linux-kernel - Re: [PATCH RFC v1] pidfd: fix a race in setting exit

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Date:   Fri, 19 Jul 2019 18:53:46 +0200
From:   Christian Brauner <christian@...uner.io>
To:     Joel Fernandes <joelaf@...gle.com>
CC:     Oleg Nesterov <oleg@...hat.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH RFC v1] pidfd: fix a race in setting exit_state for pidfd polling

On July 19, 2019 6:51:20 PM GMT+02:00, Joel Fernandes <joelaf@...gle.com> wrote:
>On Fri, Jul 19, 2019 at 12:27 PM Christian Brauner
><christian@...uner.io> wrote:
>>
>> On Fri, Jul 19, 2019 at 06:14:05PM +0200, Oleg Nesterov wrote:
>> > it seems that I missed something else...
>> >
>> > On 07/17, Joel Fernandes (Google) wrote:
>> > >
>> > > @@ -1156,10 +1157,11 @@ static int wait_task_zombie(struct
>wait_opts *wo, struct task_struct *p)
>> > >             ptrace_unlink(p);
>> > >
>> > >             /* If parent wants a zombie, don't release it now */
>> > > -           state = EXIT_ZOMBIE;
>> > > +           p->exit_state = EXIT_ZOMBIE;
>> > >             if (do_notify_parent(p, p->exit_signal))
>> > > -                   state = EXIT_DEAD;
>> > > -           p->exit_state = state;
>> > > +                   p->exit_state = EXIT_DEAD;
>> > > +
>> > > +           state = p->exit_state;
>> > >             write_unlock_irq(&tasklist_lock);
>> >
>> > why do you think we also need to change wait_task_zombie() ?
>> >
>> > pidfd_poll() only needs the exit_state != 0 check, we know that it
>> > is not zero at this point. Why do we need to change exit_state
>before
>> > do_notify_parent() ?
>>
>> Oh, because of?:
>>
>>         /*
>>          * Move the task's state to DEAD/TRACE, only one thread can
>do this.
>>          */
>>         state = (ptrace_reparented(p) && thread_group_leader(p)) ?
>>                 EXIT_TRACE : EXIT_DEAD;
>>         if (cmpxchg(&p->exit_state, EXIT_ZOMBIE, state) !=
>EXIT_ZOMBIE)
>>                 return 0;
>>
>> So exit_state will definitely be set in this scenario. Good point.
>>
>
>Agreed. Christian, do you mind dropping this hunk from the patch or do
>you want me to resend the patch with the hunk dropped?

Yeah, no problem. :)