[RFC PATCH bpf-next v1 7/7] selftests/bpf: cover bpf_xdp_metadata_rx_csum in xdp_metadata

[Vladimir Vdovin <deliran@verdict.gg>]

Call bpf_xdp_metadata_rx_csum() in the xdp_metadata program and export the
status to userspace.  veth surfaces skb->ip_summed: a frame injected via
AF_XDP carries no checksum context (XDP_CSUM_NONE), while one sent through
the stack is CHECKSUM_PARTIAL (XDP_CSUM_VERIFIED).  Assert each.

Signed-off-by: Vladimir Vdovin <deliran@verdict.gg>
---
 tools/testing/selftests/bpf/prog_tests/xdp_metadata.c | 10 ++++++++++
 tools/testing/selftests/bpf/progs/xdp_metadata.c      |  9 +++++++++
 tools/testing/selftests/bpf/xdp_metadata.h            |  8 ++++++++
 3 files changed, 27 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
index 5c31054ad4a4..77f55696eb78 100644
--- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
@@ -310,6 +310,16 @@ static int verify_xsk_metadata(struct xsk *xsk, bool sent_from_af_xdp)
 	if (!ASSERT_NEQ(meta->rx_hash, 0, "rx_hash"))
 		return -1;
 
+	/* veth surfaces the checksum verdict from skb->ip_summed.  A packet
+	 * injected via AF_XDP carries no checksum context and is CHECKSUM_NONE,
+	 * while one sent through the stack is CHECKSUM_PARTIAL and reads back as
+	 * verified.
+	 */
+	if (!ASSERT_EQ(meta->rx_csum_status,
+		       sent_from_af_xdp ? XDP_META_CSUM_NONE : XDP_META_CSUM_VERIFIED,
+		       "rx_csum_status"))
+		return -1;
+
 	if (!sent_from_af_xdp) {
 		if (!ASSERT_NEQ(meta->rx_hash_type & XDP_RSS_TYPE_L4, 0, "rx_hash_type"))
 			return -1;
diff --git a/tools/testing/selftests/bpf/progs/xdp_metadata.c b/tools/testing/selftests/bpf/progs/xdp_metadata.c
index 09bb8a038d52..0089c6c5a2e4 100644
--- a/tools/testing/selftests/bpf/progs/xdp_metadata.c
+++ b/tools/testing/selftests/bpf/progs/xdp_metadata.c
@@ -33,6 +33,8 @@ extern int bpf_xdp_metadata_rx_hash(const struct xdp_md *ctx, __u32 *hash,
 extern int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
 					__be16 *vlan_proto,
 					__u16 *vlan_tci) __ksym;
+extern int bpf_xdp_metadata_rx_csum(const struct xdp_md *ctx,
+				    enum xdp_csum_status *csum_status) __ksym;
 
 SEC("xdp")
 int rx(struct xdp_md *ctx)
@@ -43,6 +45,7 @@ int rx(struct xdp_md *ctx)
 	struct udphdr *udp = NULL;
 	struct iphdr *iph = NULL;
 	struct xdp_meta *meta;
+	enum xdp_csum_status csum_status;
 	u64 timestamp = -1;
 	int ret;
 
@@ -99,6 +102,12 @@ int rx(struct xdp_md *ctx)
 	bpf_xdp_metadata_rx_vlan_tag(ctx, &meta->rx_vlan_proto,
 				     &meta->rx_vlan_tci);
 
+	ret = bpf_xdp_metadata_rx_csum(ctx, &csum_status);
+	if (ret < 0)
+		meta->rx_csum_err = ret;
+	else
+		meta->rx_csum_status = csum_status;
+
 	return bpf_redirect_map(&xsk, ctx->rx_queue_index, XDP_PASS);
 }
 
diff --git a/tools/testing/selftests/bpf/xdp_metadata.h b/tools/testing/selftests/bpf/xdp_metadata.h
index 87318ad1117a..ba1b2902b371 100644
--- a/tools/testing/selftests/bpf/xdp_metadata.h
+++ b/tools/testing/selftests/bpf/xdp_metadata.h
@@ -30,6 +30,10 @@ enum xdp_meta_field {
 	XDP_META_FIELD_VLAN_TAG	= BIT(2),
 };
 
+/* Mirror of enum xdp_csum_status (include/net/xdp.h) for userspace asserts. */
+#define XDP_META_CSUM_NONE	0
+#define XDP_META_CSUM_VERIFIED	1
+
 struct xdp_meta {
 	union {
 		__u64 rx_timestamp;
@@ -48,5 +52,9 @@ struct xdp_meta {
 		};
 		__s32 rx_vlan_tag_err;
 	};
+	union {
+		__u32 rx_csum_status;
+		__s32 rx_csum_err;
+	};
 	enum xdp_meta_field hint_valid;
 };
-- 
2.47.0