Re: [PATCH v2 1/2] fork: extend clone3() to support CLONE_SET_TID

Oleg Nesterov
2019-07-31T17:41:41Z

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.