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

[Jesper Dangaard Brouer <brouer@xxxxxxxxxx>]

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.

Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
---
 net/core/xdp.c                                     |    2 ++
 .../testing/selftests/bpf/progs/xdp_hw_metadata.c  |    6 +++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/net/core/xdp.c b/net/core/xdp.c
index a5a7ecf6391c..5ea13554c080 100644
--- a/net/core/xdp.c
+++ b/net/core/xdp.c
@@ -724,6 +724,7 @@ __diag_ignore_all("-Wmissing-prototypes",
  */
 int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
 {
+	// XXX: Question: Should we clear mem pointed to by @timestamp ?
 	return -EOPNOTSUPP;
 }
 
@@ -736,6 +737,7 @@ int bpf_xdp_metadata_rx_timestamp(const struct xdp_md *ctx, u64 *timestamp)
  */
 int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, u32 *hash)
 {
+	// XXX: Question: Should we clear mem pointed to by @hash ?
 	return -EOPNOTSUPP;
 }
 
diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
index 25b8178735ee..4c55b4d79d3d 100644
--- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
+++ b/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);
 }