Message-ID: <CAEf4BzZpEi0PQwn6oMs+fHf51VhpSvXZsNLV6Rf=9Kaqyyqy=g@mail.gmail.com>
Date: Fri, 9 Jun 2023 11:58:41 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Daniel Borkmann <daniel@...earbox.net>
Cc: Toke Høiland-Jørgensen <toke@...nel.org>, 
	Timo Beckers <timo@...line.eu>, Stanislav Fomichev <sdf@...gle.com>, ast@...nel.org, andrii@...nel.org, 
	martin.lau@...ux.dev, razor@...ckwall.org, john.fastabend@...il.com, 
	kuba@...nel.org, dxu@...uu.xyz, joe@...ium.io, davem@...emloft.net, 
	bpf@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/7] bpf: Add generic attach/detach/query API
 for multi-progs

On Fri, Jun 9, 2023 at 7:15 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 6/9/23 3:11 PM, Toke Høiland-Jørgensen wrote:
> > Timo Beckers <timo@...line.eu> writes:
> >> On 6/9/23 13:04, Toke Høiland-Jørgensen wrote:
> >>> Daniel Borkmann <daniel@...earbox.net> writes:
> [...]
> >>>>>>>>>> I'm still not sure whether the hard semantics of first/last is really
> >>>>>>>>>> useful. My worry is that some prog will just use BPF_F_FIRST which
> >>>>>>>>>> would prevent the rest of the users.. (starting with only
> >>>>>>>>>> F_BEFORE/F_AFTER feels 'safer'; we can iterate later on if we really
> >>>>>>>>>> need first/laste).
> >>>>>>>>> Without FIRST/LAST some scenarios cannot be guaranteed to be safely
> >>>>>>>>> implemented. E.g., if I have some hard audit requirements and I need
> >>>>>>>>> to guarantee that my program runs first and observes each event, I'll
> >>>>>>>>> enforce BPF_F_FIRST when attaching it. And if that attachment fails,
> >>>>>>>>> then server setup is broken and my application cannot function.
> >>>>>>>>>
> >>>>>>>>> In a setup where we expect multiple applications to co-exist, it
> >>>>>>>>> should be a rule that no one is using FIRST/LAST (unless it's
> >>>>>>>>> absolutely required). And if someone doesn't comply, then that's a bug
> >>>>>>>>> and has to be reported to application owners.
> >>>>>>>>>
> >>>>>>>>> But it's not up to the kernel to enforce this cooperation by
> >>>>>>>>> disallowing FIRST/LAST semantics, because that semantics is critical
> >>>>>>>>> for some applications, IMO.
> >>>>>>>> Maybe that's something that should be done by some other mechanism?
> >>>>>>>> (and as a follow up, if needed) Something akin to what Toke
> >>>>>>>> mentioned with another program doing sorting or similar.
> >>>>>>> The goal of this API is to avoid needing some extra special program to
> >>>>>>> do this sorting
> >>>>>>>
> >>>>>>>> Otherwise, those first/last are just plain simple old priority bands;
> >>>>>>>> only we have two now, not u16.
> >>>>>>> I think it's different. FIRST/LAST has to be used judiciously, of
> >>>>>>> course, but when they are needed, they will have no alternative.
> >>>>>>>
> >>>>>>> Also, specifying FIRST + LAST is the way to say "I want my program to
> >>>>>>> be the only one attached". Should we encourage such use cases? No, of
> >>>>>>> course. But I think it's fair  for users to be able to express this.
> >>>>>>>
> >>>>>>>> I'm mostly coming from the observability point: imagine I have my fancy
> >>>>>>>> tc_ingress_tcpdump program that I want to attach as a first program to debug
> >>>>>>>> some issue, but it won't work because there is already a 'first' program
> >>>>>>>> installed.. Or the assumption that I'd do F_REPLACE | F_FIRST ?
> >>>>>>> If your production setup requires that some important program has to
> >>>>>>> be FIRST, then yeah, your "let me debug something" program shouldn't
> >>>>>>> interfere with it (assuming that FIRST requirement is a real
> >>>>>>> requirement and not someone just thinking they need to be first; but
> >>>>>>> that's up to user space to decide). Maybe the solution for you in that
> >>>>>>> case would be freplace program installed on top of that stubborn FIRST
> >>>>>>> program? And if we are talking about local debugging and development,
> >>>>>>> then you are a sysadmin and you should be able to force-detach that
> >>>>>>> program that is getting in the way.
> >>>>>> I'm not really concerned about our production environment. It's pretty
> >>>>>> controlled and restricted and I'm pretty certain we can avoid doing
> >>>>>> something stupid. Probably the same for your env.
> >>>>>>
> >>>>>> I'm mostly fantasizing about upstream world where different users don't
> >>>>>> know about each other and start doing stupid things like F_FIRST where
> >>>>>> they don't really have to be first. It's that "used judiciously" part
> >>>>>> that I'm a bit skeptical about :-D
> >>>> But in the end how is that different from just attaching themselves blindly
> >>>> into the first position (e.g. with before and relative_fd as 0 or the fd/id
> >>>> of the current first program) - same, they don't really have to be first.
> >>>> How would that not result in doing something stupid? ;) To add to Andrii's
> >>>> earlier DDoS mitigation example ... think of K8s environment: one project
> >>>> is implementing DDoS mitigation with BPF, another one wants to monitor/
> >>>> sample traffic to user space with BPF. Both install as first position by
> >>>> default (before + 0). In K8s, there is no built-in Pod dependency management
> >>>> so you cannot guarantee whether Pod A comes up before Pod B. So you'll end
> >>>> up in a situation where sometimes the monitor runs before the DDoS mitigation
> >>>> and on some other nodes it's vice versa. The other case where this gets
> >>>> broken (assuming a node where we get first the DDoS mitigation, then the
> >>>> monitoring) is when you need to upgrade one of the Pods: monitoring Pod
> >>>> gets a new stable update and is being re-rolled out, then it inserts
> >>>> itself before the DDoS mitigation mechanism, potentially causing outage.
> >>>> With the first/last mechanism these two situations cannot happen. The DDoS
> >>>> mitigation software uses first and the monitoring uses before + 0, then no
> >>>> matter the re-rollouts or the ordering in which Pods come up, it's always
> >>>> at the expected/correct location.
> >>> I'm not disputing that these kinds of policy issues need to be solved
> >>> somehow. But adding the first/last pinning as part of the kernel hooks
> >>> doesn't solve the policy problem, it just hard-codes a solution for one
> >>> particular instance of the problem.
> >>>
> >>> Taking your example from above, what happens when someone wants to
> >>> deploy those tools in reverse order? Say the monitoring tool counts
> >>> packets and someone wants to also count the DDOS traffic; but the DDOS
> >>> protection tool has decided for itself (by setting the FIRST) flag that
> >>> it can *only* run as the first program, so there is no way to achieve
> >>> this without modifying the application itself.
> >>>
> >>>>>> Because even with this new ordering scheme, there still should be
> >>>>>> some entity to do relative ordering (systemd-style, maybe CNI?).
> >>>>>> And if it does the ordering, I don't really see why we need
> >>>>>> F_FIRST/F_LAST.
