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Date: Fri, 10 Jun 2022 12:07:50 -0700
From: Joanne Koong <joannelkoong@...il.com>
To: Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc: bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Jamal Hadi Salim <jhs@...atatu.com>,
Vlad Buslov <vladbu@...dia.com>,
Cong Wang <xiyou.wangcong@...il.com>,
Jesper Dangaard Brouer <brouer@...hat.com>,
Toke Høiland-Jørgensen <toke@...hat.com>,
netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 0/7] Add bpf_link based TC-BPF API
On Fri, Jun 10, 2022 at 10:23 AM Joanne Koong <joannelkoong@...il.com> wrote:
>
> On Fri, Jun 10, 2022 at 5:58 AM Kumar Kartikeya Dwivedi
> <memxor@...il.com> wrote:
> >
> > On Fri, Jun 10, 2022 at 05:54:27AM IST, Joanne Koong wrote:
> > > On Thu, Jun 3, 2021 at 11:31 PM Kumar Kartikeya Dwivedi
> > > <memxor@...il.com> wrote:
> > > >
> > > > This is the second (non-RFC) version.
> > > >
> > > > This adds a bpf_link path to create TC filters tied to cls_bpf classifier, and
> > > > introduces fd based ownership for such TC filters. Netlink cannot delete or
> > > > replace such filters, but the bpf_link is severed on indirect destruction of the
> > > > filter (backing qdisc being deleted, or chain being flushed, etc.). To ensure
> > > > that filters remain attached beyond process lifetime, the usual bpf_link fd
> > > > pinning approach can be used.
> > > >
> > > > The individual patches contain more details and comments, but the overall kernel
> > > > API and libbpf helper mirrors the semantics of the netlink based TC-BPF API
> > > > merged recently. This means that we start by always setting direct action mode,
> > > > protocol to ETH_P_ALL, chain_index as 0, etc. If there is a need for more
> > > > options in the future, they can be easily exposed through the bpf_link API in
> > > > the future.
> > > >
> > > > Patch 1 refactors cls_bpf change function to extract two helpers that will be
> > > > reused in bpf_link creation.
> > > >
> > > > Patch 2 exports some bpf_link management functions to modules. This is needed
> > > > because our bpf_link object is tied to the cls_bpf_prog object. Tying it to
> > > > tcf_proto would be weird, because the update path has to replace offloaded bpf
> > > > prog, which happens using internal cls_bpf helpers, and would in general be more
> > > > code to abstract over an operation that is unlikely to be implemented for other
> > > > filter types.
> > > >
> > > > Patch 3 adds the main bpf_link API. A function in cls_api takes care of
> > > > obtaining block reference, creating the filter object, and then calls the
> > > > bpf_link_change tcf_proto op (only supported by cls_bpf) that returns a fd after
> > > > setting up the internal structures. An optimization is made to not keep around
> > > > resources for extended actions, which is explained in a code comment as it wasn't
> > > > immediately obvious.
> > > >
> > > > Patch 4 adds an update path for bpf_link. Since bpf_link_update only supports
> > > > replacing the bpf_prog, we can skip tc filter's change path by reusing the
> > > > filter object but swapping its bpf_prog. This takes care of replacing the
> > > > offloaded prog as well (if that fails, update is aborted). So far however,
> > > > tcf_classify could do normal load (possibly torn) as the cls_bpf_prog->filter
> > > > would never be modified concurrently. This is no longer true, and to not
> > > > penalize the classify hot path, we also cannot impose serialization around
> > > > its load. Hence the load is changed to READ_ONCE, so that the pointer value is
> > > > always consistent. Due to invocation in a RCU critical section, the lifetime of
> > > > the prog is guaranteed for the duration of the call.
> > > >
> > > > Patch 5, 6 take care of updating the userspace bits and add a bpf_link returning
> > > > function to libbpf.
> > > >
> > > > Patch 7 adds a selftest that exercises all possible problematic interactions
> > > > that I could think of.
> > > >
> > > > Design:
> > > >
> > > > This is where in the object hierarchy our bpf_link object is attached.
> > > >
> > > > ┌─────┐
> > > > │ │
> > > > │ BPF │
> > > > program
> > > > │ │
> > > > └──▲──┘
> > > > ┌───────┐ │
> > > > │ │ ┌──────┴───────┐
> > > > │ mod ├─────────► cls_bpf_prog │
> > > > ┌────────────────┐ │cls_bpf│ └────┬───▲─────┘
> > > > │ tcf_block │ │ │ │ │
> > > > └────────┬───────┘ └───▲───┘ │ │
> > > > │ ┌─────────────┐ │ ┌─▼───┴──┐
> > > > └──────────► tcf_chain │ │ │bpf_link│
> > > > └───────┬─────┘ │ └────────┘
> > > > │ ┌─────────────┐ │
> > > > └──────────► tcf_proto ├────┘
> > > > └─────────────┘
> > > >
> > > > The bpf_link is detached on destruction of the cls_bpf_prog. Doing it this way
> > > > allows us to implement update in a lightweight manner without having to recreate
> > > > a new filter, where we can just replace the BPF prog attached to cls_bpf_prog.
