netdev - Re: [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program based on kernel version

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Date:   Wed, 20 Jan 2021 18:30:05 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        ast@...nel.org, daniel@...earbox.net, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     magnus.karlsson@...el.com, maciej.fijalkowski@...el.com,
        kuba@...nel.org, jonathan.lemon@...il.com, maximmi@...dia.com,
        davem@...emloft.net, hawk@...nel.org, john.fastabend@...il.com,
        ciara.loftus@...el.com, weqaar.a.janjua@...el.com,
        Marek Majtyka <alardam@...il.com>
Subject: Re: [PATCH bpf-next v2 5/8] libbpf, xsk: select AF_XDP BPF program
 based on kernel version

Björn Töpel <bjorn.topel@...el.com> writes:

> On 2021-01-20 16:11, Toke Høiland-Jørgensen wrote:
>> Björn Töpel <bjorn.topel@...el.com> writes:
>> 
>>> On 2021-01-20 14:25, Björn Töpel wrote:
>>>> On 2021-01-20 13:52, Toke Høiland-Jørgensen wrote:
>>>>> Björn Töpel <bjorn.topel@...il.com> writes:
>>>>>
>>>>>> From: Björn Töpel <bjorn.topel@...el.com>
>>>>>>
>>>>>> Add detection for kernel version, and adapt the BPF program based on
>>>>>> kernel support. This way, users will get the best possible performance
>>>>>> from the BPF program.
>>>>>
>>>>> Please do explicit feature detection instead of relying on the kernel
>>>>> version number; some distro kernels are known to have a creative notion
>>>>> of their own version, which is not really related to the features they
>>>>> actually support (I'm sure you know which one I'm referring to ;)).
>>>>>
>>>>
>>>> Right. For a *new* helper, like bpf_redirect_xsk, we rely on rejection
>>>> from the verifier to detect support. What about "bpf_redirect_map() now
>>>> supports passing return value as flags"? Any ideas how to do that in a
>>>> robust, non-version number-based scheme?
>>>>
>>>
>>> Just so that I understand this correctly. Red^WSome distro vendors
>>> backport the world, and call that franken kernel, say, 3.10. Is that
>>> interpretation correct? My hope was that wasn't the case. :-(
>> 
>> Yup, indeed. All kernels shipped for the entire lifetime of RHEL8 think
>> they are v4.18.0... :/
>> 
>> I don't think we're the only ones doing it (there are examples in the
>> embedded world as well, for instance, and not sure about the other
>> enterprise distros), but RHEL is probably the most extreme example.
>> 
>> We could patch the version check in the distro-supplied version of
>> libbpf, of course, but that doesn't help anyone using upstream versions,
>> and given the prevalence of vendoring libbpf, I fear that going with the
>> version check will just result in a bad user experience...
>>
>
> Ok! Thanks for clearing that out!
>
>>> Would it make sense with some kind of BPF-specific "supported
>>> features" mechanism? Something else with a bigger scope (whole
>>> kernel)?
>> 
>> Heh, in my opinion, yeah. Seems like we'll finally get it for XDP, but
>> for BPF in general the approach has always been probing AFAICT.
>> 
>> For the particular case of arguments to helpers, I suppose the verifier
>> could technically validate value ranges for flags arguments, say. That
>> would be nice as an early reject anyway, but I'm not sure if it is
>> possible to add after-the-fact without breaking existing programs
>> because the verifier can't prove the argument is within the valid range.
>> And of course it doesn't help you with compatibility with
>> already-released kernels.
>>
>
> Hmm, think I have a way forward. I'll use BPF_PROG_TEST_RUN.
>
> If the load fail for the new helper, fallback to bpf_redirect_map(). Use
> BPF_PROG_TEST_RUN to make sure that "action via flags" passes.
>
> That should work for you guys as well, right? I'll take a stab at it.

Yup, think so - SGTM! :)

-Toke