linux-kernel - Re: [PATCH] sctp: Make "Invalid Stream Identifier" ERROR follows SACK when bundling

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Date:	Tue, 24 Jul 2012 07:38:02 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	xufeng zhang <xufeng.zhang@...driver.com>
Cc:	xufengzhang.main@...il.com, vyasevich@...il.com, sri@...ibm.com,
	davem@...emloft.net, linux-sctp@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sctp: Make "Invalid Stream Identifier" ERROR follows
 SACK when bundling

On Tue, Jul 24, 2012 at 09:50:18AM +0800, xufeng zhang wrote:
> On 07/23/2012 08:14 PM, Neil Horman wrote:
> >On Mon, Jul 23, 2012 at 10:30:34AM +0800, xufeng zhang wrote:
> >>On 07/23/2012 08:49 AM, Neil Horman wrote:
> >>>Not sure I understand how you came into this error.  If we get an invalid
> >>>stream, we issue an SCTP_REPORT_TSN side effect, followed by an SCTP_CMD_REPLY
> >>>which sends the error chunk.  The reply goes through
> >>>sctp_outq_tail->sctp_outq_chunk->sctp_outq_transmit_chunk->sctp_outq_append_chunk.
> >>>That last function checks to see if a sack is already part of the packet, and if
> >>>there isn't one, appends one, using the updated tsn map.
> >>Yes, you are right, but consider the invalid stream identifier's
> >>DATA chunk is the first
> >>DATA chunk in the association which will need SACK immediately.
> >>Here is what I thought of the scenario:
> >>     sctp_sf_eat_data_6_2()
> >>         -->sctp_eat_data()
> >>             -->sctp_make_op_error()
> >>             -->sctp_add_cmd_sf(commands, SCTP_CMD_REPLY, SCTP_CHUNK(err))
> >>             -->sctp_outq_tail()          /* First enqueue ERROR chunk */
> >>         -->sctp_add_cmd_sf(commands, SCTP_CMD_GEN_SACK, SCTP_FORCE())
> >>             -->sctp_gen_sack()
> >>                 -->sctp_make_sack()
> >>                 -->sctp_add_cmd_sf(commands, SCTP_CMD_REPLY,
> >>SCTP_CHUNK(sack))
> >>                 -->sctp_outq_tail()          /* Then enqueue SACK chunk */
> >>
> >>So SACK chunk is enqueued after ERROR chunk.
> >Ah, I see.  Since the ERROR and SACK chunks are both control chunks, and since
> >we explicitly add the SACK to the control queue instead of going through the
> >bundle path in sctp_packet_append_chunk the ordering gets wrong.
> >
> >Ok, so the problem makes sense.  I think the soultion could be alot easier
> >though.  IIRC SACK chunks always live at the head of a packet, so why not just
> >special case it in sctp_outq_tail?  I.e. instead of doing a list_add_tail, in
> >the else clause of sctp_outq_tail check the chunk_hdr->type to see if its
> >SCTP_CID_SACK.  If it is, use list_add_head rather than list_add_tail.  I think
> >that will fix up both the COOKIE_ECHO and ESTABLISHED cases, won't it?  And then
> >you won't have keep track of extra state in the packet configuration.
> Yes, it's a good idea, but I think the premise is not correct:
> RFC 4960 page 57:
> "D) Upon reception of the COOKIE ECHO chunk, endpoint "Z" will reply
>    with a COOKIE ACK chunk after building a TCB and moving to the
>    ESTABLISHED state. A COOKIE ACK chunk may be bundled with any
>    pending DATA chunks (and/or SACK chunks), *but the COOKIE ACK chunk
>    MUST be the first chunk in the packet*."
> 
> So we can't put SACK chunk always at the head of the packet.
> 
Ok, Fair point, but that just changes the ordering a bit to:
COOKIE_ACK
SACK
OTHER CONTROL CHUNKS

What about something like this?  Its completely untested, and I'm sure it can be
cleaned up a bunch, but this keeps us from having to add additional state to the
packet structure.


diff --git a/net/sctp/outqueue.c b/net/sctp/outqueue.c
index e7aa177c..eeac32f 100644
--- a/net/sctp/outqueue.c
+++ b/net/sctp/outqueue.c
@@ -300,7 +300,7 @@ void sctp_outq_free(struct sctp_outq *q)
 int sctp_outq_tail(struct sctp_outq *q, struct sctp_chunk *chunk)
 {
 	int error = 0;
-
+	struct sctp_chunk *cptr;
 	SCTP_DEBUG_PRINTK("sctp_outq_tail(%p, %p[%s])\n",
 			  q, chunk, chunk && chunk->chunk_hdr ?
 			  sctp_cname(SCTP_ST_CHUNK(chunk->chunk_hdr->type))
@@ -344,7 +344,21 @@ int sctp_outq_tail(struct sctp_outq *q, struct sctp_chunk *chunk)
 			break;
 		}
 	} else {
-		list_add_tail(&chunk->list, &q->control_chunk_list);
+		list_del_init(&chunk->list);
+		if (chunk->chunk_hdr->type == SCTP_CID_COOKIE_ACK)
+			list_add_head(&chunk->list, &q->control_chunk_list);
+		else if (!list_empty(&q->control_chunk_list) &&
+		    chunk->chunk_hdr->type == SCTP_CID_SACK) {
+			list_for_each_entry(cptr, &q->control_chunk_list, list) {
+				if (cptr->chunk_hdr->type == SCTP_CID_COOKIE_ACK)
+					continue;
+				list_add(&chunk->list, &cptr->list);
+				break;
+			}
+		} 
+
+		if (list_empty(&chunk->list))
+			list_add_tail(&chunk->list, &q->control_chunk_list);
 		SCTP_INC_STATS(SCTP_MIB_OUTCTRLCHUNKS);
 	}
 
> 
> Thanks,
> Xufeng Zhang
> >Regards
> >Neil
> >
> >
> 
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