netlink: Pass extack to dump handlers

Declare extack in netlink_dump and pass to dump handlers via
netlink_callback. Add any extack message after the dump_done_errno
allowing error messages to be returned. This will be useful when
strict checking is done on dump requests, returning why the dump
fails EINVAL.
Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Christian Brauner <christian@brauner.io>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netlink.h

@@ -176,6 +176,7 @@ struct netlink_callback {
 	void			*data;
 	/* the module that dump function belong to */
 	struct module		*module;
+	struct netlink_ext_ack	*extack;
 	u16			family;
 	u16			min_dump_alloc;
 	unsigned int		prev_seq, seq;

net/netlink/af_netlink.c

@@ -2171,6 +2171,7 @@ EXPORT_SYMBOL(__nlmsg_put);
 static int netlink_dump(struct sock *sk)
 {
 	struct netlink_sock *nlk = nlk_sk(sk);
+	struct netlink_ext_ack extack = {};
 	struct netlink_callback *cb;
 	struct sk_buff *skb = NULL;
 	struct nlmsghdr *nlh;
@@ -2222,8 +2223,11 @@ static int netlink_dump(struct sock *sk)
 	skb_reserve(skb, skb_tailroom(skb) - alloc_size);
 	netlink_skb_set_owner_r(skb, sk);
 
-	if (nlk->dump_done_errno > 0)
+	if (nlk->dump_done_errno > 0) {
+		cb->extack = &extack;
 		nlk->dump_done_errno = cb->dump(skb, cb);
+		cb->extack = NULL;
+	}
 
 	if (nlk->dump_done_errno > 0 ||
 	    skb_tailroom(skb) < nlmsg_total_size(sizeof(nlk->dump_done_errno))) {
@@ -2246,6 +2250,12 @@ static int netlink_dump(struct sock *sk)
 	memcpy(nlmsg_data(nlh), &nlk->dump_done_errno,
 	       sizeof(nlk->dump_done_errno));
 
+	if (extack._msg && nlk->flags & NETLINK_F_EXT_ACK) {
+		nlh->nlmsg_flags |= NLM_F_ACK_TLVS;
+		if (!nla_put_string(skb, NLMSGERR_ATTR_MSG, extack._msg))
+			nlmsg_end(skb, nlh);
+	}
+
 	if (sk_filter(sk, skb))
 		kfree_skb(skb);
 	else