Subject: [PATCH net-next v2] ethtool: Clarify len/n_stats fields in/out semantics

From: Gal Pressman <[email-masked]>
Date: Mon, 12 Jan 2026 13:57:08 +0200

Document that the 'len' field in ethtool_gstrings and 'n_stats' field in
ethtool_stats optionally serve dual purposes: on entry they specify the
number of items requested, and on return they indicate the number
actually returned (which is not necessarily the same).

Reviewed-by: Dragos Tatulea <[email-masked]>
Signed-off-by: Gal Pressman <[email-masked]>
---
v1->v2: https://lore.kernel.org/all/[email-masked]/
* Reword comments (Jakub).
---
 include/uapi/linux/ethtool.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/uapi/linux/ethtool.h b/include/uapi/linux/ethtool.h
index eb7ff2602fbb..de810f1c3bfb 100644
--- a/include/uapi/linux/ethtool.h
+++ b/include/uapi/linux/ethtool.h
@@ -1101,6 +1101,13 @@ enum ethtool_module_fw_flash_status {
  * Users must use %ETHTOOL_GSSET_INFO to find the number of strings in
  * the string set.  They must allocate a buffer of the appropriate
  * size immediately following this structure.
+ *
+ * Setting @len on input is optional (though preferred), but must be zeroed
+ * otherwise.
+ * When set, @len will return the requested count if it matches the actual
+ * count; otherwise, it will be zero.
+ * This prevents issues when the number of strings is different than the
+ * userspace allocation.
  */
 struct ethtool_gstrings {
 	__u32	cmd;
@@ -1184,6 +1191,13 @@ struct ethtool_test {
  * number of statistics that will be returned.  They must allocate a
  * buffer of the appropriate size (8 * number of statistics)
  * immediately following this structure.
+ *
+ * Setting @n_stats on input is optional (though preferred), but must be zeroed
+ * otherwise.
+ * When set, @n_stats will return the requested count if it matches the actual
+ * count; otherwise, it will be zero.
+ * This prevents issues when the number of stats is different than the
+ * userspace allocation.
  */
 struct ethtool_stats {
 	__u32	cmd;
-- 
2.40.1