rds: Accept peer connection reject messages due to incompatible version

rds: Accept peer connection reject messages due to incompatible version

Prior to
commit d021fabf525ff ("rds: rdma: add consumer reject")

function "rds_rdma_cm_event_handler_cmn" would always honor a rejected
connection attempt by issuing a "rds_conn_drop".

The commit mentioned above added a "break", eliminating
the "fallthrough" case and made the "rds_conn_drop" rather conditional:

Now it only happens if a "consumer defined" reject (i.e. "rdma_reject")
carries an integer-value of "1" inside "private_data":

  if (!conn)
    break;
    err = (int *)rdma_consumer_reject_data(cm_id, event, &len);
    if (!err || (err && ((*err) == RDS_RDMA_REJ_INCOMPAT))) {
      pr_warn("RDS/RDMA: conn <%pI6c, %pI6c> rejected, dropping connection\n",
              &conn->c_laddr, &conn->c_faddr);
              conn->c_proposed_version = RDS_PROTOCOL_COMPAT_VERSION;
              rds_conn_drop(conn);
    }
    rdsdebug("Connection rejected: %s\n",
             rdma_reject_msg(cm_id, event->status));
    break;
    /* FALLTHROUGH */
A number of issues are worth mentioning here:
   #1) Previous versions of the RDS code simply rejected a connection
       by calling "rdma_reject(cm_id, NULL, 0);"
       So the value of the payload in "private_data" will not be "1",
       but "0".

   #2) Now the code has become dependent on host byte order and sizing.
       If one peer is big-endian, the other is little-endian,
       or there's a difference in sizeof(int) (e.g. ILP64 vs LP64),
       the *err check does not work as intended.

   #3) There is no check for "len" to see if the data behind *err is even valid.
       Luckily, it appears that the "rdma_reject(cm_id, NULL, 0)" will always
       carry 148 bytes of zeroized payload.
       But that should probably not be relied upon here.

   #4) With the added "break;",
       we might as well drop the misleading "/* FALLTHROUGH */" comment.

This commit does _not_ address issue #2, as the sender would have to
agree on a byte order as well.

Here is the sequence of messages in this observed error-scenario:
   Host-A is pre-QoS changes (excluding the commit mentioned above)
   Host-B is post-QoS changes (including the commit mentioned above)

   #1 Host-B
      issues a connection request via function "rds_conn_path_transition"
      connection state transitions to "RDS_CONN_CONNECTING"

   #2 Host-A
      rejects the incompatible connection request (from #1)
      It does so by calling "rdma_reject(cm_id, NULL, 0);"

   #3 Host-B
      receives an "RDMA_CM_EVENT_REJECTED" event (from #2)
      But since the code is changed in the way described above,
      it won't drop the connection here, simply because "*err == 0".

   #4 Host-A
      issues a connection request

   #5 Host-B
      receives an "RDMA_CM_EVENT_CONNECT_REQUEST" event
      and ends up calling "rds_ib_cm_handle_connect".
      But since the state is already in "RDS_CONN_CONNECTING"
      (as of #1) it will end up issuing a "rdma_reject" without
      dropping the connection:
         if (rds_conn_state(conn) == RDS_CONN_CONNECTING) {
             /* Wait and see - our connect may still be succeeding */
             rds_ib_stats_inc(s_ib_connect_raced);
         }
         goto out;

   #6 Host-A
      receives an "RDMA_CM_EVENT_REJECTED" event (from #5),
      drops the connection and tries again (goto #4) until it gives up.

Tested-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: Gerd Rausch <gerd.rausch@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>