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

["Serge E. Hallyn" <serue@us.ibm.com>]

Quoting Pavel Machek (pavel@ucw.cz):
> > 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?

Yup, we might be restarting an app using a nested pid namespace, in which
case restart would specify pids for 2 (or more) of the innermost containers.

thanks,
-serge