linux-kernel - Re: [PATCH 0/3] bpf: kernel/bpf/task_iter.c: don't abuse next

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Message-ID: <41039b2a-6122-4392-96e2-4a833f41496d@linux.dev>
Date:   Thu, 16 Nov 2023 06:52:16 -0500
From:   Yonghong Song <yonghong.song@...ux.dev>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Chuyi Zhou <zhouchuyi@...edance.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kui-Feng Lee <kuifeng@...com>, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH 0/3] bpf: kernel/bpf/task_iter.c: don't abuse
 next_thread()


On 11/16/23 4:54 AM, Oleg Nesterov wrote:
> On 11/15, Yonghong Song wrote:
>> On 11/14/23 11:32 AM, Oleg Nesterov wrote:
>>> Compile tested.
>>>
>>> Every lockless usage of next_thread() was wrong, bpf/task_iter.c is
>>> the last user and is no exception.
>> It would be great if you can give more information in the commit message
>> about why the usage of next_thread() is wrong in bpf/task_iter.c.
> I tried to explain the problems in the changelogs:
>
> 1/3:
> 	task_group_seq_get_next() can return the group leader twice if it races
> 	with mt-thread exec which changes the group->leader's pid.
>
> 2/3:
> 	bpf_iter_task_next() can loop forever, "kit->pos == kit->task" can never
> 	happen if kit->pos execs.
>
>> IIUC, some information is presented in :
>>    https://lore.kernel.org/all/20230824143112.GA31208@redhat.com/
> Yes, Linus and Eric suggest to simply kill next_thread(). I am not
> sure, this needs another discussion.
>
> But as for bpf/task_iter.c... Even _if_ the usage was correct, this
> code simply doesn't need the "circular" next_thread(), NULL at the
> end simplifies the code.
>
>> Also, please add 'bpf' in the subject tag ([PATCH bpf 0/3]) to
>> make it clear the patch should be applied to bpf tree.
> OK, will do next time. Or should I resend this series with 'bpf'
> in the subject tag?

There is no need then. We can wait for maintainers who may or
may not have additional requests.


>
> Thanks,
>
> Oleg.
>