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:   Sat, 9 Oct 2021 16:43:56 +0800
From:   Zhang Changzhong <zhangchangzhong@...wei.com>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
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 2021/10/8 19:00, Oleksij Rempel wrote:
> 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"?

Yes.

Regards,
Changzhong

> 
> Regards,
> Oleksij
>