+ /*
+ * Don't try to enable a channel if the session has been started at
+ * some point in time before. The tracer does not allow it.
+ */
+ if (session->has_been_started) {
+ ret = LTTNG_ERR_TRACE_ALREADY_STARTED;
+ goto error;
+ }
+
+ /*
+ * The ringbuffer (both in user space and kernel) behave badly in overwrite
+ * mode and with less than 2 subbuf so block it right away and send back an
+ * invalid attribute error.
+ */
+ if (attr->attr.overwrite && attr->attr.num_subbuf < 2) {
+ ret = LTTNG_ERR_INVALID;
+ goto error;
+ }
+