linux-kernel - Re: task_is_descendant() cleanup

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Date:   Wed, 25 Jan 2017 13:59:58 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Pavel Tikhomirov <ptikhomirov@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Konstantin Khorenko <khorenko@...tuozzo.com>
Subject: Re: task_is_descendant() cleanup

On Mon, Jan 23, 2017 at 4:52 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 01/23, Oleg Nesterov wrote:
>>
>> Btw task_is_descendant() looks wrong at first glance.
>
> No, I missed the 2nd ->group_leader dereference. Still this function looks
> overcomplicated and the usage of thread_group_leader/group_leader just add
> the unnecessary confusion. It can be simplified a little bit:
>
>         static int task_is_descendant(struct task_struct *parent,
>                                       struct task_struct *child)
>         {
>                 int rc = 0;
>                 struct task_struct *walker;
>
>                 if (!parent || !child)
>                         return 0;
>
>                 rcu_read_lock();
>                 for (walker = child; walker->pid; walker = rcu_dereference(walker->real_parent))
>                         if (same_thread_group(parent, walker)) {
>                                 rc = 1;
>                                 break;
>                         }
>                 rcu_read_unlock();
>
>                 return rc;
>         }
>
> Kees, I can send a patch if you think this very minor cleanup makes any sense.

Err, isn't checking same_thread_group() at every level more expensive
than what I currently have?

-Kees

-- 
Kees Cook
Nexus Security