netlink: support all extack types in dumps

Note that when this commit message refers to netlink dump
it only means the actual dumping part, the parsing / dump
start is handled by the same code as "doit".

Commit 4a19edb ("netlink: Pass extack to dump handlers")
added support for returning extack messages from dump handlers,
but left out other extack info, e.g. bad attribute.

This used to be fine because until YNL we had little practical
use for the machine readable attributes, and only messages were
used in practice.

YNL flips the preference 180 degrees, it's now much more useful
to point to a bad attr with NL_SET_BAD_ATTR() than type
an English message saying "attribute XYZ is $reason-why-bad".

Support all of extack. The fact that extack only gets added if
it fits remains unaddressed.

Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240420023543.3300306-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: kuba-moo

net/netlink/af_netlink.c

@@ -2198,7 +2198,7 @@ netlink_ack_tlv_len(struct netlink_sock *nlk, int err,
 
 static void
 netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
-		     struct nlmsghdr *nlh, int err,
+		     const struct nlmsghdr *nlh, int err,
 		     const struct netlink_ext_ack *extack)
 {
 	if (extack->_msg)
@@ -2214,7 +2214,7 @@ netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
 	    !WARN_ON((u8 *)extack->bad_attr < in_skb->data ||
 		     (u8 *)extack->bad_attr >= in_skb->data + in_skb->len))
 		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_OFFS,
-				    (u8 *)extack->bad_attr - (u8 *)nlh));
+				    (u8 *)extack->bad_attr - (const u8 *)nlh));
 	if (extack->policy)
 		netlink_policy_dump_write_attr(skb, extack->policy,
 					       NLMSGERR_ATTR_POLICY);
@@ -2225,7 +2225,7 @@ netlink_ack_tlv_fill(struct sk_buff *in_skb, struct sk_buff *skb,
 	    !WARN_ON((u8 *)extack->miss_nest < in_skb->data ||
 		     (u8 *)extack->miss_nest > in_skb->data + in_skb->len))
 		WARN_ON(nla_put_u32(skb, NLMSGERR_ATTR_MISS_NEST,
-				    (u8 *)extack->miss_nest - (u8 *)nlh));
+				    (u8 *)extack->miss_nest - (const u8 *)nlh));
 }
 
 /*
@@ -2238,6 +2238,7 @@ static int netlink_dump_done(struct netlink_sock *nlk, struct sk_buff *skb,
 			     struct netlink_ext_ack *extack)
 {
 	struct nlmsghdr *nlh;
+	size_t extack_len;
 
 	nlh = nlmsg_put_answer(skb, cb, NLMSG_DONE, sizeof(nlk->dump_done_errno),
 			       NLM_F_MULTI | cb->answer_flags);
@@ -2247,10 +2248,14 @@ static int netlink_dump_done(struct netlink_sock *nlk, struct sk_buff *skb,
 	nl_dump_check_consistent(cb, nlh);
 	memcpy(nlmsg_data(nlh), &nlk->dump_done_errno, sizeof(nlk->dump_done_errno));
 
-	if (extack->_msg && test_bit(NETLINK_F_EXT_ACK, &nlk->flags)) {
+	extack_len = netlink_ack_tlv_len(nlk, nlk->dump_done_errno, extack);
+	if (extack_len) {
 		nlh->nlmsg_flags |= NLM_F_ACK_TLVS;
-		if (!nla_put_string(skb, NLMSGERR_ATTR_MSG, extack->_msg))
+		if (skb_tailroom(skb) >= extack_len) {
+			netlink_ack_tlv_fill(cb->skb, skb, cb->nlh,
+					     nlk->dump_done_errno, extack);
 			nlmsg_end(skb, nlh);
+		}
 	}
 
 	return 0;