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    SubjectRe: [RFC][v4][PATCH 7/7]: Define clone_with_pids syscall

    > Subject: [RFC][v4][PATCH 7/7]: Define clone_with_pids syscall
    > Container restart requires that a task have the same pid it had when it was
    > checkpointed. When containers are nested the tasks within the containers
    > exist in multiple pid namespaces and hence have multiple pids to specify
    > during restart.
    > clone_with_pids(), intended for use during restart, is the same as clone(),
    > except that it takes a 'target_pid_set' paramter. This parameter lets caller
    > choose specific pid numbers for the child process, in the process's active
    > and ancestor pid namespaces. (Descendant pid namespaces in general don't
    > matter since processes don't have pids in them anyway, but see comments
    > in copy_target_pids() regarding CLONE_NEWPID).

    This should go to documentation/manpage somewhere.

    > Unlike clone(), clone_with_pids() needs CAP_SYS_ADMIN, at least for now, to
    > prevent unprivileged processes from misusing this interface.
    > Call clone_with_pids as follows:
    > pid_t pids[] = { 0, 77, 99 };
    > struct pid_set pid_set;
    > pid_set.num_pids = sizeof(pids) / sizeof(int);
    > pid_set.pids = &pids;
    > syscall(__NR_clone_with_pids, flags, stack, NULL, NULL, NULL, &pid_set);
    > If a target-pid is 0, the kernel continues to assign a pid for the process in
    > that namespace. In the above example, pids[0] is 0, meaning the kernel will
    > assign next available pid to the process in init_pid_ns. But kernel will assign
    > pid 77 in the child pid namespace 1 and pid 99 in pid namespace 2. If either
    > 77 or 99 are taken, the system call fails with -EBUSY.
    > If 'pid_set.num_pids' exceeds the current nesting level of pid namespaces,
    > the system call fails with -EINVAL.

    Does it make sense to set the pid in anything but innermost container?

    (cesky, pictures)

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