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

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Message-ID: <20231116095439.GC18748@redhat.com>
Date:   Thu, 16 Nov 2023 10:54:39 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Yonghong Song <yonghong.song@...ux.dev>
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 11/15, 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?

Thanks,

Oleg.