selftests/bpf: Add light skeleton test for kfunc detection.

Add light skeleton test for kfunc detection and denylist it for s390.

Signed-off-by: Alexei Starovoitov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/DENYLIST.s390x

···
 iters/testmod_seq*                       # s390x doesn't support kfuncs in modules yet
 kprobe_multi_bench_attach                # bpf_program__attach_kprobe_multi_opts unexpected error: -95
 kprobe_multi_test                        # relies on fentry
+ksyms_btf/weak_ksyms*                    # test_ksyms_weak__open_and_load unexpected error: -22                        (kfunc)
 ksyms_module                             # test_ksyms_module__open_and_load unexpected error: -9                       (?)
 ksyms_module_libbpf                      # JIT does not support calling kernel function                                (kfunc)
 ksyms_module_lskel                       # test_ksyms_module_lskel__open_and_load unexpected error: -9                 (?)

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

···
 /* test existing weak symbols can be resolved. */
 extern const struct rq runqueues __ksym __weak; /* typed */
 extern const void bpf_prog_active __ksym __weak; /* typeless */
+struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym __weak;
+void bpf_testmod_test_mod_kfunc(int i) __ksym __weak;
 
 
 /* non-existent weak symbols. */
···
 
 /* typed symbols, default to zero. */
 extern const int bpf_link_fops2 __ksym __weak;
+void invalid_kfunc(void) __ksym __weak;
 
 SEC("raw_tp/sys_enter")
 int pass_handler(const void *ctx)
···
 
 	if (&bpf_link_fops2) /* can't happen */
 		out__non_existent_typed = (__u64)bpf_per_cpu_ptr(&bpf_link_fops2, 0);
+
+	if (!bpf_ksym_exists(bpf_task_acquire))
+		/* dead code won't be seen by the verifier */
+		bpf_task_acquire(0);
+
+	if (!bpf_ksym_exists(bpf_testmod_test_mod_kfunc))
+		/* dead code won't be seen by the verifier */
+		bpf_testmod_test_mod_kfunc(0);
+
+	if (bpf_ksym_exists(invalid_kfunc))
+		/* dead code won't be seen by the verifier */
+		invalid_kfunc();
 
 	return 0;
 }