Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup() kfunc

[Hao Luo]

On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@xxxxxxxxxxxxx> wrote:
>
<...>
> ---
>  kernel/bpf/helpers.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index bb6b4637ebf2..453cbd312366 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2149,6 +2149,25 @@ __bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
>                 return NULL;
>         return cgrp;
>  }
> +
> +/**
> + * bpf_task_under_cgroup - wrap task_under_cgroup_hierarchy() as a kfunc, test
> + * task's membership of cgroup ancestry.
> + * @task: the task to be tested
> + * @ancestor: possible ancestor of @task's cgroup
> + *
> + * Tests whether @task's default cgroup hierarchy is a descendant of @ancestor.
> + * It follows all the same rules as cgroup_is_descendant, and only applies
> + * to the default hierarchy.
> + */
> +__bpf_kfunc long bpf_task_under_cgroup(struct task_struct *task,
> +                                      struct cgroup *ancestor)
> +{
> +       if (unlikely(!ancestor || !task))
> +               return -EINVAL;
> +
> +       return task_under_cgroup_hierarchy(task, ancestor);
> +}
>  #endif /* CONFIG_CGROUPS */
>

I wonder in what situation a null 'task' or 'ancestor' can be passed.
Please call out in the comment that the returned value can be a
negative error, so that writing if(bpf_task_under_cgroup()) may cause
surprising results.

Hao