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Subject[PATCH v2 2/2] pidns: Expose task pid_ns_for_children to userspace
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pid_ns_for_children set by a task is known only to the task itself,
and it's impossible to identify it from outside.

It's a big problem for checkpoint/restore software like CRIU,
because it can't correctly handle tasks, that do setns(CLONE_NEWPID)
in proccess of their work.

This patch solves the problem, and it 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]

v2: Do not allow to get namespace if there is no child reaper created,
as other tasks need initializations it did (e.g., pid_namespace::proc_mnt),
and we don't want they race.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
fs/proc/namespaces.c | 1 +
include/linux/proc_ns.h | 1 +
kernel/pid_namespace.c | 34 ++++++++++++++++++++++++++++++++++
3 files changed, 36 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 de461aa0bf9a..d1f3e9f558b8 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -374,6 +374,29 @@ 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);
+
+ if (ns) {
+ read_lock(&tasklist_lock);
+ if (!ns->child_reaper) {
+ put_pid_ns(ns);
+ ns = NULL;
+ }
+ read_unlock(&tasklist_lock);
+ }
+
+ return ns ? &ns->ns : NULL;
+}
+
static void pidns_put(struct ns_common *ns)
{
put_pid_ns(to_pid_ns(ns));
@@ -443,6 +466,17 @@ 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);
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