netdev - Re: [PATCH bpf-next v3 6/8] bpf: Add XDP_REDIRECT support to XDP for bpf_prog

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Message-ID: <CAADnVQJunh7KTKJe3F_tO0apqLHtOMFqGAB-V28ORh6o5JUTUQ@mail.gmail.com>
Date:   Mon, 13 Dec 2021 16:02:36 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 6/8] bpf: Add XDP_REDIRECT support to XDP for bpf_prog_run()

On Mon, Dec 13, 2021 at 8:26 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Alexei Starovoitov <alexei.starovoitov@...il.com> writes:
>
> > On Sat, Dec 11, 2021 at 10:43 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
> >> +
> >> +static void bpf_test_run_xdp_teardown(struct bpf_test_timer *t)
> >> +{
> >> +       struct xdp_mem_info mem = {
> >> +               .id = t->xdp.pp->xdp_mem_id,
> >> +               .type = MEM_TYPE_PAGE_POOL,
> >> +       };
> >
> > pls add a new line.
> >
> >> +       xdp_unreg_mem_model(&mem);
> >> +}
> >> +
> >> +static bool ctx_was_changed(struct xdp_page_head *head)
> >> +{
> >> +       return (head->orig_ctx.data != head->ctx.data ||
> >> +               head->orig_ctx.data_meta != head->ctx.data_meta ||
> >> +               head->orig_ctx.data_end != head->ctx.data_end);
> >
> > redundant ()
> >
> >>         bpf_test_timer_enter(&t);
> >>         old_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
> >>         do {
> >>                 run_ctx.prog_item = &item;
> >> -               if (xdp)
> >> +               if (xdp && xdp_redirect) {
> >> +                       ret = bpf_test_run_xdp_redirect(&t, prog, ctx);
> >> +                       if (unlikely(ret < 0))
> >> +                               break;
> >> +                       *retval = ret;
> >> +               } else if (xdp) {
> >>                         *retval = bpf_prog_run_xdp(prog, ctx);
> >
> > Can we do this unconditionally without introducing a new uapi flag?
> > I mean "return bpf_redirect()" was a nop under test_run.
> > What kind of tests might break if it stops being a nop?
>
> Well, I view the existing mode of bpf_prog_test_run() with XDP as a way
> to write XDP unit tests: it allows you to submit a packet, run your XDP
> program on it, and check that it returned the right value and did the
> right modifications. This means if you XDP program does 'return
> bpf_redirect()', userspace will still get the XDP_REDIRECT value and so
> it can check correctness of your XDP program.
>
> With this flag the behaviour changes quite drastically, in that it will
> actually put packets on the wire instead of getting back the program
> return. So I think it makes more sense to make it a separate opt-in
> mode; the old behaviour can still be useful for checking XDP program
> behaviour.

Ok that all makes sense.
How about using prog_run to feed the data into proper netdev?
XDP prog may or may not attach to it (this detail is tbd) and
prog_run would use prog_fd and ifindex to trigger RX (yes, receive)
in that netdev. XDP prog will execute and will be able to perform
all actions (not only XDP_REDIRECT).
XDP_PASS would pass the packet to the stack, etc.