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sysctl: add the kernel.ns_last_pid control

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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>
This commit is contained in:
Pavel Emelyanov 2012-01-12 17:20:27 -08:00 committed by Linus Torvalds
parent f5138e4221
commit b8f566b04d
3 changed files with 42 additions and 1 deletions
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8
Documentation/sysctl/kernel.txt
View File

@ -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,

4
kernel/pid.c
View File

@ -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.
*

31
kernel/pid_namespace.c
View File

@ -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;
}