> >>>>> I can see I'm a bit late to the party, but FWIW I agree with this:
> >>>>> FIRST/LAST will definitely be abused if we add it. It also seems to me
> >> It's in the prisoners' best interest to collaborate (and they do! see
> >> https://www.youtube.com/watch?v=YK7GyEJdJGo), except the current
> >> prio system is limiting and turns out to be really fragile in practice.
> >>
> >> If your tool wants to attach to tc prio 1 and there's already a prog
> >> attached,
> >> the most reliable option is basically to blindly replace the attachment,
> >> unless
> >> you have the possibility to inspect the attached prog and try to figure
> >> out if it
> >> belongs to another tool. This is fragile in and of itself, and only
> >> possible on
> >> more recent kernels iirc.
> >>
> >> With tcx, Cilium could make an initial attachment using F_FIRST and simply
> >> update a link at well-known path on subsequent startups. If there's no
> >> existing
> >> link, and F_FIRST is taken, bail out with an error. The owner of the
> >> existing
> >> F_FIRST program can be queried and logged; we know for sure the program
> >> doesn't belong to Cilium, and we have no interest in detaching it.
> >
> > That's conflating the benefit of F_FIRST with that of bpf_link, though;
> > you can have the replace thing without the exclusive locking.
> >
> >>>> See above on the issues w/o the first/last. How would you work around them
> >>>> in practice so they cannot happen?
> >>> By having an ordering configuration that is deterministic. Enforced by
> >>> the system-wide management daemon by whichever mechanism suits it. We
> >>> could implement a minimal reference policy agent that just reads a
> >>> config file in /etc somewhere, and *that* could implement FIRST/LAST
> >>> semantics.
> >> I think this particular perspective is what's deadlocking this discussion.
> >> To me, it looks like distros and hyperscalers are in the same boat with
> >> regards to the possibility of coordination between tools. Distros are only
> >> responsible for the tools they package themselves, and hyperscalers
> >> run a tight ship with mostly in-house tooling already. When it comes to
> >> projects out in the wild, that all goes out the window.
> >
> > Not really: from the distro PoV we absolutely care about arbitrary
> > combinations of programs with different authors. Which is why I'm
> > arguing against putting anything into the kernel where the first program
> > to come along can just grab a hook and lock everyone out.
> >
> > My assumption is basically this: A system administrator installs
> > packages A and B that both use the TC hook. The developers of A and B
> > have never heard about each other. It should be possible for that admin
> > to run A and B in whichever order they like, without making any changes
> > to A and B themselves.
>
> I would come with the point of view of the K8s cluster operator or platform
> engineer, if you will. Someone deeply familiar with K8s, but not necessarily
> knowing about kernel internals. I know my org needs to run container A and
> container B, so I'll deploy the daemon-sets for both and they get deployed
> into my cluster. That platform engineer might have never heard of BPF or might
> not even know that container A or container B ships software with BPF. As
> mentioned, K8s itself has no concept of Pod ordering as its paradigm is that
> everything is loosely coupled. We are now expecting from that person to make
> a concrete decision about some BPF kernel internals on various hooks in which
> order they should be executed given if they don't then the system becomes
> non-deterministic. I think that is quite a big burden and ask to understand.
> Eventually that person will say that he/she cannot make this technical decision
> and that only one of the two containers can be deployed. I agree with you that
> there should be an option for a technically versed person to be able to change
> ordering to avoid lock out, but I don't think it will fly asking users to come
> up on their own with policies of BPF software in the wild ... similar as you
> probably don't want having to deal with writing systemd unit files for software
> xyz before you can use your laptop. It's a burden. You expect this to magically
> work by default and only if needed for good reasons to make custom changes.
> Just the one difference is that the latter ships with the OS (a priori known /
> tight-ship analogy).
>
> >> Regardless of merit or feasability of a system-wide bpf management
> >> daemon for k8s, there _is no ordering configuration possible_. K8s is not
> >> a distro where package maintainers (or anyone else, really) can coordinate
> >> on correctly defining priority of each of the tools they ship. This is
> >> effectively
> >> the prisoner's dilemma. I feel like most of the discussion so far has been
> >> very hand-wavy in 'user space should solve it'. Well, we are user space, and
> >> we're here trying to solve it. :)
> >>
> >> A hypothetical policy/gatekeeper/ordering daemon doesn't possess
> >> implicit knowledge about which program needs to go where in the chain,
> >> nor is there an obvious heuristic about how to order things. Maintaining
> >> such a configuration for all cloud-native tooling out there that possibly
> >> uses bpf is simply impossible, as even a tool like Cilium can change
> >> dramatically from one release to the next. Having to manage this too
> >> would put a significant burden on velocity and flexibility for arguably
> >> little benefit to the user.
> >>
> >> So, daemon/kernel will need to be told how to order things, preferably by
> >> the tools (Cilium/datadog-agent) themselves, since the user/admin of the
> >> system cannot be expected to know where to position the hundreds of progs
> >> loaded by Cilium and how they might interfere with other tools. Figuring
> >> this out is the job of the tool, daemon or not.
> >>
> >> The prisoners _must_ communicate (so, not abuse F_FIRST) for things to
> >> work correctly, and it's 100% in their best interest in doing so. Let's not
> >> pretend like we're able to solve game theory on this mailing list. :)
> >> We'll have to settle for the next-best thing: give user space a safe and
> >> clear
> >> API to allow it to coordinate and make the right decisions.
> >
> > But "always first" is not a meaningful concept. It's just what we have
> > today (everyone picks priority 1), except now if there are two programs
> > that want the same hook, it will be the first program that wins the
> > contest (by locking the second one out), instead of the second program
> > winning (by overriding the first one) as is the case with the silent
> > override semantics we have with TC today. So we haven't solved the
> > problem, we've just shifted the breakage.
>
> Fwiw, it's deterministic, and I think this 1000x better than silently
> having a non-deterministic deployment where the two programs ship with
> before + 0. That is much harder to debug.
>

