staging/lustre/ptlrpc: Fix assertion failure of null_alloc_rs()

lustre_get_emerg_rs() set the size of the reply buffer to zero
by mistake, which will cause LBUG in null_alloc_rs() when memory
pressure is high. This patch fix this problem and adds a size
check to avoid the problem of insufficient buffer size.
Signed-off-by: Li Xi <lixi@ddn.com>
Signed-off-by: Patrick Farrell <paf@cray.com>
Reviewed-on: http://review.whamcloud.com/8200
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3680Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/ptlrpc/pack_generic.c

@@ -300,6 +300,7 @@ lustre_get_emerg_rs(struct ptlrpc_service_part *svcpt)
 	spin_unlock(&svcpt->scp_rep_lock);
 
 	memset(rs, 0, svcpt->scp_service->srv_max_reply_size);
+	rs->rs_size = svcpt->scp_service->srv_max_reply_size;
 	rs->rs_svcpt = svcpt;
 	rs->rs_prealloc = 1;
 out:

drivers/staging/lustre/lustre/ptlrpc/sec.c

@@ -2086,8 +2086,18 @@ int sptlrpc_svc_alloc_rs(struct ptlrpc_request *req, int msglen)
 
 	rc = policy->sp_sops->alloc_rs(req, msglen);
 	if (unlikely(rc == -ENOMEM)) {
+		struct ptlrpc_service_part *svcpt = req->rq_rqbd->rqbd_svcpt;
+		if (svcpt->scp_service->srv_max_reply_size <
+		   msglen + sizeof(struct ptlrpc_reply_state)) {
+			/* Just return failure if the size is too big */
+			CERROR("size of message is too big (%zd), %d allowed",
+				msglen + sizeof(struct ptlrpc_reply_state),
+				svcpt->scp_service->srv_max_reply_size);
+			return -ENOMEM;
+		}
+
 		/* failed alloc, try emergency pool */
-		rs = lustre_get_emerg_rs(req->rq_rqbd->rqbd_svcpt);
+		rs = lustre_get_emerg_rs(svcpt);
 		if (rs == NULL)
 			return -ENOMEM;