ethtool: stats: Use struct_group() to clear all stats at once

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring fields.

Add struct_group() to mark region of struct stats_reply_data that should
be initialized, which can now be done in a single memset() call.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/stats.c

@@ -14,10 +14,12 @@ struct stats_req_info {
 
 struct stats_reply_data {
 	struct ethnl_reply_data		base;
+	struct_group(stats,
 		struct ethtool_eth_phy_stats	phy_stats;
 		struct ethtool_eth_mac_stats	mac_stats;
 		struct ethtool_eth_ctrl_stats	ctrl_stats;
 		struct ethtool_rmon_stats	rmon_stats;
+	);
 	const struct ethtool_rmon_hist_range	*rmon_ranges;
 };
 
@@ -117,10 +119,7 @@ static int stats_prepare_data(const struct ethnl_req_info *req_base,
 	/* Mark all stats as unset (see ETHTOOL_STAT_NOT_SET) to prevent them
 	 * from being reported to user space in case driver did not set them.
 	 */
-	memset(&data->phy_stats, 0xff, sizeof(data->phy_stats));
-	memset(&data->mac_stats, 0xff, sizeof(data->mac_stats));
-	memset(&data->ctrl_stats, 0xff, sizeof(data->ctrl_stats));
-	memset(&data->rmon_stats, 0xff, sizeof(data->rmon_stats));
+	memset(&data->stats, 0xff, sizeof(data->stats));
 
 	if (test_bit(ETHTOOL_STATS_ETH_PHY, req_info->stat_mask) &&
 	    dev->ethtool_ops->get_eth_phy_stats)