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 10:25:40 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Björn Töpel <bjorn.topel@...el.com>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Björn Töpel <bjorn.topel@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>, maximmi@...dia.com,
        "David S. Miller" <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Ciara Loftus <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 Wed, Jan 20, 2021 at 7:27 AM Björn Töpel <bjorn.topel@...el.com> wrote:
>
> >> 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.

+1 to Toke's point. No version checks please.
One way to detect is to try prog_load. Search for FEAT_* in libbpf.
Another approach is to scan vmlinux BTF for necessary helpers.
Currently libbpf is relying on the former.
I think going forward would be good to detect features via BTF.
It's going to be much faster and won't create noise for audit that
could be looking at prog_load calls.