Re: [RFC net-next] bonding: netlink error message support for options

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 17 May 2022 14:11:14 -0700 Jay Vosburgh wrote:
> 	If I'm reading the code correctly, rtnl isn't held that long.
> Once the ->doit() returns, rtnl is dropped, but the copy happens later:
> 
> rtnetlink_rcv()
> 	netlink_rcv_skb(skb, &rtnetlink_rcv_msg)
> 		rtnetlink_rcv_msg()	[ as cb(skb, nlh, &extack) ]
> 			rtnl_lock()
> 			link->doit()	[ rtnl_setlink, rtnl_newlink, et al ]
> 			rtnl_unlock()
> 		netlink_ack()
> 
> inside netlink_ack():
> 
>         if (nlk_has_extack && extack) {
>                 if (extack->_msg) {
>                         WARN_ON(nla_put_string(skb, NLMSGERR_ATTR_MSG,
>                                                extack->_msg));
>                 }

Indeed.

> 	Even if the strings have to be constant (via NL_SET_ERR_MSG),
> adding extack messages is likely still an improvement.

At a quick glance it seems like the major use of the printf here is to
point at a particular option. If options are carried in individual
attributes pointing at the right attribute with NL_SET_ERR_MSG_ATTR()
should also be helpful. Maybe that's stating the obvious.