
To:     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

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).

Best regards,
Changzhong

> 
> Kind regards,
> Kurt
>> +	}
>>  
>>  	switch (session->last_cmd) {
>>  	case 0xff:
>> @@ -1904,7 +1907,7 @@ static void j1939_xtp_rx_dat_one(struct j1939_session *session,
>>   out_session_cancel:
>>  	kfree_skb(se_skb);
>>  	j1939_session_timers_cancel(session);
>> -	j1939_session_cancel(session, J1939_XTP_ABORT_FAULT);
>> +	j1939_session_cancel(session, abort);
>>  	j1939_session_put(session);
>>  }
>>  
>> -- 
>> 2.9.5
>>
> .
>