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

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Date:   Mon, 24 Oct 2016 14:22:10 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Oleg Nesterov <oleg@...hat.com>
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 Mon, Oct 24, 2016 at 02:10:31PM +0200, Oleg Nesterov wrote:
> --- x/kernel/pid.c
> +++ x/kernel/pid.c
> @@ -526,8 +526,11 @@ pid_t __task_pid_nr_ns(struct task_struc
>  	if (!ns)
>  		ns = task_active_pid_ns(current);
>  	if (likely(pid_alive(task))) {
> -		if (type != PIDTYPE_PID)
> +		if (type != PIDTYPE_PID) {
> +			if (type == PIDTYPE_TGID)
> +				type = PIDTYPE_PID;
>  			task = task->group_leader;
> +		}

Aah, that makes much more sense ;-)

>  		nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);
>  	}
>  	rcu_read_unlock();


Still, I wonder if returning 0 is the right thing. 0 is a 'valid' PID
for the init/idle task.

And we still have the re-use issue for the TID, because when we get here
TID is already unhashed too afaict, it just doesn't explode because we
don't deref freed memory.