selftests/bpf: Cover write access to skb metadata via dynptr

Add tests what exercise writes to skb metadata in two ways:
1. indirectly, using bpf_dynptr_write helper,
2. directly, using a read-write dynptr slice.

Signed-off-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Reviewed-by: Jesse Brandeburg <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://patch.msgid.link/20250814-skb-metadata-thru-dynptr-v7-7-8a39e636e0fb@cloudflare.com

Author: jsitnicki

tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c

@@ -269,7 +269,8 @@ void test_xdp_context_veth(void)
 }
 
 static void test_tuntap(struct bpf_program *xdp_prog,
-			struct bpf_program *tc_prog,
+			struct bpf_program *tc_prio_1_prog,
+			struct bpf_program *tc_prio_2_prog,
 			struct bpf_map *result_map)
 {
 	LIBBPF_OPTS(bpf_tc_hook, tc_hook, .attach_point = BPF_TC_INGRESS);
@@ -302,11 +303,20 @@ static void test_tuntap(struct bpf_program *xdp_prog,
 	if (!ASSERT_OK(ret, "bpf_tc_hook_create"))
 		goto close;
 
-	tc_opts.prog_fd = bpf_program__fd(tc_prog);
+	tc_opts.prog_fd = bpf_program__fd(tc_prio_1_prog);
 	ret = bpf_tc_attach(&tc_hook, &tc_opts);
 	if (!ASSERT_OK(ret, "bpf_tc_attach"))
 		goto close;
 
+	if (tc_prio_2_prog) {
+		LIBBPF_OPTS(bpf_tc_opts, tc_opts, .handle = 1, .priority = 2,
+			    .prog_fd = bpf_program__fd(tc_prio_2_prog));
+
+		ret = bpf_tc_attach(&tc_hook, &tc_opts);
+		if (!ASSERT_OK(ret, "bpf_tc_attach"))
+			goto close;
+	}
+
 	ret = bpf_xdp_attach(tap_ifindex, bpf_program__fd(xdp_prog),
 			     0, NULL);
 	if (!ASSERT_GE(ret, 0, "bpf_xdp_attach"))
@@ -341,13 +351,29 @@ void test_xdp_context_tuntap(void)
 		return;
 
 	if (test__start_subtest("data_meta"))
-		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls,
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.ing_cls,
+			    NULL, /* tc prio 2 */
 			    skel->maps.test_result);
 	if (test__start_subtest("dynptr_read"))
-		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls_dynptr_read,
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.ing_cls_dynptr_read,
+			    NULL, /* tc prio 2 */
 			    skel->maps.test_result);
 	if (test__start_subtest("dynptr_slice"))
-		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls_dynptr_slice,
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.ing_cls_dynptr_slice,
+			    NULL, /* tc prio 2 */
+			    skel->maps.test_result);
+	if (test__start_subtest("dynptr_write"))
+		test_tuntap(skel->progs.ing_xdp_zalloc_meta,
+			    skel->progs.ing_cls_dynptr_write,
+			    skel->progs.ing_cls_dynptr_read,
+			    skel->maps.test_result);
+	if (test__start_subtest("dynptr_slice_rdwr"))
+		test_tuntap(skel->progs.ing_xdp_zalloc_meta,
+			    skel->progs.ing_cls_dynptr_slice_rdwr,
+			    skel->progs.ing_cls_dynptr_slice,
 			    skel->maps.test_result);
 
 	test_xdp_meta__destroy(skel);

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

@@ -60,6 +60,24 @@ int ing_cls_dynptr_read(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+/* Write to metadata using bpf_dynptr_write helper */
+SEC("tc")
+int ing_cls_dynptr_write(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	__u8 *src;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	src = bpf_dynptr_slice(&data, sizeof(struct ethhdr), NULL, META_SIZE);
+	if (!src)
+		return TC_ACT_SHOT;
+
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	bpf_dynptr_write(&meta, 0, src, META_SIZE, 0);
+
+	return TC_ACT_UNSPEC; /* pass */
+}
+
 /* Read from metadata using read-only dynptr slice */
 SEC("tc")
 int ing_cls_dynptr_slice(struct __sk_buff *ctx)
@@ -82,6 +100,55 @@ int ing_cls_dynptr_slice(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+/* Write to metadata using writeable dynptr slice */
+SEC("tc")
+int ing_cls_dynptr_slice_rdwr(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	__u8 *src, *dst;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	src = bpf_dynptr_slice(&data, sizeof(struct ethhdr), NULL, META_SIZE);
+	if (!src)
+		return TC_ACT_SHOT;
+
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	dst = bpf_dynptr_slice_rdwr(&meta, 0, NULL, META_SIZE);
+	if (!dst)
+		return TC_ACT_SHOT;
+
+	__builtin_memcpy(dst, src, META_SIZE);
+
+	return TC_ACT_UNSPEC; /* pass */
+}
+
+/* Reserve and clear space for metadata but don't populate it */
+SEC("xdp")
+int ing_xdp_zalloc_meta(struct xdp_md *ctx)
+{
+	struct ethhdr *eth = ctx_ptr(ctx, data);
+	__u8 *meta;
+	int ret;
+
+	/* Drop any non-test packets */
+	if (eth + 1 > ctx_ptr(ctx, data_end))
+		return XDP_DROP;
+	if (eth->h_proto != 0)
+		return XDP_DROP;
+
+	ret = bpf_xdp_adjust_meta(ctx, -META_SIZE);
+	if (ret < 0)
+		return XDP_DROP;
+
+	meta = ctx_ptr(ctx, data_meta);
+	if (meta + META_SIZE > ctx_ptr(ctx, data))
+		return XDP_DROP;
+
+	__builtin_memset(meta, 0, META_SIZE);
+
+	return XDP_PASS;
+}
+
 SEC("xdp")
 int ing_xdp(struct xdp_md *ctx)
 {