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 16:27:12 +0100
From:   Björn Töpel <bjorn.topel@...el.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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

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.


Björn