[PATCH batadv] batman-adv: tp_meter: fix race condition in send error reporting

[Sven Eckelmann <sven@narfation.org>]

batadv_tp_sender_shutdown() previously used two separate variables to track
session state: sending (an atomic flag indicating whether the session was
active) and reason (a plain enum storing the stop reason). This introduced
a race window between the two writes: after sending was cleared to 0,
batadv_tp_send() could observe the stopped state and call
batadv_tp_sender_end() before reason was written, causing the wrong stop
reason to be reported to the caller.

Fix this by consolidating both variables into a single atomic send_result,
which holds 0 while the session is running and the stop reason once it
ends.

Fixes: 98d7a766b645 ("batman-adv: throughput meter implementation")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
---
 net/batman-adv/tp_meter.c | 40 +++++++++++++++++++++++++---------------
 net/batman-adv/types.h    | 10 +++++-----
 2 files changed, 30 insertions(+), 20 deletions(-)

diff --git a/net/batman-adv/tp_meter.c b/net/batman-adv/tp_meter.c
index ca6c3f63..a8a15ecc 100644
--- a/net/batman-adv/tp_meter.c
+++ b/net/batman-adv/tp_meter.c
@@ -419,11 +419,14 @@ static void batadv_tp_sender_cleanup(struct batadv_tp_vars *tp_vars)
 static void batadv_tp_sender_end(struct batadv_priv *bat_priv,
 				 struct batadv_tp_vars *tp_vars)
 {
+	enum batadv_tp_meter_reason reason;
 	u32 session_cookie;
 
+	reason = atomic_read(&tp_vars->send_result);
+
 	batadv_dbg(BATADV_DBG_TP_METER, bat_priv,
 		   "Test towards %pM finished..shutting down (reason=%d)\n",
-		   tp_vars->other_end, tp_vars->reason);
+		   tp_vars->other_end, reason);
 
 	batadv_dbg(BATADV_DBG_TP_METER, bat_priv,
 		   "Last timing stats: SRTT=%ums RTTVAR=%ums RTO=%ums\n",
@@ -436,7 +439,7 @@ static void batadv_tp_sender_end(struct batadv_priv *bat_priv,
 	session_cookie = batadv_tp_session_cookie(tp_vars->session,
 						  tp_vars->icmp_uid);
 
-	batadv_tp_batctl_notify(tp_vars->reason,
+	batadv_tp_batctl_notify(reason,
 				tp_vars->other_end,
 				bat_priv,
 				tp_vars->start_time,
@@ -452,10 +455,18 @@ static void batadv_tp_sender_end(struct batadv_priv *bat_priv,
 static void batadv_tp_sender_shutdown(struct batadv_tp_vars *tp_vars,
 				      enum batadv_tp_meter_reason reason)
 {
-	if (atomic_xchg(&tp_vars->sending, 0) != 1)
-		return;
+	atomic_cmpxchg(&tp_vars->send_result, 0, reason);
+}
 
-	tp_vars->reason = reason;
+/**
+ * batadv_tp_sender_stopped() - check if tp session was stopped with reason
+ * @tp_vars: the private data of the current TP meter session
+ *
+ * Return: whether stop reason was found
+ */
+static bool batadv_tp_sender_stopped(struct batadv_tp_vars *tp_vars)
+{
+	return atomic_read(&tp_vars->send_result) != 0;
 }
 
 /**
@@ -485,7 +496,7 @@ static void batadv_tp_reset_sender_timer(struct batadv_tp_vars *tp_vars)
 	/* most of the time this function is invoked while normal packet
 	 * reception...
 	 */
-	if (unlikely(atomic_read(&tp_vars->sending) == 0))
+	if (unlikely(batadv_tp_sender_stopped(tp_vars)))
 		/* timer ref will be dropped in batadv_tp_sender_cleanup */
 		return;
 
@@ -505,7 +516,7 @@ static void batadv_tp_sender_timeout(struct timer_list *t)
 	struct batadv_tp_vars *tp_vars = timer_container_of(tp_vars, t, timer);
 	struct batadv_priv *bat_priv = tp_vars->bat_priv;
 
-	if (atomic_read(&tp_vars->sending) == 0)
+	if (batadv_tp_sender_stopped(tp_vars))
 		return;
 
