Date:   Tue, 14 Dec 2021 16:47:11 +0100
From:   Matthias May <matthias.may@...termo.com>
To:     Russell Strong <russell@...ong.id.au>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: DSCP in IPv4 routing v2

On 11/24/20 4:22 PM, Guillaume Nault wrote:
> On Tue, Nov 24, 2020 at 12:41:49PM +1000, Russell Strong wrote:
>> On Mon, 23 Nov 2020 23:55:05 +0100 Guillaume Nault <gnault@...hat.com> wrote:
>>> On Sat, Nov 21, 2020 at 06:24:46PM +1000, Russell Strong wrote:
>>
>> I was wondering if one patch would be acceptable, or should it be broken
>> up?  If broken up. It would not make sense to apply 1/2 of them.
> 
> A patch series would be applied in its entirety or not applied at all.
> However, it's not acceptable to temporarily bring regressions in one
> patch and fix it later in the series. The tree has to remain
> bisectable.
> 
> Anyway, I believe there's no need to replace all the TOS macros in the
> same patch series. DSCP doesn't have to be enabled everywhere at once.
> Small, targeted, patch series are much easier to review.
> 
>>> RT_TOS didn't clear the second lowest bit, while the new IP_DSCP does.
>>> Therefore, there's no guarantee that such a blanket replacement isn't
>>> going to change existing behaviours. Replacements have to be done
>>> step by step and accompanied by an explanation of why they're safe.
>>
>> Original TOS did not use this bit until it was added in RFC1349 as "lowcost".
>> The DSCP change (RFC2474) marked these as currently unused, but worse than that,
>> with the introduction of ECN, both of those now "unused" bits are for ECN.
>> Other parts of the kernel are using those bits for ECN, so bit 1 probably
>> shouldn't be used in routing anymore as congestion could create unexpected
>> routing behaviour, i.e. fib_rules
> 
> The IETF meaning and history of these bits are well understood. But we
> can't write patches based on assumptions like "bit 1 probably shouldn't
> be used". The actual code is what matters. That's why, again, changes
> have to be done incrementally and in a reviewable manner.
> 
>>> For example some of the ip6_make_flowinfo() calls can probably
>>> erroneously mark some packets with ECT(0). Instead of masking the
>>> problem in this patch, I think it'd be better to have an explicit fix
>>> that'd mask the ECN bits in ip6_make_flowinfo() and drop the buggy
>>> RT_TOS() in the callers.
>>>
>>> Another example is inet_rtm_getroute(). It calls
>>> ip_route_output_key_hash_rcu() without masking the tos field first.
>>
>> Should rtm->tos be checked for validity in inet_rtm_valid_getroute_req? Seems
>> like it was missed.
> 
> Well, I don't think so. inet_rtm_valid_getroute_req() is supposed to
> return an error if a parameter is wrong. Verifying ->tos should have
> been done since day 1, yes. However, in practice, we've been accepting
> any value for years. That's the kind of user space behaviour that we
> can't really change. The only solution I can see is to mask the ECN
> bits silently. That way, users can still pass whatever they like (we
> won't break any script), but the result will be right (that is,
> consistent with what routing does).
> 
>>> Therefore it can return a different route than what the routing code
>>> would actually use. Like for the ip6_make_flowinfo() case, it might
>>> be better to stop relying on the callers to mask ECN bits and do that
>>> in ip_route_output_key_hash_rcu() instead.
>>
>> In this context one of the ECN bits is not an ECN bit, as can be seen by
>>
>> #define RT_FL_TOS(oldflp4) \
>>         ((oldflp4)->flowi4_tos & (IP_DSCP_MASK | RTO_ONLINK))
> 
> The RTO_ONLINK flag would have to be passed in a different way. Not a
> trivial task (many places to audit), but that looks feasible.
> 
>> It's all a bit messy and spread about.  Reducing the distributed nature of
>> the masking would be good.
> 
> Yes, that's why I'd like to stop sprinkling RT_TOS everywhere and mask
> the bits in central places when possible. Once the RT_TOS situation
> improves, adding DSCP support will be much easier.
> 
>>> I'll verify that these two problems can actually happen in practice
>>> and will send patches if necessary.
>>
>> Thanks
>>
> 

Hi Russell

Do you have any plans to continue to work on this?

BR
Matthias



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