selftests/bpf: Cover skb metadata access after change_head/tail helper

Add a test to verify that skb metadata remains accessible after calling
bpf_skb_change_head() and bpf_skb_change_tail(), which modify packet
headroom/tailroom and can trigger head reallocation.

Signed-off-by: Jakub Sitnicki <[email protected]>

Author: jsitnicki

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

@@ -497,6 +497,11 @@ void test_xdp_context_tuntap(void)
 			    skel->progs.helper_skb_adjust_room,
 			    NULL, /* tc prio 2 */
 			    &skel->bss->test_pass);
+	if (test__start_subtest("helper_skb_change_head_tail"))
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.helper_skb_change_head_tail,
+			    NULL, /* tc prio 2 */
+			    &skel->bss->test_pass);
 
 	test_xdp_meta__destroy(skel);
 }

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

@@ -611,4 +611,38 @@ int helper_skb_adjust_room(struct __sk_buff *ctx)
 	return TC_ACT_SHOT;
 }
 
+SEC("tc")
+int helper_skb_change_head_tail(struct __sk_buff *ctx)
+{
+	int err;
+
+	/* Reserve 1 extra in the front for packet data */
+	err = bpf_skb_change_head(ctx, 1, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	/* Reserve 256 extra bytes in the front to trigger head reallocation */
+	err = bpf_skb_change_head(ctx, 256, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	/* Reserve 4k extra bytes in the back to trigger head reallocation */
+	err = bpf_skb_change_tail(ctx, ctx->len + 4096, 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
 char _license[] SEC("license") = "GPL";