netdev - Re: [RFC] netlink: add NLA

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Message-ID: <20180912081621.GC29691@unicorn.suse.cz>
Date:   Wed, 12 Sep 2018 10:16:22 +0200
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [RFC] netlink: add NLA_REJECT policy type

On Wed, Sep 12, 2018 at 09:32:45AM +0200, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> In some situations some netlink attributes may be used for output
> only (kernel->userspace) or may be reserved for future use. It's
> then helpful to be able to prevent userspace from using them in
> messages sent to the kernel, since they'd otherwise be ignored and
> any future will become impossible if this happens.
> 
> Add NLA_REJECT to the policy which does nothing but reject (with
> EINVAL) validation of any messages containing this attribute.
> 
> The specific case I have in mind now is a shared nested attribute
> containing request/response data, and it would be pointless and
> potentially confusing to have userspace include response data in
> the messages that actually contain a request.

I find this feature very useful. Actually, I was a bit surprised when
I found I can't mark an attribute "forbidden" using policy.

IMHO it would be even nicer if one could also specify an error message
to use in extack if NLA_REJECT is applied; the easiest way would be
using .validation_data and passing extack to validate_nla() but I'm not
sure if it wouldn't qualify as an abuse.

Michal Kubecek