selftests/bpf: Validate CHECKSUM_COMPLETE option

Adjust skb program test to run with checksum validation.
Signed-off-by: Vadim Fedorenko <vadfed@meta.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240606145851.229116-2-vadfed@meta.com

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

@@ -11,6 +11,7 @@ static int sanity_run(struct bpf_program *prog)
 		.data_in = &pkt_v4,
 		.data_size_in = sizeof(pkt_v4),
 		.repeat = 1,
+		.flags = BPF_F_TEST_SKB_CHECKSUM_COMPLETE,
 	);
 
 	prog_fd = bpf_program__fd(prog);

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

@@ -33,6 +33,8 @@ int main_prog(struct __sk_buff *skb)
 	struct iphdr *ip = NULL;
 	struct tcphdr *tcp;
 	__u8 proto = 0;
+	int urg_ptr;
+	u32 offset;
 
 	if (!(ip = get_iphdr(skb)))
 		goto out;
@@ -48,7 +50,14 @@ int main_prog(struct __sk_buff *skb)
 	if (!tcp)
 		goto out;
 
-	return tcp->urg_ptr;
+	urg_ptr = tcp->urg_ptr;
+
+	/* Checksum validation part */
+	proto++;
+	offset = sizeof(struct ethhdr) + offsetof(struct iphdr, protocol);
+	bpf_skb_store_bytes(skb, offset, &proto, sizeof(proto), BPF_F_RECOMPUTE_CSUM);
+
+	return urg_ptr;
 out:
 	return -1;
 }