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

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Message-ID: <20161024122123.GB17007@redhat.com>
Date:   Mon, 24 Oct 2016 14:21:23 +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:
>
> On Mon, Oct 24, 2016 at 01:15:27PM +0200, Oleg Nesterov wrote:
> >
> > --- x/kernel/events/core.c
> > +++ x/kernel/events/core.c
> > @@ -1257,7 +1257,7 @@ static u32 perf_event_pid(struct perf_ev
> >  	if (event->parent)
> >  		event = event->parent;
> >
> > -	return task_tgid_nr_ns(p, event->ns);
> > +	return pid_alive(p) ? task_tgid_nr_ns(p, event->ns) : 0;
> >  }
> >
> >  static u32 perf_event_tid(struct perf_event *event, struct task_struct *p)
>
> Should we do the same for perf_event_tid() and report -1 as the pid/tid
> in the !alive case? -1 should be an obvious invalid pid since we limit
> the pid-space to less than 32 bits.

task_pid_nr_ns() is always safe, it calls __task_pid_nr_ns(). But yes,
it can return zero if called after exit_notify() and/or release_task().

And while zero is not a valid pid too, I guess it can be confused with
the idle thread's "pid" ?

Oleg.