selftests/bpf: Cover skb metadata access after bpf_skb_change_proto

Add a test to verify that skb metadata remains accessible after calling
bpf_skb_change_proto(), which modifies packet headroom to accommodate
different IP header sizes.

Signed-off-by: Jakub Sitnicki <[e-mail]>
Signed-off-by: Martin KaFai Lau <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

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

···
 			    skel->progs.helper_skb_change_head_tail,
 			    NULL, /* tc prio 2 */
 			    &skel->bss->test_pass);
+	if (test__start_subtest("helper_skb_change_proto"))
+		test_tuntap(skel->progs.ing_xdp,
+			    skel->progs.helper_skb_change_proto,
+			    NULL, /* tc prio 2 */
+			    &skel->bss->test_pass);
 
 	test_xdp_meta__destroy(skel);
 }

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

···
 #include <linux/if_ether.h>
 #include <linux/pkt_cls.h>
 
+#include <bpf/bpf_endian.h>
 #include <bpf/bpf_helpers.h>
 #include "bpf_kfuncs.h"
 
···
 
 	/* 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;
+}
+
+SEC("tc")
+int helper_skb_change_proto(struct __sk_buff *ctx)
+{
+	int err;
+
+	err = bpf_skb_change_proto(ctx, bpf_htons(ETH_P_IPV6), 0);
+	if (err)
+		goto out;
+
+	if (!check_skb_metadata(ctx))
+		goto out;
+
+	err = bpf_skb_change_proto(ctx, bpf_htons(ETH_P_IP), 0);
 	if (err)
 		goto out;