[xdp-hints] Re: [PATCH bpf-next RFC V1] selftests/bpf: xdp_hw_metadata clear metadata when -EOPNOTSUPP

[Stanislav Fomichev <sdf@google.com>]

On Fri, Jan 27, 2023 at 5:58 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Jesper Dangaard Brouer <brouer@redhat.com> writes:
>
> > The AF_XDP userspace part of xdp_hw_metadata see non-zero as a signal of
> > the availability of rx_timestamp and rx_hash in data_meta area. The
> > kernel-side BPF-prog code doesn't initialize these members when kernel
> > returns an error e.g. -EOPNOTSUPP.  This memory area is not guaranteed to
> > be zeroed, and can contain garbage/previous values, which will be read
> > and interpreted by AF_XDP userspace side.
> >
> > Tested this on different drivers. The experiences are that for most
> > packets they will have zeroed this data_meta area, but occasionally it
> > will contain garbage data.
> >
> > Example of failure tested on ixgbe:
> >  poll: 1 (0)
> >  xsk_ring_cons__peek: 1
> >  0x18ec788: rx_desc[0]->addr=100000000008000 addr=8100 comp_addr=8000
> >  rx_hash: 3697961069
> >  rx_timestamp:  9024981991734834796 (sec:9024981991.7348)
> >  0x18ec788: complete idx=8 addr=8000
> >
> > Converting to date:
> >  date -d @9024981991
> >  2255-12-28T20:26:31 CET
> >
> > I choose a simple fix in this patch. When kfunc fails or isn't supported
> > assign zero to the corresponding struct meta value.
> >
> > It's up to the individual BPF-programmer to do something smarter e.g.
> > that fits their use-case, like getting a software timestamp and marking
> > a flag that gives the type of timestamp.
> >
> > Another possibility is for the behavior of kfunc's
> > bpf_xdp_metadata_rx_timestamp and bpf_xdp_metadata_rx_hash to require
> > clearing return value pointer.
>
> I definitely think we should leave it up to the BPF programmer to react
> to failures; that's what the return code is there for, after all :)

+1

Maybe we can unconditionally memset(meta, sizeof(*meta), 0) in
tools/testing/selftests/bpf/progs/xdp_hw_metadata.c?
Since it's not a performance tool, it should be ok functionality-wise.

> -Toke
>