Re: [PATCH 2/2] pidns: Expose task pid_ns_for_children to userspace

Andrei Vagin
2017-01-17T20:00:30Z

On Sat, Jan 14, 2017 at 05:15:04PM +0300, Kirill Tkhai wrote:
> For correct checkpointing/restoring of a task from userspace
> it's need to know the task's pid_ns_for_children. Currently,
> there is no a sane way to do that (the only possible trick
> is to force the task create a new child and to analize the
> child's /proc/[pid]/ns/pid link, that is performance-stupid).
>
> The patch exposes pid_ns_for_children to ns directory
> in standard way with the name "pid_for_children":
>
> ~# ls /proc/5531/ns -l | grep pid
> lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid -> pid:[4026531836]
> lrwxrwxrwx 1 root root 0 Jan 14 16:38 pid_for_children -> pid:[4026532286]
>

Cc: linux-api, Michael Kerrisk

Acked-by: Andrei Vagin <avagin@virtuozzo.com>

> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
> fs/proc/namespaces.c | 1 +
> include/linux/proc_ns.h | 1 +
> kernel/pid_namespace.c | 25 +++++++++++++++++++++++++
> 3 files changed, 27 insertions(+)
>
> diff --git a/fs/proc/namespaces.c b/fs/proc/namespaces.c
> index 766f0c637ad1..3803b24ca220 100644
> --- a/fs/proc/namespaces.c
> +++ b/fs/proc/namespaces.c
> @@ -23,6 +23,7 @@ static const struct proc_ns_operations *ns_entries[] = {
> #endif
> #ifdef CONFIG_PID_NS
> &pidns_operations,
> + &pidns_for_children_operations,
> #endif
> #ifdef CONFIG_USER_NS
> &userns_operations,
> diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
> index 88dba3b53375..58ab28d81fc2 100644
> --- a/include/linux/proc_ns.h
> +++ b/include/linux/proc_ns.h
> @@ -27,6 +27,7 @@ extern const struct proc_ns_operations netns_operations;
> extern const struct proc_ns_operations utsns_operations;
> extern const struct proc_ns_operations ipcns_operations;
> extern const struct proc_ns_operations pidns_operations;
> +extern const struct proc_ns_operations pidns_for_children_operations;
> extern const struct proc_ns_operations userns_operations;
> extern const struct proc_ns_operations mntns_operations;
> extern const struct proc_ns_operations cgroupns_operations;
> diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
> index df9e8e9e0be7..cbe950d4a11e 100644
> --- a/kernel/pid_namespace.c
> +++ b/kernel/pid_namespace.c
> @@ -369,6 +369,20 @@ static struct ns_common *pidns_get(struct task_struct *task)
> return ns ? &ns->ns : NULL;
> }
>
> +static struct ns_common *pidns_for_children_get(struct task_struct *task)
> +{
> + struct pid_namespace *ns = NULL;
> +
> + task_lock(task);
> + if (task->nsproxy) {
> + ns = task->nsproxy->pid_ns_for_children;
> + get_pid_ns(ns);
> + }
> + task_unlock(task);
> +
> + return ns ? &ns->ns : NULL;
> +}
> +
> static void pidns_put(struct ns_common *ns)
> {
> put_pid_ns(to_pid_ns(ns));
> @@ -438,6 +452,17 @@ const struct proc_ns_operations pidns_operations = {
> .get_parent = pidns_get_parent,
> };
>
> +const struct proc_ns_operations pidns_for_children_operations = {
> + .name = "pid_for_children",
> + .real_ns_name = "pid",
> + .type = CLONE_NEWPID,
> + .get = pidns_for_children_get,
> + .put = pidns_put,
> + .install = pidns_install,
> + .owner = pidns_owner,
> + .get_parent = pidns_get_parent,
> +};
> +
> static __init int pid_namespaces_init(void)
> {
> pid_ns_cachep = KMEM_CACHE(pid_namespace, SLAB_PANIC);
>