Hello Jakub,

quoted

For other set_link_af users IPV4 and IPV6 both make a trick here by
using nla_parse_nested_deprecated, which will check the NLA_F_NESTED
then able to use this type field as family value. This patch simply port
the MCTP code also to deprecated parser to make it work.

Did you test this? It's a suspiciously detailed and yet seemingly
incorrect explanation..

Yes, I hacked the iproute tool and add below code

https://github.com/iproute2/iproute2/blob/main/ip/iplink.c#L801

853a854,861

		} else if (matches(*argv, "mctptest") == 0) {
			struct rtattr *afs, *afsmctp;
			afs = addattr_nest(&req->n, sizeof(*req), IFLA_AF_SPEC);
			afsmctp = addattr_nest(&req->n, sizeof(*req), AF_MCTP);
			addattr32(&req->n, sizeof(*req),
                                 IFLA_MCTP_NET, 0);
			addattr_nest_end(&req->n, afsmctp);
			addattr_nest_end(&req->n, afs);

according to the implementation of the `addrgenmode`.

In short, the package format should be like
     
  L         T        L      T  
+---+--------------+---+---------+-------------
|   | IFLA_AF_SPEC |   | AF_MCTP | .....
+---+--------------+---+---------+-------------
                   ^
                   |
                   af

Therefore, the code

  nla_for_each_nested(af, tb[IFLA_AF_SPEC], rem) {
    const struct rtnl_af_ops *af_ops;
    BUG_ON(!(af_ops = rtnl_af_lookup(nla_type(af)))); <= (1)
    err = af_ops->set_link_af(dev, af, extack);       <= (2)

(1) will get AF_MCTP with nla_type(...) call, and (2) will then passing `af` to
`mctp_set_link_af`, which will expect the `NLA_F_NESTED` flag.

With the hacked iproute tools, simple type command like

`ip link set eth0 mtu 1024 mctptest` will hit the function mctp_set_link_af and
with my debugger, I found that the nla_parse_nested always return error.

Another elegant solution is to change how (1) calls rtnl_af_lookup, we can introduce
another family attribute and avoid to use nla_type(..) which is so suspicious.
But this will have to change also other set_link_af implementations (ipv4, ipv6).

-- 
pv-bot: s
pw-bot: cr

Regards
Lin