Re: [PATCH bpf-next v3 6/8] bpf: Add XDP_REDIRECT support to XDP for bpf_prog_run()

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:

> On Mon, Dec 13, 2021 at 8:26 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Alexei Starovoitov <alexei.starovoitov@gmail.com> writes:
>>
>> > On Sat, Dec 11, 2021 at 10:43 AM Toke Høiland-Jørgensen <toke@redhat.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.

Great!

> 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.

Hmm, that's certainly an interesting idea! I don't think we can actually
run the XDP hook on the netdev itself (since that is deep in the
driver), but we can emulate it: we just need to do what this version of
the patch is doing, but add handling of the other return codes.

XDP_PASS could be supported by basically copying what cpumap is doing
(turn the frames into skbs and call netif_receive_skb_list()), but
XDP_TX would have to be implemented via ndo_xdp_xmit(), so it becomes
equivalent to a REDIRECT back to the same interface. That's probably OK,
though, right?

I'll try this out for the next version, thanks for the idea!

-Toke