selftests/bpf: xdp_hw_metadata clear metadata when -EOPNOTSUPP

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.

Fixes: 297a3f12 ("selftests/bpf: Simple program to dump XDP RX metadata")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/167527271027.937063.5177725618616476592.stgit@firesoul

tools/testing/selftests/bpf/progs/xdp_hw_metadata.c

@@ -70,10 +70,14 @@ int rx(struct xdp_md *ctx)
 	}
 
 	if (!bpf_xdp_metadata_rx_timestamp(ctx, &meta->rx_timestamp))
-		bpf_printk("populated rx_timestamp with %u", meta->rx_timestamp);
+		bpf_printk("populated rx_timestamp with %llu", meta->rx_timestamp);
+	else
+		meta->rx_timestamp = 0; /* Used by AF_XDP as not avail signal */
 
 	if (!bpf_xdp_metadata_rx_hash(ctx, &meta->rx_hash))
 		bpf_printk("populated rx_hash with %u", meta->rx_hash);
+	else
+		meta->rx_hash = 0; /* Used by AF_XDP as not avail signal */
 
 	return bpf_redirect_map(&xsk, ctx->rx_queue_index, XDP_PASS);
 }