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    SubjectRe: [RFC][PATCH 0/3] fork: Add the ability to create tasks with given pids
    On 11/17/2011 07:49 PM, Oleg Nesterov wrote:
    > On 11/17, Pavel Emelyanov wrote:
    >> Gentlemen, please, find some time for this, your ACK/NACK on the API proposal
    >> is required badly.
    > Please.
    >> The proposal is to introduce the CLONE_CHILD_USEPIDS flag for clone() syscall
    >> and pass the pids values in the child_tidptr. In order not to introduce the
    >> hole for the pid-reuse attack, using this flag will result in EPERM in case
    >> the pid namespace we're trying to create pid in has at least one pid (except
    >> for the init's one) generated with regular fork()/clone().
    >> Currently Tejun and Oleg are worrying only about the intrusiveness of this
    >> approach, although Oleg agrees, that it solves all the problems it should. The
    >> previous attempts to implement the similar stuff stopped, but no objections
    >> against this were expressed. So the decision of whether it's OK to go this
    >> way or not is required.
    > Yes, personally I'd prefer /proc/set_last_pid (or something similar) which
    > simply writes to pid_ns->last_pid. Perhaps it is less convenient from the
    > user-space pov (serialization, security) but it is much simpler.

    Yes, this is also possible. I have a working prototype of /proc/sys/kernel/ns_last_pid
    with the security issue solved, but setting sysctl then cloning seems more obfuscating
    to me than just passing an array of pids to clone.

    > OTOH, I do not pretend I understand the user-space needs, so I won't argue.
    > This series seems correct, the bugs we discussed are fixed.
    > But. Speaking of API, it differs a bit compared to the previous version...
    >> The API will be used like in the code below
    >> /* restore new pid namespace with an init in it */
    >> pid = clone(CLONE_NEWPID);
    > Yes, CLONE_NEWPID | CLONE_CHILD_USEPIDS is not possible.

    It should be. If we (in theory, but) restore two pid namespaces with one being
    a child of another we will have to create an init of the child ns with predefined
    pid in the parent ns.

    > Then how the array of pids in child_tidptr[] can be useful? If CLONE_NEWPID
    > can't restore the pid_nr's in the parent namespaces, then probably this
    > doesn't makes sense at all?
    > IOW. I think we should either allow CLONE_NEWPID | CLONE_CHILD_USEPIDS
    > (with additional check in set_pidmap() to ensure that CLONE_NEWPID
    > comes with child_tidptr[0] == 1), or we should treat the "overloaded"
    > child_tidptr as a simple pid_t.

    The child_tidptr[0] == 1 check will also work. Currently I check for the
    ns->child_reaper being NULL instead.

    > Again, I won't insist. Just I want to be sure we do not miss something
    > adding the new API.
    > Oleg.

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