bpf: selftest: Trigger a DCE on the whole subprog

This patch adds a test to trigger the DCE to remove
the whole subprog to ensure the verifier  does not
depend on a stable subprog index.  The DCE is done
by testing a global const.
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211106014020.651638-1-kafai@fb.com

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

@@ -23,6 +23,16 @@ struct callback_ctx {
 	int output;
 };
 
+const volatile int bypass_unused = 1;
+
+static __u64
+unused_subprog(struct bpf_map *map, __u32 *key, __u64 *val,
+	       struct callback_ctx *data)
+{
+	data->output = 0;
+	return 1;
+}
+
 static __u64
 check_array_elem(struct bpf_map *map, __u32 *key, __u64 *val,
 		 struct callback_ctx *data)
@@ -54,6 +64,8 @@ int test_pkt_access(struct __sk_buff *skb)
 
 	data.output = 0;
 	bpf_for_each_map_elem(&arraymap, check_array_elem, &data, 0);
+	if (!bypass_unused)
+		bpf_for_each_map_elem(&arraymap, unused_subprog, &data, 0);
 	arraymap_output = data.output;
 
 	bpf_for_each_map_elem(&percpu_map, check_percpu_elem, (void *)0, 0);