Re: [PATCH 0/3] bpf: kernel/bpf/task_iter.c: don't abuse next_thread()

[Yonghong Song]

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@xxxxxxxxxx/
> 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.