net: sctp: fix suboptimal edge-case on non-active active/retrans path selection

In SCTP, selection of active (T.ACT) and retransmission (T.RET)
transports is being done whenever transport control operations
(UP, DOWN, PF, ...) are engaged through sctp_assoc_control_transport().

Commits 4c47af4d ("net: sctp: rework multihoming retransmission
path selection to rfc4960") and a7288c4d ("net: sctp: improve
sctp_select_active_and_retran_path selection") have both improved
it towards a more fine-grained and optimal path selection.

Currently, the selection algorithm for T.ACT and T.RET is as follows:

1) Elect the two most recently used ACTIVE transports T1, T2 for
   T.ACT, T.RET, where T.ACT<-T1 and T1 is most recently used
2) In case primary path T.PRI not in {T1, T2} but ACTIVE, set
   T.ACT<-T.PRI and T.RET<-T1
3) If only T1 is ACTIVE from the set, set T.ACT<-T1 and T.RET<-T1
4) If none is ACTIVE, set T.ACT<-best(T.PRI, T.RET, T3) where
   T3 is the most recently used (if avail) in PF, set T.RET<-T.PRI

Prior to above commits, 4) was simply a camp on T.ACT<-T.PRI and
T.RET<-T.PRI, ignoring possible paths in PF. Camping on T.PRI is
still slightly suboptimal as it can lead to the following scenario:

Setup:
        <A>                                <B>
    T1: p1p1 (10.0.10.10) <==>  .'`)  <==> p1p1 (10.0.10.12)  <= T.PRI
    T2: p1p2 (10.0.10.20) <==> (_ . ) <==> p1p2 (10.0.10.22)

    net.sctp.rto_min = 1000
    net.sctp.path_max_retrans = 2
    net.sctp.pf_retrans = 0
    net.sctp.hb_interval = 1000

T.PRI is permanently down, T2 is put briefly into PF state (e.g. due to
link flapping). Here, the first time transmission is sent over PF path
T2 as it's the only non-INACTIVE path, but the retransmitted data-chunks
are sent over the INACTIVE path T1 (T.PRI), which is not good.

After the patch, it's choosing better transports in both cases by
modifying step 4):

4) If none is ACTIVE, set T.ACT_new<-best(T.ACT_old, T3) where T3 is
   the most recently used (if avail) in PF, set T.RET<-T.ACT_new

This will still select a best possible path in PF if available (which
can also include T.PRI/T.RET), and set both T.ACT/T.RET to it.

In case sctp_assoc_control_transport() *just* put T.ACT_old into INACTIVE
as it transitioned from ACTIVE->PF->INACTIVE and stays in INACTIVE just
for a very short while before going back ACTIVE, it will guarantee that
this path will be reselected for T.ACT/T.RET since T3 (PF) is not
available.

Previously, this was not possible, as we would only select between T.PRI
and T.RET, and a possible T3 would be NULL due to the fact that we have
just transitioned T3 in sctp_assoc_control_transport() from PF->INACTIVE
and would select a suboptimal path when T.PRI/T.RET have worse properties.

In the case that T.ACT_old permanently went to INACTIVE during this
transition and there's no PF path available, plus T.PRI and T.RET are
INACTIVE as well, we would now camp on T.ACT_old, but if everything is
being INACTIVE there's really not much we can do except hoping for a
successful HB to bring one of the transports back up again and, thus
cause a new selection through sctp_assoc_control_transport().

Now both tests work fine:

Case 1:

 1. T1 S(ACTIVE) T.ACT
    T2 S(ACTIVE) T.RET

 2. T1 S(ACTIVE) T.ACT, T.RET
    T2 S(PF)

 3. T1 S(ACTIVE) T.ACT, T.RET
    T2 S(INACTIVE)

 5. T1 S(PF) T.ACT, T.RET
    T2 S(INACTIVE)

[ 5.1 T1 S(INACTIVE) T.ACT, T.RET
      T2 S(INACTIVE) ]

 6. T1 S(ACTIVE) T.ACT, T.RET
    T2 S(INACTIVE)

 7. T1 S(ACTIVE) T.ACT
    T2 S(ACTIVE) T.RET

Case 2:

 1. T1 S(ACTIVE) T.ACT
    T2 S(ACTIVE) T.RET

 2. T1 S(PF)
    T2 S(ACTIVE) T.ACT, T.RET

 3. T1 S(INACTIVE)
    T2 S(ACTIVE) T.ACT, T.RET

 5. T1 S(INACTIVE)
    T2 S(PF) T.ACT, T.RET

[ 5.1 T1 S(INACTIVE)
      T2 S(INACTIVE) T.ACT, T.RET ]

 6. T1 S(INACTIVE)
    T2 S(ACTIVE) T.ACT, T.RET

 7. T1 S(ACTIVE) T.ACT
    T2 S(ACTIVE) T.RET
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sctp/associola.c

@@ -1356,14 +1356,11 @@ static void sctp_select_active_and_retran_path(struct sctp_association *asoc)
 		trans_sec = trans_pri;
 
 	/* If we failed to find a usable transport, just camp on the
-	 * primary or retran, even if they are inactive, if possible
-	 * pick a PF iff it's the better choice.
+	 * active or pick a PF iff it's the better choice.
 	 */
 	if (trans_pri == NULL) {
-		trans_pri = sctp_trans_elect_best(asoc->peer.primary_path,
-						  asoc->peer.retran_path);
-		trans_pri = sctp_trans_elect_best(trans_pri, trans_pf);
-		trans_sec = asoc->peer.primary_path;
+		trans_pri = sctp_trans_elect_best(asoc->peer.active_path, trans_pf);
+		trans_sec = trans_pri;
 	}
 
 	/* Set the active and retran transports. */