rtnetlink: avoid frame size warning in rtnl_newlink()

Standard kernel compilation produces the following warning:

net/core/rtnetlink.c: In function ‘rtnl_newlink’:
net/core/rtnetlink.c:3232:1: warning: the frame size of 1288 bytes is larger than 1024 bytes [-Wframe-larger-than=]
 }
  ^

This should not really be an issue, as rtnl_newlink() stack is
generally quite shallow.

Fix the warning by allocating attributes with kmalloc() in a wrapper
and passing it down to rtnl_newlink(), avoiding complexities on error
paths.

Alternatively we could kmalloc() some structure within rtnl_newlink(),
slave attributes look like a good candidate.  In practice it adds to
already rather high complexity and length of the function.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/rtnetlink.c

@@ -2971,14 +2971,13 @@ static int rtnl_group_changelink(const struct sk_buff *skb,
 	return 0;
 }
 
-static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
-			struct netlink_ext_ack *extack)
+static int __rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
+			  struct nlattr **attr, struct netlink_ext_ack *extack)
 {
 	struct nlattr *slave_attr[RTNL_SLAVE_MAX_TYPE + 1];
 	unsigned char name_assign_type = NET_NAME_USER;
 	struct nlattr *linkinfo[IFLA_INFO_MAX + 1];
 	const struct rtnl_link_ops *m_ops = NULL;
-	struct nlattr *attr[RTNL_MAX_TYPE + 1];
 	struct net_device *master_dev = NULL;
 	struct net *net = sock_net(skb->sk);
 	const struct rtnl_link_ops *ops;
@@ -3226,6 +3225,21 @@ static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 	goto out;
 }
 
+static int rtnl_newlink(struct sk_buff *skb, struct nlmsghdr *nlh,
+			struct netlink_ext_ack *extack)
+{
+	struct nlattr **attr;
+	int ret;
+
+	attr = kmalloc_array(RTNL_MAX_TYPE + 1, sizeof(*attr), GFP_KERNEL);
+	if (!attr)
+		return -ENOMEM;
+
+	ret = __rtnl_newlink(skb, nlh, attr, extack);
+	kfree(attr);
+	return ret;
+}
+
 static int rtnl_getlink(struct sk_buff *skb, struct nlmsghdr *nlh,
 			struct netlink_ext_ack *extack)
 {