linux-kernel - Re: hit a KASan bug related to Perf during stress test

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Message-ID: <20161024153908.GA26135@redhat.com>
Date:   Mon, 24 Oct 2016 17:39:08 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Ni, BaoleX" <baolex.ni@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "acme@...nel.org" <acme@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "Liu, Chuansheng" <chuansheng.liu@...el.com>
Subject: Re: hit a KASan bug related to Perf during stress test

On 10/24, Peter Zijlstra wrote:
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -1257,7 +1257,14 @@ static u32 perf_event_pid(struct perf_event *event, struct task_struct *p)
>  	if (event->parent)
>  		event = event->parent;
>  
> -	return task_tgid_nr_ns(p, event->ns);
> +	/*
> +	 * It is possible the task already got unhashed, in which case we
> +	 * cannot determine the current->group_leader/real_parent.
> +	 *
> +	 * Also, report -1 to indicate unhashed, so as not to confused with
> +	 * 0 for the idle task.
> +	 */
> +	return pid_alive(p) ? task_tgid_nr_ns(p, event->ns) : ~0;
>  }

Yes, but this _looks_ racy unless p == current. I mean, pid_alive() makes
task_tgid_nr_ns() safe, but task_tgid_nr_ns() still can return zero _if_
it can race with the exiting task.

>  static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)
> @@ -1268,7 +1275,7 @@ static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)
>  	if (event->parent)
>  		event = event->parent;
>
> -	return task_pid_nr_ns(p, event->ns);
> +	return pid_alive(p) ? task_pid_nr_ns(p, event->ns) : ~0;

The same.

However. At first glance the only case when p != current is copy_process(),
right? And in this case the new child can't go away. So I think this patch
is fine.

Oleg.