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Date:   Fri, 5 May 2023 15:18:27 +0800
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Subject: Re: Re: [PATCH bpf-next v6 1/2] bpf: Add bpf_task_under_cgroup()
 kfunc
To:     Hao Luo <haoluo@google.com>
Cc:     martin.lau@linux.dev, ast@kernel.org, daniel@iogearbox.net,
        andrii@kernel.org, song@kernel.org, yhs@fb.com,
        john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
        jolsa@kernel.org, davem@davemloft.net, edumazet@google.com,
        kuba@kernel.org, pabeni@redhat.com, mykolal@fb.com,
        shuah@kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-kselftest@vger.kernel.org, yangzhenze@bytedance.com,
        wangdongdong.6@bytedance.com
References: <20230505060818.60037-1-zhoufeng.zf@bytedance.com>
 <20230505060818.60037-2-zhoufeng.zf@bytedance.com>
 <CA+khW7g_gq1N=cNHC-5WG2nZ8a-wHSpwg_fc5=dQpkweGvROqA@mail.gmail.com>
From:   Feng Zhou <zhoufeng.zf@bytedance.com>
In-Reply-To: <CA+khW7g_gq1N=cNHC-5WG2nZ8a-wHSpwg_fc5=dQpkweGvROqA@mail.gmail.com>
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在 2023/5/5 14:58, Hao Luo 写道:
> On Thu, May 4, 2023 at 11:08 PM Feng zhou <zhoufeng.zf@bytedance.com> 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

Hmm, you are right. As kfunc, the NULL value of the parameter is judged, 
and bpf verify will prompt the developer to add it. There is really no 
need to add this part of the judgment. See other people's opinions.