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Commit b8f566b0 authored by Pavel Emelyanov's avatar Pavel Emelyanov Committed by Linus Torvalds
Browse files

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: default avatarPavel Emelyanov <xemul@parallels.com>
Acked-by: default avatarTejun 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f5138e42

Documentation/sysctl/kernel.txt

+8 −0
Original line number Diff line number Diff line
@@ -415,6 +415,14 @@ PIDs of value pid_max or larger are not allocated. 


==============================================================



ns_last_pid:



The last pid allocated in the current (the one task using this sysctl

lives in) pid namespace. When selecting a pid for a next task on fork

kernel tries to allocate a number starting from this one.



==============================================================



powersave-nap: (PPC only)



If set, Linux-PPC will use the 'nap' mode of powersaving,

kernel/pid.c

+3 −1
Original line number Diff line number Diff line
@@ -137,7 +137,9 @@ static int pid_before(int base, int a, int b) 
}



/*

 * We might be racing with someone else trying to set pid_ns->last_pid.

 * We might be racing with someone else trying to set pid_ns->last_pid

 * at the pid allocation time (there's also a sysctl for this, but racing

 * with this one is OK, see comment in kernel/pid_namespace.c about it).

 * We want the winner to have the "later" value, because if the

 * "earlier" value prevails, then a pid may get reused immediately.

 *

kernel/pid_namespace.c

+31 −0
Original line number Diff line number Diff line
@@ -191,9 +191,40 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns) 
	return;

}



static int pid_ns_ctl_handler(struct ctl_table *table, int write,

		void __user *buffer, size_t *lenp, loff_t *ppos)

{

	struct ctl_table tmp = *table;



	if (write && !capable(CAP_SYS_ADMIN))

		return -EPERM;



	/*

	 * Writing directly to ns' last_pid field is OK, since this field

	 * is volatile in a living namespace anyway and a code writing to

	 * it should synchronize its usage with external means.

	 */



	tmp.data = &current->nsproxy->pid_ns->last_pid;

	return proc_dointvec(&tmp, write, buffer, lenp, ppos);

}



static struct ctl_table pid_ns_ctl_table[] = {

	{

		.procname = "ns_last_pid",

		.maxlen = sizeof(int),

		.mode = 0666, /* permissions are checked in the handler */

		.proc_handler = pid_ns_ctl_handler,

	},

	{ }

};



static struct ctl_path kern_path[] = { { .procname = "kernel", }, { } };



static __init int pid_namespaces_init(void)

{

	pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC);

	register_sysctl_paths(kern_path, pid_ns_ctl_table);

	return 0;

}