[PATCH bpf-next 2/2] selftests/bpf: Enhance XDP Rx Metadata Handling

[Song Yoong Siang]

Introduce the XDP_METADATA_SIZE macro to ensure that user applications can
consistently retrieve the correct location of struct xdp_meta.

Prior to this commit, the XDP program adjusted the data_meta backward by
the size of struct xdp_meta, while the user application retrieved the data
by calculating backward from the data pointer. This approach only worked if
xdp_buff->data_meta was equal to xdp_buff->data before calling
bpf_xdp_adjust_meta.

With the introduction of XDP_METADATA_SIZE, both the XDP program and user
application now calculate and identify the location of struct xdp_meta from
the data pointer. This ensures the implementation remains functional even
when there is device-reserved metadata, making the tests more portable
across different NICs.

Signed-off-by: Song Yoong Siang <yoong.siang.song@xxxxxxxxx>
---
 tools/testing/selftests/bpf/prog_tests/xdp_metadata.c |  2 +-
 tools/testing/selftests/bpf/progs/xdp_hw_metadata.c   | 10 +++++++++-
 tools/testing/selftests/bpf/progs/xdp_metadata.c      |  8 +++++++-
 tools/testing/selftests/bpf/xdp_hw_metadata.c         |  2 +-
 tools/testing/selftests/bpf/xdp_metadata.h            |  7 +++++++
 5 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
index 19f92affc2da..8d6c2633698b 100644
--- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
@@ -302,7 +302,7 @@ static int verify_xsk_metadata(struct xsk *xsk, bool sent_from_af_xdp)
 
 	/* custom metadata */
 
-	meta = data - sizeof(struct xdp_meta);
+	meta = data - XDP_METADATA_SIZE;
 
 	if (!ASSERT_NEQ(meta->rx_timestamp, 0, "rx_timestamp"))
 		return -1;
diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
index 330ece2eabdb..72242ac1cdcd 100644
--- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
+++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
@@ -27,6 +27,7 @@ extern int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
 SEC("xdp.frags")
 int rx(struct xdp_md *ctx)
 {
+	int metalen_used, metalen_to_adjust;
 	void *data, *data_meta, *data_end;
 	struct ipv6hdr *ip6h = NULL;
 	struct udphdr *udp = NULL;
@@ -72,7 +73,14 @@ int rx(struct xdp_md *ctx)
 		return XDP_PASS;
 	}
 
-	err = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
+	metalen_used = ctx->data - ctx->data_meta;
+	metalen_to_adjust = XDP_METADATA_SIZE - metalen_used;
+	if (metalen_to_adjust < (int)sizeof(struct xdp_meta)) {
+		__sync_add_and_fetch(&pkts_skip, 1);
+		return XDP_PASS;
+	}
+
+	err = bpf_xdp_adjust_meta(ctx, -metalen_to_adjust);
 	if (err) {
 		__sync_add_and_fetch(&pkts_fail, 1);
 		return XDP_PASS;
diff --git a/tools/testing/selftests/bpf/progs/xdp_metadata.c b/tools/testing/selftests/bpf/progs/xdp_metadata.c
index 09bb8a038d52..a0ba4ef4bbd8 100644
--- a/tools/testing/selftests/bpf/progs/xdp_metadata.c
+++ b/tools/testing/selftests/bpf/progs/xdp_metadata.c
@@ -37,6 +37,7 @@ extern int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
 SEC("xdp")
 int rx(struct xdp_md *ctx)
 {
+	int metalen_used, metalen_to_adjust;
 	void *data, *data_meta, *data_end;
 	struct ipv6hdr *ip6h = NULL;
 	struct ethhdr *eth = NULL;
@@ -73,7 +74,12 @@ int rx(struct xdp_md *ctx)
 
 	/* Reserve enough for all custom metadata. */
 
-	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
+	metalen_used = ctx->data - ctx->data_meta;
+	metalen_to_adjust = XDP_METADATA_SIZE - metalen_used;
+	if (metalen_to_adjust < (int)sizeof(struct xdp_meta))
+		return XDP_DROP;
+
+	ret = bpf_xdp_adjust_meta(ctx, -metalen_to_adjust);
 	if (ret != 0)
 		return XDP_DROP;
 
diff --git a/tools/testing/selftests/bpf/xdp_hw_metadata.c b/tools/testing/selftests/bpf/xdp_hw_metadata.c
index 3d8de0d4c96a..a529d55d4ff4 100644
--- a/tools/testing/selftests/bpf/xdp_hw_metadata.c
+++ b/tools/testing/selftests/bpf/xdp_hw_metadata.c
@@ -223,7 +223,7 @@ static void verify_xdp_metadata(void *data, clockid_t clock_id)
 {
 	struct xdp_meta *meta;
 
-	meta = data - sizeof(*meta);
+	meta = data - XDP_METADATA_SIZE;
 
 	if (meta->hint_valid & XDP_META_FIELD_RSS)
 		printf("rx_hash: 0x%X with RSS type:0x%X\n",
diff --git a/tools/testing/selftests/bpf/xdp_metadata.h b/tools/testing/selftests/bpf/xdp_metadata.h
index 87318ad1117a..2dfd3bf5e7bb 100644
--- a/tools/testing/selftests/bpf/xdp_metadata.h
+++ b/tools/testing/selftests/bpf/xdp_metadata.h
@@ -50,3 +50,10 @@ struct xdp_meta {
 	};
 	enum xdp_meta_field hint_valid;
 };
+
+/* XDP_METADATA_SIZE must be at least the size of struct xdp_meta. An additional
+ * 32 bytes of padding is included as a conservative measure to accommodate any
+ * metadata areas reserved by Ethernet devices. If the device-reserved metadata
+ * exceeds 32 bytes, this value will need adjustment.
+ */
+#define XDP_METADATA_SIZE	(sizeof(struct xdp_meta) + 32)
-- 
2.34.1