 	/* if the user waited long enough...shutdown the test */
@@ -664,7 +675,7 @@ static void batadv_tp_recv_ack(struct batadv_priv *bat_priv,
 	if (unlikely(!tp_vars))
 		return;
 
-	if (unlikely(atomic_read(&tp_vars->sending) == 0))
+	if (unlikely(batadv_tp_sender_stopped(tp_vars)))
 		goto out;
 
 	/* old ACK? silently drop it.. */
@@ -830,21 +841,21 @@ static int batadv_tp_send(void *arg)
 
 	if (unlikely(tp_vars->role != BATADV_TP_SENDER)) {
 		err = BATADV_TP_REASON_DST_UNREACHABLE;
-		tp_vars->reason = err;
+		batadv_tp_sender_shutdown(tp_vars, err);
 		goto out;
 	}
 
 	orig_node = batadv_orig_hash_find(bat_priv, tp_vars->other_end);
 	if (unlikely(!orig_node)) {
 		err = BATADV_TP_REASON_DST_UNREACHABLE;
-		tp_vars->reason = err;
+		batadv_tp_sender_shutdown(tp_vars, err);
 		goto out;
 	}
 
 	primary_if = batadv_primary_if_get_selected(bat_priv);
 	if (unlikely(!primary_if)) {
 		err = BATADV_TP_REASON_DST_UNREACHABLE;
-		tp_vars->reason = err;
+		batadv_tp_sender_shutdown(tp_vars, err);
 		goto out;
 	}
 
@@ -863,7 +874,7 @@ static int batadv_tp_send(void *arg)
 	queue_delayed_work(batadv_event_workqueue, &tp_vars->finish_work,
 			   msecs_to_jiffies(tp_vars->test_length));
 
-	while (atomic_read(&tp_vars->sending) != 0) {
+	while (!batadv_tp_sender_stopped(tp_vars)) {
 		if (unlikely(!batadv_tp_avail(tp_vars, payload_len))) {
 			batadv_tp_wait_available(tp_vars, payload_len);
 			continue;
@@ -886,8 +897,7 @@ static int batadv_tp_send(void *arg)
 				   "Meter: %s() cannot send packets (%d)\n",
 				   __func__, err);
 			/* ensure nobody else tries to stop the thread now */
-			if (atomic_xchg(&tp_vars->sending, 0) == 1)
-				tp_vars->reason = err;
+			batadv_tp_sender_shutdown(tp_vars, err);
 			break;
 		}
 
@@ -1009,7 +1019,7 @@ void batadv_tp_start(struct batadv_priv *bat_priv, const u8 *dst,
 	ether_addr_copy(tp_vars->other_end, dst);
 	kref_init(&tp_vars->refcount);
 	tp_vars->role = BATADV_TP_SENDER;
-	atomic_set(&tp_vars->sending, 1);
+	atomic_set(&tp_vars->send_result, 0);
 	memcpy(tp_vars->session, session_id, sizeof(session_id));
 	tp_vars->icmp_uid = icmp_uid;
 
diff --git a/net/batman-adv/types.h b/net/batman-adv/types.h
index 739439e2..a7d1ea3a 100644
--- a/net/batman-adv/types.h
+++ b/net/batman-adv/types.h
@@ -1320,15 +1320,15 @@ struct batadv_tp_vars {
 	/** @role: receiver/sender modi */
 	enum batadv_tp_meter_role role;
 
-	/** @sending: sending binary semaphore: 1 if sending, 0 is not */
-	atomic_t sending;
+	/**
+	 * @stop_reason: 0 when sending is ongoing and otherwise
+	 * enum batadv_tp_meter_reason
+	 */
+	atomic_t send_result;
 
 	/** @receiving: receiving binary semaphore: 1 if receiving, 0 is not */
 	atomic_t receiving;
 
-	/** @reason: reason for a stopped session */
-	enum batadv_tp_meter_reason reason;
-
 	/** @finish_work: work item for the finishing procedure */
 	struct delayed_work finish_work;
 

---
base-commit: 57c6af492c1948145db835bb3ea2980472558298
change-id: 20260513-tp-reason-missing-f1aa51f3f374

Best regards,
--  
Sven Eckelmann <sven@narfation.org>