Re: [PATCH bpf-next v7 3/3] selftests/bpf: Add selftest for XDP_REDIRECT in bpf_prog_run()

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Jan 7, 2022 at 1:54 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> +
> +#define NUM_PKTS 1000000

It takes 7 seconds on my kvm with kasan and lockdep
and will take much longer in BPF CI.
So it needs to be lower otherwise CI will struggle.

> +       /* The XDP program we run with bpf_prog_run() will cycle through all
> +        * three xmit (PASS/TX/REDIRECT) return codes starting from above, and
> +        * ending up with PASS, so we should end up with two packets on the dst
> +        * iface and NUM_PKTS-2 in the TC hook. We match the packets on the UDP
> +        * payload.
> +        */

could you keep cycling through all return codes?
That should make the test stronger.

> +
> +       /* We enable forwarding in the test namespace because that will cause
> +        * the packets that go through the kernel stack (with XDP_PASS) to be
> +        * forwarded back out the same interface (because of the packet dst
> +        * combined with the interface addresses). When this happens, the
> +        * regular forwarding path will end up going through the same
> +        * veth_xdp_xmit() call as the XDP_REDIRECT code, which can cause a
> +        * deadlock if it happens on the same CPU. There's a local_bh_disable()
> +        * in the test_run code to prevent this, but an earlier version of the
> +        * code didn't have this, so we keep the test behaviour to make sure the
> +        * bug doesn't resurface.
> +        */
> +       SYS("sysctl -qw net.ipv6.conf.all.forwarding=1");

Does it mean that without forwarding=1 the kernel will dead lock ?!