netdev - Re: [RFC 4/5] netlink: prepare validate extack setting for recursion

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Date:   Wed, 19 Sep 2018 11:15:25 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Jiri Benc <jbenc@...hat.com>
Cc:     netdev@...r.kernel.org
Subject: Re: [RFC 4/5] netlink: prepare validate extack setting for recursion

On Wed, 2018-09-19 at 11:10 +0200, Jiri Benc wrote:
> On Tue, 18 Sep 2018 15:12:11 +0200, Johannes Berg wrote:
> >  static int validate_nla(const struct nlattr *nla, int maxtype,
> >  			const struct nla_policy *policy,
> > -			const char **error_msg)
> > +			struct netlink_ext_ack *extack, bool *extack_set)
> 
> Can't the extack_set be included in the struct netlink_ext_ack? One less
> parameter to pass around and the NL_SET_* macros could check this on
> their own.

No, I don't think it can or should.

For one, having the NL_SET_* macros check it on their own will already
not work - as we discussed over in the NLA_REJECT thread, we do need to
be able to override the data, e.g. if somebody does

NL_SET_ERR_MSG(extack, "warning: deprecated command");
err = nla_parse(..., extack);

and nla_parse() sets a new message. Thus, hiding all the logic in there
already will not work, and is also IMHO rather unexpected. Normally,
*later* messages should win, not *earlier* ones - and that doesn't
require any tracking, just setting them unconditionally.

It's just a side effect of how we do the recursive validation here that
we want *earlier* messages to win (but only within this code piece - if
a previous message was set, we want it to be overwritten by nla_parse!).

It might be possible to do this differently, in theory, but all the ways
I've tried to come up with so far made the code vastly more complex.

johannes