netdev - Re: [PATCH net-next 1/6] netlink: add support for ext

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Date:   Wed, 24 Aug 2022 09:36:09 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com,
        pabeni@...hat.com, mkubecek@...e.cz
Subject: Re: [PATCH net-next 1/6] netlink: add support for ext_ack missing
 attributes

On Wed, 24 Aug 2022 10:09:55 +0200 Johannes Berg wrote:
> On Tue, 2022-08-23 at 21:50 -0700, Jakub Kicinski wrote:
> > The @offset points to the
> > nest which should have contained the attribute  
> 
> I find this a bit tedious, tbh. You already kernel-side have patch 2 and
> patch 3 that pass different things here.
> 
> Maybe it would be better to have this point to the _end_ of the {nest,
> message} header, which - if there are any - would be equivalent to the
> first sibling attribute?

Pointing at the start of a nest is easier because I can reuse the same
"attr walking" logic in user space as for finding invalid attributes 
to find the nest.

> Though I guess one way or the other userspace has to have an if that
> asks whether or not it's in a nest or the top-level namespace.
> 
> Hmm.
> 
> How about we just _remove_ the NLMSGERR_ATTR_MISS_NEST attribute if it's
> not missing in a nested attribute? That would make sense from the naming
> too:
>  * NLMSGERR_ATTR_MISS_TYPE - which attribute type you missed
>  * NLMSGERR_ATTR_MISS_NEST - which nesting you missed it in, _if any_
> 
> 
> And that way the if simplifies down to something like
> 
> 	if (tb[NLMSGERR_ATTR_MISS_NEST])
> 
> in the consumer too, and you don't need GENL_REQ_ATTR_CHECK() at all,
> you just pass NULL to the second argument of NL_REQ_ATTR_CHECK().

Sounds good!