netdev - Re: [PATCH net-next 0/4] inet: Separate DSCP from ECN bits using new dscp

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Date:   Fri, 17 Dec 2021 18:55:43 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Guillaume Nault <gnault@...hat.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Russell Strong <russell@...ong.id.au>
Subject: Re: [PATCH net-next 0/4] inet: Separate DSCP from ECN bits using
 new dscp_t type

>> > Note that there's no equivalent of patch 3 for IPv6 (ip route), since
>> > the tos/dsfield option is silently ignored for IPv6 routes.
>> 
>> Shouldn't we just start rejecting them, like for v4?
>
> I had some thoughs about that, but didn't talk about them in the cover
> letter since I felt there was already enough edge cases to discuss, and
> this one wasn't directly related to this series (the problem is there
> regardless of this RFC).
>
> So, on the one hand, we have this old policy of ignoring unknown
> netlink attributes, so it looks consistent to also ignore unused
> structure fields.
>
> On the other hand, ignoring rtm_tos leads to a different behaviour than
> what was requested. So it certainly makes sense to at least warn the
> user. But a hard fail may break existing programs that don't clear
> rtm_tos by mistake.
>
> I'm not too sure which approach is better.

So I guess you could argue that those applications were broken in the
first place, and so an explicit reject would only expose this? Do you
know of any applications that actually *function* while doing what you
describe?

One thought could be to add the rejection but be prepared to back it out
if it does turn out (during the -rc phase) that it breaks something?

-Toke