netdev - Re: [PATCH net-next v2 1/6] net: dcb: add new pcp selector to app object

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Date:   Tue, 4 Oct 2022 12:52:35 +0200
From:   Petr Machata <petrm@...dia.com>
To:     Petr Machata <petrm@...dia.com>
CC:     Jakub Kicinski <kuba@...nel.org>,
        Daniel Machon <daniel.machon@...rochip.com>,
        <netdev@...r.kernel.org>, <davem@...emloft.net>,
        <maxime.chevallier@...tlin.com>, <thomas.petazzoni@...tlin.com>,
        <edumazet@...gle.com>, <pabeni@...hat.com>,
        <lars.povlsen@...rochip.com>, <Steen.Hegelund@...rochip.com>,
        <UNGLinuxDriver@...rochip.com>, <joe@...ches.com>,
        <linux@...linux.org.uk>, <horatiu.vultur@...rochip.com>,
        <Julia.Lawall@...ia.fr>, <vladimir.oltean@....com>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH net-next v2 1/6] net: dcb: add new pcp selector to app
 object


Petr Machata <petrm@...dia.com> writes:

> Jakub Kicinski <kuba@...nel.org> writes:
>
>> On Mon, 3 Oct 2022 09:52:59 +0200 Petr Machata wrote:
>>> I assumed the policy is much more strict with changes like this. If you
>>> think it's OK, I'm fine with it as well.
>>> 
>>> The userspace (lldpad in particular) is doing the opposite thing BTW:
>>> assuming everything in the nest is a DCB_ATTR_IEEE_APP. When we start
>>> emitting the new attribute, it will get confused.
>>
>> Can you add an attribute or a flag to the request which would turn
>> emitting the new attrs on?
>
> The question is whether it's better to do it anyway. My opinion is that
> if a userspace decides to make assumptions about the contents of a TLV,
> and neglects to validate the actual TLV type, it's on them, and I'm not
> obligated to keep them working. We know about this case, but really any
> attribute addition at all could potentially trip some userspace if they
> expected something else at this offset.

And re the flag: I think struct dcbmsg.dcb_pad was meant to be the place
to keep flags when the need arises, but it is not validated anywhere, so
we cannot use it. It could be a new command, but I'm not a fan. So if we
need to discriminate userspaces, I think it should be a new attribute.