bpf: Disable bpf_refcount_acquire kfunc calls until race conditions are fixed

As reported by Kumar in [0], the shared ownership implementation for BPF
programs has some race conditions which need to be addressed before it
can safely be used. This patch does so in a minimal way instead of
ripping out shared ownership entirely, as proper fixes for the issues
raised will follow ASAP, at which point this patch's commit can be
reverted to re-enable shared ownership.

The patch removes the ability to call bpf_refcount_acquire_impl from BPF
programs. Programs can only bump refcount and obtain a new owning
reference using this kfunc, so removing the ability to call it
effectively disables shared ownership.

Instead of changing success / failure expectations for
bpf_refcount-related selftests, this patch just disables them from
running for now.

  [0]: https://lore.kernel.org/bpf/d7hyspcow5wtjcmw4fugdgyp3fwhljwuscp3xyut5qnwivyeru@ysdq543otzv2/Reported-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Link: https://lore.kernel.org/r/20230424204321.2680232-1-davemarchevsky@fb.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -10509,7 +10509,10 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 				verbose(env, "arg#%d doesn't point to a type with bpf_refcount field\n", i);
 				return -EINVAL;
 			}
-
+			if (rec->refcount_off >= 0) {
+				verbose(env, "bpf_refcount_acquire calls are disabled for now\n");
+				return -EINVAL;
+			}
 			meta->arg_refcount_acquire.btf = reg->btf;
 			meta->arg_refcount_acquire.btf_id = reg->btf_id;
 			break;

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

@@ -9,10 +9,8 @@
 
 void test_refcounted_kptr(void)
 {
-	RUN_TESTS(refcounted_kptr);
 }
 
 void test_refcounted_kptr_fail(void)
 {
-	RUN_TESTS(refcounted_kptr_fail);
 }