netdev - Re: [xdp-hints] Re: [PATCH bpf-next v3 00/12] xdp: hints via kfuncs

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Date:   Fri, 09 Dec 2022 01:14:13 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
        yhs@...com, john.fastabend@...il.com, kpsingh@...nel.org,
        haoluo@...gle.com, jolsa@...nel.org,
        David Ahern <dsahern@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org
Subject: Re: [xdp-hints] Re: [PATCH bpf-next v3 00/12] xdp: hints via kfuncs

Stanislav Fomichev <sdf@...gle.com> writes:

> On Thu, Dec 8, 2022 at 2:29 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>>
>> Stanislav Fomichev <sdf@...gle.com> writes:
>>
>> > Please see the first patch in the series for the overall
>> > design and use-cases.
>> >
>> > Changes since v3:
>> >
>> > - Rework prog->bound_netdev refcounting (Jakub/Marin)
>> >
>> >   Now it's based on the offload.c framework. It mostly fits, except
>> >   I had to automatically insert a HT entry for the netdev. In the
>> >   offloaded case, the netdev is added via a call to
>> >   bpf_offload_dev_netdev_register from the driver init path; with
>> >   a dev-bound programs, we have to manually add (and remove) the entry.
>> >
>> >   As suggested by Toke, I'm also prohibiting putting dev-bound programs
>> >   into prog-array map; essentially prohibiting tail calling into it.
>> >   I'm also disabling freplace of the dev-bound programs. Both of those
>> >   restrictions can be loosened up eventually.
>>
>> I thought it would be a shame that we don't support at least freplace
>> programs from the get-go (as that would exclude libxdp from taking
>> advantage of this). So see below for a patch implementing this :)
>>
>> -Toke
>
> Damn, now I need to write a selftest :-)
> But seriously, thank you for taking care of this, will try to include
> preserving SoB!

Cool, thanks! I just realised I made on mistake in the attach check,
though:

[...]

>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>> index b345a273f7d0..606e6de5f716 100644
>> --- a/kernel/bpf/syscall.c
>> +++ b/kernel/bpf/syscall.c
>> @@ -3021,6 +3021,14 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>>                         goto out_put_prog;
>>                 }
>>
>> +               if (bpf_prog_is_dev_bound(tgt_prog->aux) &&
>> +                   (bpf_prog_is_offloaded(tgt_prog->aux) ||
>> +                    !bpf_prog_is_dev_bound(prog->aux) ||
>> +                    !bpf_offload_dev_match(prog, tgt_prog->aux->offload->netdev))) {

This should switch the order of the is_dev_bound() checks, like:

+               if (bpf_prog_is_dev_bound(prog->aux) &&
+                   (bpf_prog_is_offloaded(tgt_prog->aux) ||
+                    !bpf_prog_is_dev_bound(tgt_prog->aux) ||
+                    !bpf_offload_dev_match(prog, tgt_prog->aux->offload->netdev))) {

I.e., first check bpf_prog_is_dev_bound(prog->aux) (the program being
attached), and only perform the other checks if we're attaching
something that has been verified as being dev-bound. It should be fine
to attach a non-devbound function to a devbound parent program (since
that non-devbound function can't call any of the kfuncs).

-Toke