selftests/bpf: Add test for non-NULLable PTR_TO_BTF_IDs

In a recent patch, we taught the verifier that trusted PTR_TO_BTF_ID can
never be NULL. This prevents the verifier from incorrectly failing to
load certain programs where it gets confused and thinks a reference
isn't dropped because it incorrectly assumes that a branch exists in
which a NULL PTR_TO_BTF_ID pointer is never released.

This patch adds a testcase that verifies this cannot happen.
Signed-off-by: David Vernet <void@manifault.com>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/r/20230602150112.1494194-2-void@manifault.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -70,5 +70,6 @@ void test_cpumask(void)
 		verify_success(cpumask_success_testcases[i]);
 	}
 
+	RUN_TESTS(cpumask_success);
 	RUN_TESTS(cpumask_failure);
 }

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

@@ -5,6 +5,7 @@
 #include <bpf/bpf_tracing.h>
 #include <bpf/bpf_helpers.h>
 
+#include "bpf_misc.h"
 #include "cpumask_common.h"
 
 char _license[] SEC("license") = "GPL";
@@ -426,3 +427,26 @@ int BPF_PROG(test_global_mask_rcu, struct task_struct *task, u64 clone_flags)
 
 	return 0;
 }
+
+SEC("tp_btf/task_newtask")
+__success
+int BPF_PROG(test_refcount_null_tracking, struct task_struct *task, u64 clone_flags)
+{
+	struct bpf_cpumask *mask1, *mask2;
+
+	mask1 = bpf_cpumask_create();
+	mask2 = bpf_cpumask_create();
+
+	if (!mask1 || !mask2)
+		goto free_masks_return;
+
+	bpf_cpumask_test_cpu(0, (const struct cpumask *)mask1);
+	bpf_cpumask_test_cpu(0, (const struct cpumask *)mask2);
+
+free_masks_return:
+	if (mask1)
+		bpf_cpumask_release(mask1);
+	if (mask2)
+		bpf_cpumask_release(mask2);
+	return 0;
+}