Totally agree. Silent overriding of the BPF program while user-space
is still running (and completely unaware) is MUCH WORSE than not being
able to start if your spot is taken.

The latter is explicit and gives a lot of signal early on. The former
is confusing and results in hours of painful guessing on what's going
on.

How is this even controversial?


> >> To circle back to the observability case: in offline discussions with
> >> Daniel,
> >> I've mentioned the need for 'shadow' progs that only collect data and
> >> pump it to user space, attached at specific points in the chain (still
> >> within tcx!).
> >> Their retcodes would be ignored, and context modifications would be
> >> rejected, so attaching multiple to the same hook can always succeed,
> >> much like cgroup multi. Consider the following:
> >>
> >> To attach a shadow prog before F_FIRST, a caller could use F_BEFORE |
> >> F_FIRST |
> >> F_RDONLY. Attaching between first and the 'relative' section: F_AFTER |
> >> F_FIRST |
> >> F_RDONLY, etc. The rdonly flag could even be made redundant if a new prog/
> >> attach type is added for progs like these.
> >>
> >> This is still perfectly possible to implement on top of Daniel's
> >> proposal, and
> >> to me looks like it could address many of the concerns around ordering of
> >> progs I've seen in this thread, many mention data exfiltration.
> >
> > It may well be that semantics like this will turn out to be enough. Or
> > it may not (I personally believe we'll need something more expressive
> > still, and where the system admin has the option to override things; but
> > I may turn out to be wrong). Ultimately, my main point wrt this series
> > is that this kind of policy decision can be added later, and it's better
> > to merge the TCX infrastructure without it, instead of locking ourselves
> > into an API that is way too limited today. TCX (and in-kernel XDP
> > multiprog) has value without it, so let's merge that first and iterate
> > on the policy aspects.
>
> That's okay and I'll do that for v3 to move on.
>
> I feel we might repeat the same discussion with no good solution for K8s
> users once we come back to this point again.
>
> Thanks,
> Daniel

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