From: Karel Zak <kzak@redhat.com>
Date: Mon, 15 Jun 2026 11:15:01 +0000 (+0200)
Subject: libmount: detect fanotify queue overflow in monitor
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=a9ce458c65aacdf1676987f6fc15371643907976;p=thirdparty%2Futil-linux.git

libmount: detect fanotify queue overflow in monitor

When the fanotify event queue overflows under sustained mount churn,
the kernel sends a FAN_Q_OVERFLOW event. The fanotify_next_fs() function
did not check for this, causing it to access invalid memory (the
overflow event carries no fanotify_event_info_mnt payload) and silently
drop the overflow notification. The consumer never learned that events
were lost, so its mount-unit view could permanently diverge from the
kernel.

Detect FAN_Q_OVERFLOW, discard the remaining buffered events (they are
unreliable after an overflow), and return -EOVERFLOW so the caller can
perform a full rescan.

Signed-off-by: Karel Zak <kzak@redhat.com>
---

diff --git a/libmount/src/monitor.c b/libmount/src/monitor.c
index e99607a3c..fd0aa628a 100644
--- a/libmount/src/monitor.c
+++ b/libmount/src/monitor.c
@@ -482,19 +482,27 @@ int mnt_monitor_event_cleanup(struct libmnt_monitor *mn)
  * (as returned by mnt_monitor_next_change()). If the event does not provide
  * details, it returns -ENOTSUP.
  *
+ * If the fanotify event queue has overflowed (events were lost due to
+ * sustained mount churn), this function returns -EOVERFLOW. In this case,
+ * the remaining buffered events are discarded and the caller should
+ * perform a full rescan to recover a consistent mount state.
+ *
  * <informalexample>
  *  <programlisting>
  *   while (mnt_monitor_next_change(mn, NULL, &type) == 0) {
  *     if (type == MNT_MONITOR_TYPE_FANOTIFY) {
- *       while (mnt_monitor_event_next_fs(mn, fs) == 0) {
+ *       int rc;
+ *       while ((rc = mnt_monitor_event_next_fs(mn, fs)) == 0) {
  *         printf("ID=%ju\n", mnt_fs_get_uniq_id(fs));
  *       }
+ *       if (rc == -EOVERFLOW)
+ *         full_rescan();
  *     }
  *   }
  *  </programlisting>
  * </informalexample>
  *
- * Returns: 0 on success, 1 no more data, <0 on error
+ * Returns: 0 on success, 1 no more data, -EOVERFLOW on queue overflow, <0 on error
  *
  * Since: 2.42
  */
diff --git a/libmount/src/monitor_fanotify.c b/libmount/src/monitor_fanotify.c
index 20d20ff3a..6d3b4617a 100644
--- a/libmount/src/monitor_fanotify.c
+++ b/libmount/src/monitor_fanotify.c
@@ -197,6 +197,12 @@ static int fanotify_next_fs(struct libmnt_monitor *mn, struct monitor_entry *me,
 	    meta->vers != FANOTIFY_METADATA_VERSION)
 		goto nothing;
 
+	if (meta->mask & FAN_Q_OVERFLOW) {
+		DBG_OBJ(MONITOR, mn, ul_debug("fanotify queue overflow"));
+		data->remaining = 0;
+		return -EOVERFLOW;
+	}
+
 	thislen = meta->event_len - sizeof(*meta);
 
 	mnt = (struct fanotify_event_info_mnt *)