ethtool: add a stricter length check

There has been a few errors in the ethtool reply size calculations,
most of those are hard to trigger during basic testing because of
skb size rounding up and netdev names being shorter than max.
Add a more precise check.

This change will affect the value of payload length displayed in
case of -EMSGSIZE but that should be okay, "payload length" isn't
a well defined term here.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/netlink.c

@@ -315,9 +315,9 @@ static int ethnl_default_doit(struct sk_buff *skb, struct genl_info *info)
 	struct ethnl_req_info *req_info = NULL;
 	const u8 cmd = info->genlhdr->cmd;
 	const struct ethnl_request_ops *ops;
+	int hdr_len, reply_len;
 	struct sk_buff *rskb;
 	void *reply_payload;
-	int reply_len;
 	int ret;
 
 	ops = ethnl_default_requests[cmd];
@@ -346,15 +346,20 @@ static int ethnl_default_doit(struct sk_buff *skb, struct genl_info *info)
 	ret = ops->reply_size(req_info, reply_data);
 	if (ret < 0)
 		goto err_cleanup;
-	reply_len = ret + ethnl_reply_header_size();
+	reply_len = ret;
 	ret = -ENOMEM;
-	rskb = ethnl_reply_init(reply_len, req_info->dev, ops->reply_cmd,
+	rskb = ethnl_reply_init(reply_len + ethnl_reply_header_size(),
+				req_info->dev, ops->reply_cmd,
 				ops->hdr_attr, info, &reply_payload);
 	if (!rskb)
 		goto err_cleanup;
+	hdr_len = rskb->len;
 	ret = ops->fill_reply(rskb, req_info, reply_data);
 	if (ret < 0)
 		goto err_msg;
+	WARN_ONCE(rskb->len - hdr_len > reply_len,
+		  "ethnl cmd %d: calculated reply length %d, but consumed %d\n",
+		  cmd, reply_len, rskb->len - hdr_len);
 	if (ops->cleanup_data)
 		ops->cleanup_data(reply_data);