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Date:   Wed, 7 Aug 2019 16:26:10 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Adrian Reber <areber@redhat.com>
Cc:     Christian Brauner <christian@brauner.io>,
        Eric Biederman <ebiederm@xmission.com>,
        Pavel Emelianov <xemul@virtuozzo.com>,
        Jann Horn <jannh@google.com>,
        Dmitry Safonov <0x7f454c46@gmail.com>,
        linux-kernel@vger.kernel.org, Andrei Vagin <avagin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@gmail.com>
Subject: Re: [PATCH v3 1/2] fork: extend clone3() to support CLONE_SET_TID
Message-ID: <20190807142609.GC24112@redhat.com>
References: <20190806191551.22192-1-areber@redhat.com>
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On 08/06, Adrian Reber wrote:
>
> +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
>  {
>  	struct pid *pid;
>  	enum pid_type type;
> @@ -186,12 +186,35 @@ struct pid *alloc_pid(struct pid_namespace *ns)
>  		if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
>  			pid_min = RESERVED_PIDS;
>  
> -		/*
> -		 * Store a null pointer so find_pid_ns does not find
> -		 * a partially initialized PID (see below).
> -		 */
> -		nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
> -				      pid_max, GFP_ATOMIC);
> +		if (set_tid) {
> +			/*
> +			 * Also fail if a PID != 1 is requested
> +			 * and no PID 1 exists.
> +			 */
> +			if ((set_tid >= pid_max) || ((set_tid != 1) &&
> +				(idr_is_empty(&tmp->idr)))) {

too many parentheses ;) this is purely cosmetic, up to you, but to me

			if (set_tid >= pid_max ||
			   (set_tid != 1 && idr_is_empty(&tmp->idr))) {

looks a bit more readable.


> +				spin_unlock_irq(&pidmap_lock);
> +				retval = -EINVAL;
> +				goto out_free;

This doesn't look right, you need idr_preload_end() before goto out_free.

But I'd suggest to simply do

			nr = -EINVAL;
			if (set_tid < pid_max &&
			   (set_tid != 1 || idr_is_empty(&tmp->idr)))
				nr = idr_alloc(&tmp->idr, NULL, set_tid,
						set_tid + 1, GFP_ATOMIC);

			...

this is more robust.

Oleg.