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

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Date:   Mon, 11 Oct 2021 09:18:42 +0200
From:   Kurt Van Dijck <dev.kurt@...dijck-laurijssen.be>
To:     Oleksij Rempel <o.rempel@...gutronix.de>
Cc:     Zhang Changzhong <zhangchangzhong@...wei.com>,
        Maxime Jayat <maxime.jayat@...ile-devices.fr>,
        Robin van der Gracht <robin@...tonic.nl>,
        linux-kernel@...r.kernel.org,
        Oleksij Rempel <linux@...pel-privat.de>,
        netdev@...r.kernel.org, Marc Kleine-Budde <mkl@...gutronix.de>,
        kernel@...gutronix.de, Oliver Hartkopp <socketcan@...tkopp.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-can@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH net] can: j1939: j1939_xtp_rx_dat_one(): cancel session
 if receive TP.DT with error length

On Mon, 11 Oct 2021 08:35:07 +0200, Oleksij Rempel wrote:
> On Sat, Oct 09, 2021 at 04:43:56PM +0800, Zhang Changzhong wrote:
> > 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.
> > >>
> > >>>
> > >>>>
[...]
> > >>>
> > >>> 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.
> 
> OK, then I have some problems to understand this part:
> - 5.10.2.4 Connection Closure
>   The “connection abort” message is not allowed to be used by responders in the
>   case of a global destination (i.e. BAM).
> 
> My assumption would be: In case of broadcast transfer, multiple MCU are
> receivers. If one of MCU was not able to get complete TP.DT, it should
> not abort BAM for all.

There is indeed no action defined to abort at BAM.

> 
> So, "DUT discards the BAM transport" sounds for me as local action.
> Complete TP would be dropped locally.

exact.

Kurt