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

Exercise reading from SKB metadata area in two new ways:
1. indirectly, with bpf_dynptr_read(), and
2. directly, with bpf_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-v7-6-8a39e636e0fb@cloudflare.com

Author: jsitnicki

tools/testing/selftests/bpf/bpf_kfuncs.h

@@ -19,6 +19,9 @@ extern int bpf_dynptr_from_skb(struct __sk_buff *skb, __u64 flags,
 extern int bpf_dynptr_from_xdp(struct xdp_md *xdp, __u64 flags,
 			       struct bpf_dynptr *ptr__uninit) __ksym __weak;
 
+extern int bpf_dynptr_from_skb_meta(struct __sk_buff *skb, __u64 flags,
+				    struct bpf_dynptr *ptr__uninit) __ksym __weak;
+
 /* Description
  *  Obtain a read-only pointer to the dynptr's data
  * Returns

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

@@ -171,6 +171,18 @@ static void assert_test_result(const struct bpf_map *result_map)
 		     "test_result map contains test payload");
 }
 
+static bool clear_test_result(struct bpf_map *result_map)
+{
+	const __u8 v[sizeof(test_payload)] = {};
+	const __u32 k = 0;
+	int err;
+
+	err = bpf_map__update_elem(result_map, &k, sizeof(k), v, sizeof(v), BPF_ANY);
+	ASSERT_OK(err, "update test_result");
+
+	return err == 0;
+}
+
 void test_xdp_context_veth(void)
 {
 	LIBBPF_OPTS(bpf_tc_hook, tc_hook, .attach_point = BPF_TC_INGRESS);
@@ -268,6 +280,9 @@ static void test_tuntap(struct bpf_program *xdp_prog,
 	int tap_ifindex;
 	int ret;
 
+	if (!clear_test_result(result_map))
+		return;
+
 	ns = netns_new(TAP_NETNS, true);
 	if (!ASSERT_OK_PTR(ns, "create and open ns"))
 		return;
@@ -328,6 +343,12 @@ void test_xdp_context_tuntap(void)
 	if (test__start_subtest("data_meta"))
 		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls,
 			    skel->maps.test_result);
+	if (test__start_subtest("dynptr_read"))
+		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls_dynptr_read,
+			    skel->maps.test_result);
+	if (test__start_subtest("dynptr_slice"))
+		test_tuntap(skel->progs.ing_xdp, skel->progs.ing_cls_dynptr_slice,
+			    skel->maps.test_result);
 
 	test_xdp_meta__destroy(skel);
 }

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

@@ -1,8 +1,10 @@
+#include <stdbool.h>
 #include <linux/bpf.h>
 #include <linux/if_ether.h>
 #include <linux/pkt_cls.h>
 
 #include <bpf/bpf_helpers.h>
+#include "bpf_kfuncs.h"
 
 #define META_SIZE 32
 
@@ -40,6 +42,46 @@ int ing_cls(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+/* Read from metadata using bpf_dynptr_read helper */
+SEC("tc")
+int ing_cls_dynptr_read(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr meta;
+	const __u32 zero = 0;
+	__u8 *dst;
+
+	dst = bpf_map_lookup_elem(&test_result, &zero);
+	if (!dst)
+		return TC_ACT_SHOT;
+
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	bpf_dynptr_read(dst, META_SIZE, &meta, 0, 0);
+
+	return TC_ACT_SHOT;
+}
+
+/* Read from metadata using read-only dynptr slice */
+SEC("tc")
+int ing_cls_dynptr_slice(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr meta;
+	const __u32 zero = 0;
+	__u8 *dst, *src;
+
+	dst = bpf_map_lookup_elem(&test_result, &zero);
+	if (!dst)
+		return TC_ACT_SHOT;
+
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	src = bpf_dynptr_slice(&meta, 0, NULL, META_SIZE);
+	if (!src)
+		return TC_ACT_SHOT;
+
+	__builtin_memcpy(dst, src, META_SIZE);
+
+	return TC_ACT_SHOT;
+}
+
 SEC("xdp")
 int ing_xdp(struct xdp_md *ctx)
 {