selftests/bpf: Make sure stashed kptr in local kptr is freed recursively

When dropping a local kptr, any kptr stashed into it is supposed to be
freed through bpf_obj_free_fields->__bpf_obj_drop_impl recursively. Add a
test to make sure it happens.

The test first stashes a referenced kptr to "struct task" into a local
kptr and gets the reference count of the task. Then, it drops the local
kptr and reads the reference count of the task again. Since
bpf_obj_free_fields and __bpf_obj_drop_impl will go through the local kptr
recursively during bpf_obj_drop, the dtor of the stashed task kptr should
eventually be called. The second reference count should be one less than
the first one.

Signed-off-by: Amery Hung <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 SEC("tp_btf/task_newtask")
 int BPF_PROG(test_task_xchg_release, struct task_struct *task, u64 clone_flags)
 {
-	struct task_struct *kptr;
+	struct task_struct *kptr, *acquired;
 	struct __tasks_kfunc_map_value *v, *local;
+	int refcnt, refcnt_after_drop;
 	long status;
 
 	if (!is_test_kfunc_task())
···
 		return 0;
 	}
 
+	/* Stash a copy into local kptr and check if it is released recursively */
+	acquired = bpf_task_acquire(kptr);
+	if (!acquired) {
+		err = 7;
+		bpf_obj_drop(local);
+		bpf_task_release(kptr);
+		return 0;
+	}
+	bpf_probe_read_kernel(&refcnt, sizeof(refcnt), &acquired->rcu_users);
+
+	acquired = bpf_kptr_xchg(&local->task, acquired);
+	if (acquired) {
+		err = 8;
+		bpf_obj_drop(local);
+		bpf_task_release(kptr);
+		bpf_task_release(acquired);
+		return 0;
+	}
+
 	bpf_obj_drop(local);
+
+	bpf_probe_read_kernel(&refcnt_after_drop, sizeof(refcnt_after_drop), &kptr->rcu_users);
+	if (refcnt != refcnt_after_drop + 1) {
+		err = 9;
+		bpf_task_release(kptr);
+		return 0;
+	}
+
 	bpf_task_release(kptr);
 
 	return 0;