ethtool: fix the check logic of at least one channel for RX/TX

The command "ethtool -L <intf> combined 0" may clean the RX/TX channel
count and skip the error path, since the attrs
tb[ETHTOOL_A_CHANNELS_RX_COUNT] and tb[ETHTOOL_A_CHANNELS_TX_COUNT]
are NULL in this case when recent ethtool is used.

Tested using ethtool v5.10.

Fixes: 7be92514b99c ("ethtool: check if there is at least one channel for TX/RX in the core")
Signed-off-by: Yinjun Zhang <[e-mail]>
Signed-off-by: Simon Horman <[e-mail]>
Signed-off-by: Louis Peens <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ethtool/channels.c

···
 	struct ethtool_channels channels = {};
 	struct ethnl_req_info req_info = {};
 	struct nlattr **tb = info->attrs;
-	const struct nlattr *err_attr;
+	u32 err_attr, max_rx_in_use = 0;
 	const struct ethtool_ops *ops;
 	struct net_device *dev;
-	u32 max_rx_in_use = 0;
 	int ret;
 
 	ret = ethnl_parse_header_dev_get(&req_info,
···
 
 	/* ensure new channel counts are within limits */
 	if (channels.rx_count > channels.max_rx)
-		err_attr = tb[ETHTOOL_A_CHANNELS_RX_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_RX_COUNT;
 	else if (channels.tx_count > channels.max_tx)
-		err_attr = tb[ETHTOOL_A_CHANNELS_TX_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_TX_COUNT;
 	else if (channels.other_count > channels.max_other)
-		err_attr = tb[ETHTOOL_A_CHANNELS_OTHER_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_OTHER_COUNT;
 	else if (channels.combined_count > channels.max_combined)
-		err_attr = tb[ETHTOOL_A_CHANNELS_COMBINED_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_COMBINED_COUNT;
 	else
-		err_attr = NULL;
+		err_attr = 0;
 	if (err_attr) {
 		ret = -EINVAL;
-		NL_SET_ERR_MSG_ATTR(info->extack, err_attr,
+		NL_SET_ERR_MSG_ATTR(info->extack, tb[err_attr],
 				    "requested channel count exceeds maximum");
 		goto out_ops;
 	}
 
 	/* ensure there is at least one RX and one TX channel */
 	if (!channels.combined_count && !channels.rx_count)
-		err_attr = tb[ETHTOOL_A_CHANNELS_RX_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_RX_COUNT;
 	else if (!channels.combined_count && !channels.tx_count)
-		err_attr = tb[ETHTOOL_A_CHANNELS_TX_COUNT];
+		err_attr = ETHTOOL_A_CHANNELS_TX_COUNT;
 	else
-		err_attr = NULL;
+		err_attr = 0;
 	if (err_attr) {
 		if (mod_combined)
-			err_attr = tb[ETHTOOL_A_CHANNELS_COMBINED_COUNT];
+			err_attr = ETHTOOL_A_CHANNELS_COMBINED_COUNT;
 		ret = -EINVAL;
-		NL_SET_ERR_MSG_ATTR(info->extack, err_attr, "requested channel counts would result in no RX or TX channel being configured");
+		NL_SET_ERR_MSG_ATTR(info->extack, tb[err_attr],
+				    "requested channel counts would result in no RX or TX channel being configured");
 		goto out_ops;
 	}