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@xxxxxxxxxx>
>> ---
>> 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
>>
> .
>
