linux-kernel - Re: perf_event_pid() (Was: [PATCH] pids: make task_tgid_nr

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Date:   Mon, 21 Aug 2017 21:18:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Troy Kensinger <tkensinger@...gle.com>,
        "security@...nel.org" <security@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        linux-kernel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: perf_event_pid() (Was: [PATCH] pids: make task_tgid_nr_ns() safe)

On Mon, Aug 21, 2017 at 05:36:27PM +0200, Oleg Nesterov wrote:
> On 08/21, Oleg Nesterov wrote:
> >
> > This was reported many times, and this was even mentioned in commit
> > 52ee2dfdd4f5 "pids: refactor vnr/nr_ns helpers to make them safe" but
> > somehow nobody bothered to fix the obvious problem: task_tgid_nr_ns()
> > is not safe because task->group_leader points to nowhere after the
> > exiting task passes exit_notify(), rcu_read_lock() can not help.
> 
> Peter,
> 
> we already discussed this before, but I can't recall the result...
> 
> perf_event_pid() can hit this problem too, with this patch the problem
> goes away but (with or without this patch) we do not want to report zero
> pids to avoid the confusion with idle threads, right?
> 
> So do you think the patch below makes sense or we do not care?

Yeah, works for me. While rare I think it makes sense to distinguish
between a recently dead and the idle tasks.