Re: [PATCH net 3/4] sctp: don't treat an I-FORWARD-TSN as an association reset

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- [Medium] After this patch, sctp_intl_skip() is the only code that emits
  SCTP_PARTIAL_DELIVERY_ABORTED when an I-FORWARD-TSN reaches the…

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sctp: don't treat an I-FORWARD-TSN as an association reset

This commit removes the ftsn == max_tsn_seen check from
sctp_report_iftsn(). That check made every I-FORWARD-TSN reaching the
leading edge abort partial delivery on all streams and flush the ulpq.
The SSN/TSN reset paths in stream.c now call abort_pd() explicitly, and
the leading-edge abort is left to sctp_intl_skip().

> diff --git a/net/sctp/stream_interleave.c b/net/sctp/stream_interleave.c
> index 77fe0db5ff774..f49b534ff78fd 100644
> --- a/net/sctp/stream_interleave.c
> +++ b/net/sctp/stream_interleave.c
> @@ -1225,9 +1225,6 @@ static void sctp_report_iftsn(struct sctp_ulpq *ulpq, __u32 ftsn)
>  {
>  	/* Move the Cumulattive TSN Ack ahead. */
>  	sctp_tsnmap_skip(&ulpq->asoc->peer.tsn_map, ftsn);
> -	/* abort only when it's for all data */
> -	if (ftsn == sctp_tsnmap_get_max_tsn_seen(&ulpq->asoc->peer.tsn_map))
> -		sctp_intl_abort_pd(ulpq, GFP_ATOMIC);
>  }

[Severity: Medium]
With this call gone, sctp_intl_skip() now sends
SCTP_PARTIAL_DELIVERY_ABORTED for an I-FORWARD-TSN at the leading edge.
Is the pdapi_seq it reports still the message that was being partially
delivered?

The removed path, sctp_intl_abort_pd(), reported the in-progress MID:

net/sctp/stream_interleave.c:sctp_intl_abort_pd() {
    ...
		if (sin->pd_mode_uo) {
			sin->pd_mode_uo = 0;

			mid = sin->mid_uo;
			sctp_intl_stream_abort_pd(ulpq, sid, mid, 0x1, gfp);
		}

		if (sin->pd_mode) {
			sin->pd_mode = 0;

			mid = sin->mid;
			sctp_intl_stream_abort_pd(ulpq, sid, mid, 0, gfp);
    ...
}

sctp_intl_skip() passes the skip entry MID instead:

net/sctp/stream_interleave.c:sctp_intl_skip() {
    ...
		if (sin->pd_mode_uo && !MID_lt(mid, sin->mid_uo)) {
			sin->pd_mode_uo = 0;
			sctp_intl_stream_abort_pd(ulpq, sid, mid, 0x1,
						  GFP_ATOMIC);
		}
    ...
	if (sin->pd_mode) {
		sin->pd_mode = 0;
		sctp_intl_stream_abort_pd(ulpq, sid, mid, 0x0, GFP_ATOMIC);
	}
    ...
}

The skip MID is the largest MID skipped on that stream, so it can be
greater than the in-progress MID. sctp_generate_iftsn() merges abandoned
chunks with the same stream and U flag into one entry and keeps the last
MID:

	skip_pos = sctp_get_skip_pos(&ftsn_skip_arr[0], nskips,
				     sid, flags);
	...
	ftsn_skip_arr[skip_pos].mid = mid;

sctp_ulpevent_make_pdapi() then copies this value into the user-visible
field with pd->pdapi_seq = seq.

Here is an example. The receiver is partially delivering ordered MID 3
on stream S. The sender abandons MIDs 3 and 4 on S and sends nothing
after them. The I-FORWARD-TSN then carries a single entry {S, mid=4}.
Before this patch userspace got pdapi_seq=3. Now it gets pdapi_seq=4,
a message that was never partially delivered.

sctp_intl_skip() already reported the skip MID for I-FORWARD-TSNs that
do not reach the leading edge. This patch extends that to the
leading-edge case, which the commit message describes as common. The
code is still the same at the end of the series.

Could sctp_intl_skip() pass sin->mid_uo (unordered) or sin->mid
(ordered) to sctp_intl_stream_abort_pd() for the notification, and
keep using the skip MID for the flush and sctp_mid_skip()?

>  
>  static void sctp_handle_fwdtsn(struct sctp_ulpq *ulpq, struct sctp_chunk *chunk)

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