[CRIU] Restore may fail due to PID number overflow

[Evgenii Shatokhin]

20.02.2016 17:33, Pavel Emelyanov пишет:
> On 02/20/2016 05:22 PM, Evgenii Shatokhin wrote:
>> Hi,
>>
>> When CRIU is used to checkpoint and then restore a number of processes
>> (live zdtm tests, actually) running in their own pid namespace, restore
>> fails with the following error in rare cases:
>>
>>      Error (cr-restore.c:1573): Pid 300 do not match expected 32768
>>
>> I am new to CRIU and cannot say right now how to fix this properly, so
>> your suggestions are appreciated.
>>
>> As far as I can see in the code, the problem is in
>> pstree.c, prepare_pstree_ids():
>> -------------
>> 	/* Try to find helpers, who should be connected to the leader */
>> 	list_for_each_entry(child, &helpers, sibling) {
>> 		if (child->state != TASK_HELPER)
>> 			continue;
>>
>> 		if (child->sid != item->sid)
>> 			continue;
>>
>> 		child->pgid = item->pgid;
>> 		child->pid.virt = ++max_pid;
>> 		child->parent = item;
>> 		list_move(&child->sibling, &item->children);
>>
>> 		pr_info("Attach %d to the task %d\n",
>> 				child->pid.virt, item->pid.virt);
>>
>> 		break;
>> 	}
>> -------------
>>
>> max_pid may become 32768 after the increment, and this value is saved in
>> child->pid.virt.
>>
>> However, when that process is spawned, the OS cannot give it PID number
>> greater than or equal to the maximum (/proc/sys/kernel/pid_max contains
>> 32768 in that case). Thus the OS gives it the smallest unused PID number
>> not less than 300, as it should.
>>
>> When that process executes restore_task_with_children() (cr-restore.c),
>> it compares its stored and real PID numbers, sees the mismatch and
>> reports failure:
>> -------------
>> pid = getpid();
>> if (current->pid.virt != pid) {
>> 	pr_err("Pid %d do not match expected %d\n", pid,
>> 		current->pid.virt);
>> 	set_task_cr_err(EEXIST);
>> 	goto err;
>> }
>> -------------
>>
>> If I hack prepare_pstree_ids() as follows, the problem is gone, but,
>> obviously, this is not a proper solution:
>> -------------
>>                           child->pgid = item->pgid;
>> -                       child->pid.virt = ++max_pid;
>> +
>> +                       max_pid++;
>> +                       if (max_pid == 32768)
>> +                               max_pid = 300;
>> +
>> +                       child->pid.virt = max_pid;
>>                           child->parent = item;
>> -------------
>>
>> As for the maximum value of PIDs, one can get it from
>> /proc/sys/kernel/pid_max, I suppose.
>
> Well, the intention of this code is to find _unused_ pid :) It was the
> simplest way to do it, not the correct one.
>
>> The tricky part is how to find the smallest unused PID number >= 300 at
>> that point. Any ideas?
>
> There's a patch set from Andrey that should fix this issue.
>
> https://lists.openvz.org/pipermail/criu/2016-February/025584.html

The patch set helps, indeed. That particular problem seems to be gone now.

Thanks for the hint!

Regards,
Evgenii