Add a test to verify that skb metadata remains accessible after calling
bpf_skb_vlan_push() and bpf_skb_vlan_pop(), which modify the packet
headroom.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
 .../bpf/prog_tests/xdp_context_test_run.c          |  6 +++
 tools/testing/selftests/bpf/progs/test_xdp_meta.c  | 43 ++++++++++++++++++++++
 2 files changed, 49 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
index a129c3057202..97c8f876f673 100644
--- a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
@@ -478,6 +478,12 @@ void test_xdp_context_tuntap(void)
 		test_tuntap_mirred(skel->progs.ing_xdp,
 				   skel->progs.clone_meta_dynptr_rw_before_meta_dynptr_write,
 				   &skel->bss->test_pass);
+	/* Tests for BPF helpers which touch headroom */
+	if (test__start_subtest("helper_skb_vlan_push_pop"))
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.helper_skb_vlan_push_pop,
+			    NULL, /* tc prio 2 */
+			    &skel->bss->test_pass);
 
 	test_xdp_meta__destroy(skel);
 }
diff --git a/tools/testing/selftests/bpf/progs/test_xdp_meta.c b/tools/testing/selftests/bpf/progs/test_xdp_meta.c
index ee89e1124cd8..41e1d76e90a0 100644
--- a/tools/testing/selftests/bpf/progs/test_xdp_meta.c
+++ b/tools/testing/selftests/bpf/progs/test_xdp_meta.c
@@ -44,6 +44,16 @@ static bool check_metadata(const char *file, int line, __u8 *meta_have)
 
 #define check_metadata(meta_have) check_metadata(__FILE__, __LINE__, meta_have)
 
+static bool check_skb_metadata(const char *file, int line, struct __sk_buff *skb)
+{
+	__u8 *data_meta = ctx_ptr(skb, data_meta);
+	__u8 *data = ctx_ptr(skb, data);
+
+	return data_meta + META_SIZE <= data && (check_metadata)(file, line, data_meta);
+}
+
+#define check_skb_metadata(skb) check_skb_metadata(__FILE__, __LINE__, skb)
+
 SEC("tc")
 int ing_cls(struct __sk_buff *ctx)
 {
@@ -525,4 +535,37 @@ int clone_meta_dynptr_rw_before_meta_dynptr_write(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+SEC("tc")
+int helper_skb_vlan_push_pop(struct __sk_buff *ctx)
+{
+	int err;
+
+	/* bpf_skb_vlan_push assumes HW offload for primary VLAN tag. Only
+	 * secondary tag push triggers an actual MAC header modification.
+	 */
+	err = bpf_skb_vlan_push(ctx, 0, 42);
+	if (err)
+		goto out;
+	err = bpf_skb_vlan_push(ctx, 0, 207);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	err = bpf_skb_vlan_pop(ctx);
+	if (err)
+		goto out;
+	err = bpf_skb_vlan_pop(ctx);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
 char _license[] SEC("license") = "GPL";

-- 
2.43.0