linux-kernel - Re: [PATCH bpf-next v5 2/5] selftests/bpf: Add tests for open-coded style process file iterator

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Message-ID:
 <AM6PR03MB508062C9203BCD7C63BC5206993E2@AM6PR03MB5080.eurprd03.prod.outlook.com>
Date: Wed, 11 Dec 2024 21:27:44 +0000
From: Juntong Deng <juntong.deng@...look.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Christian Brauner <brauner@...nel.org>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 John Fastabend <john.fastabend@...il.com>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eddy Z <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>, snorcht@...il.com,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v5 2/5] selftests/bpf: Add tests for open-coded
 style process file iterator

On 2024/12/10 18:51, Alexei Starovoitov wrote:
> On Tue, Dec 10, 2024 at 8:23 AM Juntong Deng <juntong.deng@...look.com> wrote:
>>
>>>> +SEC("fentry/" SYS_PREFIX "sys_nanosleep")
>>>> +int test_bpf_iter_task_file(void *ctx)
>>>> +{
>>>> +    struct bpf_iter_task_file task_file_it;
>>>> +    struct bpf_iter_task_file_item *item;
>>>> +    struct task_struct *task;
>>>> +
>>>> +    task = bpf_get_current_task_btf();
>>>> +    if (task->parent->pid != parent_pid)
>>>> +            return 0;
>>>> +
>>>> +    count++;
>>>> +
>>>> +    bpf_rcu_read_lock();
>>>
>>> What does the RCU read lock do here exactly?
>>>
>>
>> Thanks for your reply.
>>
>> This is used to solve the problem previously discussed in v3 [0].
>>
>> Task ref may be released during iteration.
>>
>> [0]:
>> https://lore.kernel.org/bpf/CAADnVQ+0LUXxmfm1YgyGDz=cciy3+dGGM-Zysq84fpAdaB74Qw@mail.gmail.com/
> 
> I think you misunderstood my comment.
> 
> "If this object _was_ RCU protected ..."
> 
> Adding rcu_read_lock doesn't make 'task' pointer RCU protected.
> That's not how RCU works.
> 
> So patch 1 doing:
> 
> item->task = task;
> 
> is not correct.
> 
> See bpf_iter_task_vma_new(). It's doing:
> kit->data->task = get_task_struct(task);
> to make sure task stays valid while iterating.
> 
> pw-bot: cr

Thanks for your reply.

Sorry for the misunderstanding.

I will fix it in the next version.