netdev - Re: [PATCH net-next 0/3] ipv4: First steps toward removing RTO

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Message-ID: <20220422110228.GB15621@debian.home>
Date:   Fri, 22 Apr 2022 13:02:28 +0200
From:   Guillaume Nault <gnault@...hat.com>
To:     David Ahern <dsahern@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        dccp@...r.kernel.org
Subject: Re: [PATCH net-next 0/3] ipv4: First steps toward removing RTO_ONLINK

On Thu, Apr 21, 2022 at 09:10:21PM -0600, David Ahern wrote:
> On 4/20/22 5:21 PM, Guillaume Nault wrote:
> > RTO_ONLINK is a flag that allows to reduce the scope of route lookups.
> > It's stored in a normally unused bit of the ->flowi4_tos field, in
> > struct flowi4. However it has several problems:
> > 
> >  * This bit is also used by ECN. Although ECN bits are supposed to be
> >    cleared before doing a route lookup, it happened that some code
> >    paths didn't properly sanitise their ->flowi4_tos. So this mechanism
> >    is fragile and we had bugs in the past where ECN bits slipped in and
> >    could end up being erroneously interpreted as RTO_ONLINK.
> > 
> >  * A dscp_t type was recently introduced to ensure ECN bits are cleared
> >    during route lookups. ->flowi4_tos is the most important structure
> >    field to convert, but RTO_ONLINK prevents such conversion, as dscp_t
> >    mandates that ECN bits (where RTO_ONLINK is stored) be zero.
> > 
> > Therefore we need to stop using RTO_ONLINK altogether. Fortunately
> > RTO_ONLINK isn't a necessity. Instead of passing a flag in ->flowi4_tos
> > to tell the route lookup function to restrict the scope, we can simply
> > initialise the scope correctly.
> > 
> 
> I believe the set looks ok. I think the fib test coverage in selftests
> could use more tests to cover tos.

Yes, this is on my todo list. I also plan to review existing tests that
cover route lookups with link scope, and extend them if necessary.

Thanks for the review.