Re: Truncate DLC to 8 instead of dropping?

[Wolfram Sang <w.sang@pengutronix.de>]

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On Fri, Aug 31, 2012 at 06:35:14PM +0200, Oliver Hartkopp wrote:
> Hello Kurt and Wolfram,
> 
> On 31.08.2012 14:50, Kurt Van Dijck wrote:
> 
> > On Fri, Aug 31, 2012 at 01:59:04PM +0200, Wolfram Sang wrote:
> >> Hi,
> >>
> >> I just noticed today that commit c7cd606f60e7679c7f9eee7010f02a6f000209c1
> >> ("can: Fix data length code handling in rx path") says regarding
> >> overlong packets:
> >>
> >> ===
> >>
> >> The ISO 11898-1 Chapter 8.4.2.3 (DLC field) says that register values > 8
> >> should be reduced to 8 without any error reporting or frame drop.
> >>
> >> ===
> >>
> >> and this is why get_can_dlc() came into existance.
> >>
> >> However, functions like can_dropped_invalid_skb() from dev.h or can_rcv() from
> >> af_can.c do check for a correct length, but drop the packet in the error case.
> >> Don't those need to be fixed?
> >>
> > 
> > Interesting point. My first idea is to acknowledge your thinking, BUT:
> > * ISO 11898-1 is about the wire format, whereas your issues address
> >   higher level interfaces. Higher level interfaces may IMHO use a stricter
> >   format that the wire format.
> 
> 
> Pointing to the wire format is a good idea.
> To me this "dlc error ignoring" requirement is implemented by the CAN
> controller. And when the dlc value in the controller register is not sanitized
> we should do it on driver level.

If I understand both of you correctly, that means the checks in the
functions I mentioned should be removed then?

Fine with me if this is consistent, I just wondered if it wasn't nice to
have a check in an upper layer in case the driver did not correctly
implement the check.

Thanks,

   Wolfram

-- 
Pengutronix e.K.                           | Wolfram Sang                |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |

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