Subject: Re: [PATCH] sctp: send abort chunk when max_retrans exceeded

From: Vlad Yasevich <[email-masked]>
Date: Tue, 20 Nov 2012 14:28:10 -0500

On 11/20/2012 02:15 PM, Neil Horman wrote:
> On Tue, Nov 20, 2012 at 01:44:23PM -0500, Vlad Yasevich wrote:
>> On 11/20/2012 12:59 PM, Neil Horman wrote:
>>> In the event that an association exceeds its max_retrans attempts, we should
>>> send an ABORT chunk indicating that we are closing the assocation as a result.
>>> Because of the nature of the error, its unlikely to be received, but its a nice
>>> clean way to close the association if it does make it through, and it will give
>>> anyone watching via tcpdump a clue as to what happened.
>>>
>>> Signed-off-by: Neil Horman <[email-masked]>
>>> CC: Vlad Yasevich <[email-masked]>
>>> CC: "David S. Miller" <[email-masked]>
>>> CC: [email-masked]
>>> ---
>>>   include/net/sctp/sm.h    |  2 ++
>>>   net/sctp/sm_make_chunk.c | 26 +++++++++++++++++++++-----
>>>   net/sctp/sm_sideeffect.c |  9 ++++++++-
>>>   3 files changed, 31 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/include/net/sctp/sm.h b/include/net/sctp/sm.h
>>> index b5887e1..2a82d13 100644
>>> --- a/include/net/sctp/sm.h
>>> +++ b/include/net/sctp/sm.h
>>> @@ -234,6 +234,8 @@ struct sctp_chunk *sctp_make_abort_violation(const struct sctp_association *,
>>>   struct sctp_chunk *sctp_make_violation_paramlen(const struct sctp_association *,
>>>   				   const struct sctp_chunk *,
>>>   				   struct sctp_paramhdr *);
>>> +struct sctp_chunk *sctp_make_violation_max_retrans(const struct sctp_association *,
>>> +						   const struct sctp_chunk *);
>>>   struct sctp_chunk *sctp_make_heartbeat(const struct sctp_association *,
>>>   				  const struct sctp_transport *);
>>>   struct sctp_chunk *sctp_make_heartbeat_ack(const struct sctp_association *,
>>> diff --git a/net/sctp/sm_make_chunk.c b/net/sctp/sm_make_chunk.c
>>> index fbe1636..d6a8c80 100644
>>> --- a/net/sctp/sm_make_chunk.c
>>> +++ b/net/sctp/sm_make_chunk.c
>>> @@ -1074,17 +1074,33 @@ struct sctp_chunk *sctp_make_violation_paramlen(
>>>   {
>>>   	struct sctp_chunk *retval;
>>>   	static const char error[] = "The following parameter had invalid length:";
>>> -	size_t payload_len = sizeof(error) + sizeof(sctp_errhdr_t) +
>>> -				sizeof(sctp_paramhdr_t);
>>> +	size_t payload_len = sizeof(error) + sizeof(sctp_errhdr_t);
>>>
>>>   	retval = sctp_make_abort(asoc, chunk, payload_len);
>>>   	if (!retval)
>>>   		goto nodata;
>>>
>>> -	sctp_init_cause(retval, SCTP_ERROR_PROTO_VIOLATION,
>>> -			sizeof(error) + sizeof(sctp_paramhdr_t));
>>> +	sctp_init_cause(retval, SCTP_ERROR_PROTO_VIOLATION, sizeof(error));
>>> +	sctp_addto_chunk(retval, sizeof(error), error);
>>> +
>>> +nodata:
>>> +	return retval;
>>> +}
>>
>> Neil
>>
>> You ended dropping the parameter information of the parameter that
>> caused the violation.  Was that intentional?
>>
> Yes, it was, because theres not really IMO a specific parameter that causes this
> abort condition.

Sure there is.  You changed sctp_make_violation_paramlen() which is 
called when we receive a protocol parameter which has an invalid length.
This triggers a violation and the parameter is report back.  This has 
nothing to do with max_rtx overflow.

The new code doesn't have to include parameter information and I am fine 
with that.

-vlad

>  If a chunk needs to be resent more than max_retrans times, we
> abort the connection, theres no specific parameter that we can point to that
> says "this caused the problem", we're just aborting because we can't get a SACK
> from the peer.  Likewise, I can't think of any information that we can include
> that would give the peer, or the anyone tcpdumping the connection an improved
> view as to why the abort happened, beyond the string this patch currently
> includes.
>
> I know I had privately sent you an early version of the patch as a rough draft
> which did include space for a param header, but that patch never filled that
> space out, since we don't have any valid information to fill it out with.
>
> Thanks & Regards
> Neil
>

Neil