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

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Date:   Thu, 21 Apr 2022 21:10:21 -0600
From:   David Ahern <dsahern@...nel.org>
To:     Guillaume Nault <gnault@...hat.com>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     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 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.