> > > >
> > > > The other way to do it would be to link the bpf_link to tcf_proto, there are
> > > > numerous downsides to this:
> > > >
> > > > 1. All filters have to embed the pointer even though they won't be using it when
> > > > cls_bpf is compiled in.
> > > > 2. This probably won't make sense to be extended to other filter types anyway.
> > > > 3. We aren't able to optimize the update case without adding another bpf_link
> > > > specific update operation to tcf_proto ops.
> > > >
> > > > The downside with tying this to the module is having to export bpf_link
> > > > management functions and introducing a tcf_proto op. Hopefully the cost of
> > > > another operation func pointer is not big enough (as there is only one ops
> > > > struct per module).
> > > >
> > > Hi Kumar,
> > >
> > > Do you have any plans / bandwidth to land this feature upstream? If
> > > so, do you have a tentative estimation for when you'll be able to work
> > > on this? And if not, are you okay with someone else working on this to
> > > get it merged in?
> > >
> >
> > I can have a look at resurrecting it later this month, if you're ok with waiting
> > until then, otherwise if someone else wants to pick this up before that it's
> > fine by me, just let me know so we avoid duplicated effort. Note that the
> > approach in v2 is dead/unlikely to get accepted by the TC maintainers, so we'd
> > have to implement the way Daniel mentioned in [0].
>
> Sounds great! We'll wait and check back in with you later this month.
>
After reading the linked thread (which I should have done before
submitting my previous reply :)), if I'm understanding it correctly,
it seems then that the work needed for tc bpf_link will be in a new
direction that's not based on the code in this v2 patchset. I'm
interested in learning more about bpf link and tc - I can pick this up
to work on. But if this was something you wanted to work on though,
please don't hesitate to let me know; I can find some other bpf link
thing to work on instead if that's the case.
> >
> > [0]: https://lore.kernel.org/bpf/15cd0a9c-95a1-9766-fca1-4bf9d09e4100@iogearbox.net
> >
> > > The reason I'm asking is because there are a few networking teams
> > > within Meta that have been requesting this feature :)
> > >
> > > Thanks,
> > > Joanne
> > >
> > > > Changelog:
> > > > ----------
> > > > v1 (RFC) -> v2
> > > > v1: https://lore.kernel.org/bpf/20210528195946.2375109-1-memxor@gmail.com
> > > >
> > > > * Avoid overwriting other members of union in bpf_attr (Andrii)
> > > > * Set link to NULL after bpf_link_cleanup to avoid double free (Andrii)
> > > > * Use __be16 to store the result of htons (Kernel Test Robot)
> > > > * Make assignment of tcf_exts::net conditional on CONFIG_NET_CLS_ACT
> > > > (Kernel Test Robot)
> > > >
> > > > Kumar Kartikeya Dwivedi (7):
> > > > net: sched: refactor cls_bpf creation code
> > > > bpf: export bpf_link functions for modules
> > > > net: sched: add bpf_link API for bpf classifier
> > > > net: sched: add lightweight update path for cls_bpf
> > > > tools: bpf.h: sync with kernel sources
> > > > libbpf: add bpf_link based TC-BPF management API
> > > > libbpf: add selftest for bpf_link based TC-BPF management API
> > > >
> > > > include/linux/bpf_types.h | 3 +
> > > > include/net/pkt_cls.h | 13 +
> > > > include/net/sch_generic.h | 6 +-
> > > > include/uapi/linux/bpf.h | 15 +
> > > > kernel/bpf/syscall.c | 14 +-
> > > > net/sched/cls_api.c | 139 ++++++-
> > > > net/sched/cls_bpf.c | 389 ++++++++++++++++--
> > > > tools/include/uapi/linux/bpf.h | 15 +
> > > > tools/lib/bpf/bpf.c | 8 +-
> > > > tools/lib/bpf/bpf.h | 8 +-
> > > > tools/lib/bpf/libbpf.c | 59 ++-
> > > > tools/lib/bpf/libbpf.h | 17 +
> > > > tools/lib/bpf/libbpf.map | 1 +
> > > > tools/lib/bpf/netlink.c | 5 +-
> > > > tools/lib/bpf/netlink.h | 8 +
> > > > .../selftests/bpf/prog_tests/tc_bpf_link.c | 285 +++++++++++++
> > > > 16 files changed, 940 insertions(+), 45 deletions(-)
> > > > create mode 100644 tools/lib/bpf/netlink.h
> > > > create mode 100644 tools/testing/selftests/bpf/prog_tests/tc_bpf_link.c
> > > >
> > > > --
> > > > 2.31.1
> > > >
> >
> > --
> > Kartikeya
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