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sysctl: add the kernel.ns_last_pid control
The sysctl works on the current task's pid namespace, getting and setting its last_pid field. Writing is allowed for CAP_SYS_ADMIN-capable tasks thus making it possible to create a task with desired pid value. This ability is required badly for the checkpoint/restore in userspace. This approach suits all the parties for now. Signed-off-by: Pavel Emelyanov <xemul@parallels.com> Acked-by: Tejun Heo <tj@kernel.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Cyrill Gorcunov <gorcunov@openvz.org> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Serge Hallyn <serue@us.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -415,6 +415,14 @@ PIDs of value pid_max or larger are not allocated.
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==============================================================
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ns_last_pid:
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The last pid allocated in the current (the one task using this sysctl
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lives in) pid namespace. When selecting a pid for a next task on fork
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kernel tries to allocate a number starting from this one.
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==============================================================
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powersave-nap: (PPC only)
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If set, Linux-PPC will use the 'nap' mode of powersaving,
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@ -137,7 +137,9 @@ static int pid_before(int base, int a, int b)
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}
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/*
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* We might be racing with someone else trying to set pid_ns->last_pid.
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* We might be racing with someone else trying to set pid_ns->last_pid
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* at the pid allocation time (there's also a sysctl for this, but racing
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* with this one is OK, see comment in kernel/pid_namespace.c about it).
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* We want the winner to have the "later" value, because if the
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* "earlier" value prevails, then a pid may get reused immediately.
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*
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@ -191,9 +191,40 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
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return;
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}
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static int pid_ns_ctl_handler(struct ctl_table *table, int write,
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void __user *buffer, size_t *lenp, loff_t *ppos)
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{
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struct ctl_table tmp = *table;
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if (write && !capable(CAP_SYS_ADMIN))
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return -EPERM;
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/*
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* Writing directly to ns' last_pid field is OK, since this field
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* is volatile in a living namespace anyway and a code writing to
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* it should synchronize its usage with external means.
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*/
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tmp.data = ¤t->nsproxy->pid_ns->last_pid;
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return proc_dointvec(&tmp, write, buffer, lenp, ppos);
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}
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static struct ctl_table pid_ns_ctl_table[] = {
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{
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.procname = "ns_last_pid",
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.maxlen = sizeof(int),
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.mode = 0666, /* permissions are checked in the handler */
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.proc_handler = pid_ns_ctl_handler,
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},
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{ }
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};
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static struct ctl_path kern_path[] = { { .procname = "kernel", }, { } };
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static __init int pid_namespaces_init(void)
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{
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pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC);
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register_sysctl_paths(kern_path, pid_ns_ctl_table);
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return 0;
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}
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