genetlink: do not parse attributes for families with zero maxattr

Commit c10e6cf8 ("net: genetlink: push attrbuf allocation and parsing
to a separate function") moved attribute buffer allocation and attribute
parsing from genl_family_rcv_msg_doit() into a separate function
genl_family_rcv_msg_attrs_parse() which, unlike the previous code, calls
__nlmsg_parse() even if family->maxattr is 0 (i.e. the family does its own
parsing). The parser error is ignored and does not propagate out of
genl_family_rcv_msg_attrs_parse() but an error message ("Unknown attribute
type") is set in extack and if further processing generates no error or
warning, it stays there and is interpreted as a warning by userspace.

Dumpit requests are not affected as genl_family_rcv_msg_dumpit() bypasses
the call of genl_family_rcv_msg_attrs_parse() if family->maxattr is zero.
Move this logic inside genl_family_rcv_msg_attrs_parse() so that we don't
have to handle it in each caller.

v3: put the check inside genl_family_rcv_msg_attrs_parse()
v2: adjust also argument of genl_family_rcv_msg_attrs_free()

Fixes: c10e6cf8 ("net: genetlink: push attrbuf allocation and parsing to a separate function")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -483,6 +483,9 @@ genl_family_rcv_msg_attrs_parse(const struct genl_family *family,
 	struct nlattr **attrbuf;
 	int err;
 
+	if (!family->maxattr)
+		return NULL;
+
 	if (parallel) {
 		attrbuf = kmalloc_array(family->maxattr + 1,
 					sizeof(struct nlattr *), GFP_KERNEL);
@@ -582,9 +585,6 @@ static int genl_family_rcv_msg_dumpit(const struct genl_family *family,
 	if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
 		return -EINVAL;
 
-	if (!family->maxattr)
-		goto no_attrs;
-
 	attrs = genl_family_rcv_msg_attrs_parse(family, nlh, extack,
 						ops, hdrlen,
 						GENL_DONT_VALIDATE_DUMP_STRICT,
@@ -649,7 +649,6 @@ static int genl_family_rcv_msg_doit(const struct genl_family *family,
 	attrbuf = genl_family_rcv_msg_attrs_parse(family, nlh, extack,
 						  ops, hdrlen,
 						  GENL_DONT_VALIDATE_STRICT,
-						  family->maxattr &&
 						  family->parallel_ops);
 	if (IS_ERR(attrbuf))
 		return PTR_ERR(attrbuf);
@@ -676,8 +675,7 @@ static int genl_family_rcv_msg_doit(const struct genl_family *family,
 		family->post_doit(ops, skb, &info);
 
 out:
-	genl_family_rcv_msg_attrs_free(family, attrbuf,
-				       family->maxattr && family->parallel_ops);
+	genl_family_rcv_msg_attrs_free(family, attrbuf, family->parallel_ops);
 
 	return err;
 }