selftests/bpf: Add test to access const void pointer argument in tracing program

Adding verifier test for accessing const void pointer argument in
tracing programs.

The test program loads 1st argument of bpf_fentry_test10 function
which is const void pointer and checks that verifier allows that.

Signed-off-by: KaFai Wan <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

net/bpf/test_run.c

···
 	return *a;
 }
 
+int noinline bpf_fentry_test10(const void *a)
+{
+	return (long)a;
+}
+
 void noinline bpf_fentry_test_sinfo(struct skb_shared_info *sinfo)
 {
 }
···
 		    bpf_fentry_test6(16, (void *)17, 18, 19, (void *)20, 21) != 111 ||
 		    bpf_fentry_test7((struct bpf_fentry_test_t *)0) != 0 ||
 		    bpf_fentry_test8(&arg) != 0 ||
-		    bpf_fentry_test9(&retval) != 0)
+		    bpf_fentry_test9(&retval) != 0 ||
+		    bpf_fentry_test10((void *)0) != 0)
 			goto out;
 		break;
 	case BPF_MODIFY_RETURN:

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

···
 "	::: __clobber_all);
 }
 
+SEC("fentry/bpf_fentry_test10")
+__description("btf_ctx_access const void pointer accept")
+__success __retval(0)
+__naked void ctx_access_const_void_pointer_accept(void)
+{
+	asm volatile ("					\
+	r2 = *(u64 *)(r1 + 0);		/* load 1st argument value (const void pointer) */\
+	r0 = 0;						\
+	exit;						\
+"	::: __clobber_all);
+}
+
 char _license[] SEC("license") = "GPL";