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Subject[PATCH] 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>

---
Documentation/sysctl/kernel.txt | 8 ++++++++
kernel/pid.c | 4 +++-
kernel/pid_namespace.c | 31 +++++++++++++++++++++++++++++++
3 files changed, 42 insertions(+), 1 deletions(-)
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 1f24636..1e9cd67 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -401,6 +401,14 @@ 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,
diff --git a/kernel/pid.c b/kernel/pid.c
index fa5f722..ce8e00d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -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.
*
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index e9c9adc..bcd3f16 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -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;
}

--
1.5.5.6

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