linux-kernel - Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET

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Date:   Wed, 31 Jul 2019 19:41:36 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Adrian Reber <areber@...hat.com>
Cc:     Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelianov <xemul@...tuozzo.com>,
        Jann Horn <jannh@...gle.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org, Andrei Vagin <avagin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>
Subject: Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID

On 07/31, Adrian Reber wrote:
>
> Extending clone3() to support CLONE_SET_TID makes it possible restore a
> process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
> race free (as long as the desired PID/TID is available).

I personally like this... but please see the question below.

> +struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
>  {
>  	struct pid *pid;
>  	enum pid_type type;
> @@ -186,12 +186,28 @@ 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_get_cursor(&tmp->idr) <= 1)))
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Ah, I forgot to mention... this should work but only because
RESERVED_PIDS > 0. How about idr_is_empty() ?


But the main question is how it can really help if ns->level > 0, unlikely
CRIU will ever need to clone the process with the same pid_nr == set_tid
in the ns->parent chain.

So may be kernel_clone_args->set_tid should be pid_t __user *set_tid_array?
Or I missed something ?

Oleg.