selftests/bpf: Add fexit-to-skb test for global funcs

Add simple fexit prog type to skb prog type test when subprogram is a global
function.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Song Liu <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/fexit_bpf2bpf.c

···
 		"fexit/test_pkt_access",
 		"fexit/test_pkt_access_subprog1",
 		"fexit/test_pkt_access_subprog2",
+		"fexit/test_pkt_access_subprog3",
 	};
 	test_fexit_bpf2bpf_common("./fexit_bpf2bpf.o",
 				  "./test_pkt_access.o",

tools/testing/selftests/bpf/progs/fexit_bpf2bpf.c

···
 	test_result_subprog2 = 1;
 	return 0;
 }
+
+__u64 test_result_subprog3 = 0;
+BPF_TRACE_3("fexit/test_pkt_access_subprog3", test_subprog3,
+	    int, val, struct sk_buff *, skb, int, ret)
+{
+	int len;
+
+	__builtin_preserve_access_index(({
+		len = skb->len;
+	}));
+	if (len != 74 || ret != 74 * val || val != 3)
+		return 0;
+	test_result_subprog3 = 1;
+	return 0;
+}
 char _license[] SEC("license") = "GPL";

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

···
 	return skb->len * val;
 }
 
+#define MAX_STACK (512 - 2 * 32)
+
+__attribute__ ((noinline))
+int get_skb_len(struct __sk_buff *skb)
+{
+	volatile char buf[MAX_STACK] = {};
+
+	return skb->len;
+}
+
+int get_skb_ifindex(int, struct __sk_buff *skb, int);
+
+__attribute__ ((noinline))
+int test_pkt_access_subprog3(int val, struct __sk_buff *skb)
+{
+	return get_skb_len(skb) * get_skb_ifindex(val, skb, 1);
+}
+
+__attribute__ ((noinline))
+int get_skb_ifindex(int val, struct __sk_buff *skb, int var)
+{
+	volatile char buf[MAX_STACK] = {};
+
+	return skb->ifindex * val * var;
+}
+
 SEC("classifier/test_pkt_access")
 int test_pkt_access(struct __sk_buff *skb)
 {
···
 	if (test_pkt_access_subprog1(skb) != skb->len * 2)
 		return TC_ACT_SHOT;
 	if (test_pkt_access_subprog2(2, skb) != skb->len * 2)
+		return TC_ACT_SHOT;
+	if (test_pkt_access_subprog3(3, skb) != skb->len * 3 * skb->ifindex)
 		return TC_ACT_SHOT;
 	if (tcp) {
 		if (((void *)(tcp) + 20) > data_end || proto != 6)