trx_if: Allow calling trx_if_flush/close from within TRXC callback (v2)

- If the llist is flushed during rx rsp callback, when the flow is
  returned to trx_ctrl_read_cb() it would access tcm which was in the
llist and end up in use-after-free.
- We need to store state on whether code path is inside the read_cb in
  order to:
-- Delay transmission of new message if callback calls trx_if_flush()
   followed by trx_ctrl_send(), since the trx_ctrl_send() at the end of
   trx_ctrl_read_cb would retransmit it again immediatelly.
-- Avoid accessing tcm pointer if the callback called trx_if_flush(),
   since it has been freed.

Related: OS#6020
Change-Id: Ibdffa4644aa3a7d219452644d3e74b411734f1df
---
M src/osmo-bts-trx/l1_if.h
M src/osmo-bts-trx/trx_if.c
2 files changed, 52 insertions(+), 7 deletions(-)

diff --git a/src/osmo-bts-trx/l1_if.h b/src/osmo-bts-trx/l1_if.h
index 18d84c2..84fd4b5 100644
--- a/src/osmo-bts-trx/l1_if.h
+++ b/src/osmo-bts-trx/l1_if.h
@@ -120,6 +120,10 @@
 	struct llist_head	trx_ctrl_list;
 	/* Latest RSPed cmd, used to catch duplicate RSPs from sent retransmissions */
 	struct trx_ctrl_msg 	*last_acked;
+	/* Whether the code path is in the middle of handling a received message. */
+	bool			in_trx_ctrl_read_cb;
+	/* Whether the l1h->trx_ctrl_list was flushed by the callback handling a received message */
+	bool			flushed_while_in_trx_ctrl_read_cb;
 
 	//struct gsm_bts_trx	*trx;
 	struct phy_instance	*phy_inst;
diff --git a/src/osmo-bts-trx/trx_if.c b/src/osmo-bts-trx/trx_if.c
index 3f9fc04..89078a3 100644
--- a/src/osmo-bts-trx/trx_if.c
+++ b/src/osmo-bts-trx/trx_if.c
@@ -269,8 +269,10 @@
 		tcm->cmd, tcm->params_len ? " " : "", tcm->params);
 	llist_add_tail(&tcm->list, &l1h->trx_ctrl_list);
 
-	/* send message, if we didn't already have pending messages */
-	if (prev == NULL)
+	/* send message, if we didn't already have pending messages.
+	 * If we are in the rx_rsp callback code path, skip sending, the
+	 * callback will do so when returning to it. */
+	if (prev == NULL && !l1h->in_trx_ctrl_read_cb)
 		trx_ctrl_send(l1h);
 
 	return 0;
@@ -673,6 +675,7 @@
 	struct trx_ctrl_rsp rsp;
 	int len, rc;
 	struct trx_ctrl_msg *tcm;
+	bool flushed;
 
 	len = recv(ofd->fd, buf, sizeof(buf) - 1, 0);
 	if (len <= 0)
@@ -722,21 +725,34 @@
 	rsp.cb = tcm->cb;
 
 	/* check for response code */
+	l1h->in_trx_ctrl_read_cb = true;
 	rc = trx_ctrl_rx_rsp(l1h, &rsp, tcm);
+	/* Reset state: */
+	flushed = l1h->flushed_while_in_trx_ctrl_read_cb;
+	l1h->flushed_while_in_trx_ctrl_read_cb = false;
+	l1h->in_trx_ctrl_read_cb = false;
+
 	if (rc == -EINVAL)
 		goto rsp_error;
 
 	/* re-schedule last cmd in rc seconds time */
 	if (rc > 0) {
-		osmo_timer_schedule(&l1h->trx_ctrl_timer, rc, 0);
+		/* The queue may have been flushed in the trx_ctrl_rx_rsp(): */
+		if (!llist_empty(&l1h->trx_ctrl_list))
+			osmo_timer_schedule(&l1h->trx_ctrl_timer, rc, 0);
 		return 0;
 	}
 
-	/* remove command from list, save it to last_acked and removed previous last_acked */
-	llist_del(&tcm->list);
-	talloc_free(l1h->last_acked);
-	l1h->last_acked = tcm;
+	if (!flushed) {
+		/* Remove command from list, save it to last_acked and removed
+		 * previous last_acked */
+		llist_del(&tcm->list);
+		talloc_free(l1h->last_acked);
+		l1h->last_acked = tcm;
+	} /* else: tcm was freed by trx_if_flush(), do not access it. */
 
+
+	/* Send next message waiting in the list: */
 	trx_ctrl_send(l1h);
 
 	return 0;
@@ -1224,6 +1240,10 @@
 
 	/* Tx queue is now empty, so there's no point in keeping the retrans timer armed: */
 	osmo_timer_del(&l1h->trx_ctrl_timer);
+
+	/* If we are in read_cb, signal to the returning code path that we freed the list. */
+	if (l1h->in_trx_ctrl_read_cb)
+		l1h->flushed_while_in_trx_ctrl_read_cb = true;
 }
 
 /*! close the TRX for given handle (data + control socket) */

