Re: [PATCH net-next v1 5/7] ethtool: add helper to prevent invalid statistics exposure to userspace

Introduce a new helper function, `ethtool_stat_add`, to update 64-bit
statistics with proper handling of the reserved value
`ETHTOOL_STAT_NOT_SET`. This ensures that statistics remain valid and
are always reported to userspace, even if the driver accidentally sets
`ETHTOOL_STAT_NOT_SET` during an update.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
---
  include/linux/ethtool.h | 16 ++++++++++++++++
  1 file changed, 16 insertions(+)

diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
index b0ed740ca749..657bd69ddaf7 100644
--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h

@@ -371,6 +371,22 @@ static inline void ethtool_stats_init(u64 *stats, unsigned int n)
  		stats[n] = ETHTOOL_STAT_NOT_SET;
  }
  
+/**
+ * ethtool_stat_add - Add a value to a u64 statistic with wraparound handling
+ * @stat: Pointer to the statistic to update
+ * @value: Value to add to the statistic
+ *
+ * Adds the specified value to a u64 statistic. If the result of the addition
+ * equals the reserved value (`ETHTOOL_STAT_NOT_SET`), it increments the result
+ * by 1 to avoid the reserved value.
+ */
+static inline void ethtool_stat_add(u64 *stat, u64 value)
+{
+	*stat += value;
+	if (*stat == ETHTOOL_STAT_NOT_SET)
+		(*stat)++;
+}
+
  /* Basic IEEE 802.3 MAC statistics (30.3.1.1.*), not otherwise exposed
   * via a more targeted API.
   */