[PATCH RESEND 0/2] Expose task pid_ns_for_children to userspace

[PATCH RESEND 0/2] Expose task pid_ns_for_children to userspace

[Kirill Tkhai]

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. If they have a custom pid_ns_for_children
before dump, they must have the same ns after restore. Otherwise,
restored task bumped into enviroment it does not expect.

This patchset solves the problem. 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]

---

Kirill Tkhai (2):
      ns: Allow ns_entries to have custom symlink content
      pidns: Expose task pid_ns_for_children to userspace


 fs/nsfs.c               |    4 +++-
 fs/proc/namespaces.c    |    1 +
 include/linux/proc_ns.h |    2 ++
 kernel/pid_namespace.c  |   25 +++++++++++++++++++++++++
 4 files changed, 31 insertions(+), 1 deletion(-)

--
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>

[PATCH RESEND 0/2] Expose task pid_ns_for_children to userspace

[Kirill Tkhai]

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. If they have a custom pid_ns_for_children
before dump, they must have the same ns after restore. Otherwise,
restored task bumped into enviroment it does not expect.

This patchset solves the problem. 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]

---

Kirill Tkhai (2):
      ns: Allow ns_entries to have custom symlink content
      pidns: Expose task pid_ns_for_children to userspace


 fs/nsfs.c               |    4 +++-
 fs/proc/namespaces.c    |    1 +
 include/linux/proc_ns.h |    2 ++
 kernel/pid_namespace.c  |   25 +++++++++++++++++++++++++
 4 files changed, 31 insertions(+), 1 deletion(-)

--
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>

[PATCH RESEND 1/2] ns: Allow ns_entries to have custom symlink content

[Kirill Tkhai]

Make possible to have link content prefix yyy
different from the link name xxx:

$ readlink /proc/[pid]/ns/xxx
yyy:[4026531838]

This will be used in next patch.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
---
 fs/nsfs.c               |    4 +++-
 include/linux/proc_ns.h |    1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 1656843e87d2..495f12b83a7b 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -195,9 +195,11 @@ int ns_get_name(char *buf, size_t size, struct task_struct *task,
 {
 	struct ns_common *ns;
 	int res = -ENOENT;
+	const char *name;
 	ns = ns_ops->get(task);
 	if (ns) {
-		res = snprintf(buf, size, "%s:[%u]", ns_ops->name, ns->inum);
+		name = ns_ops->real_ns_name ? : ns_ops->name;
+		res = snprintf(buf, size, "%s:[%u]", name, ns->inum);
 		ns_ops->put(ns);
 	}
 	return res;
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index 12cb8bd81d2d..88dba3b53375 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -14,6 +14,7 @@ struct inode;
 
 struct proc_ns_operations {
 	const char *name;
+	const char *real_ns_name;
 	int type;
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);

[PATCH RESEND 1/2] ns: Allow ns_entries to have custom symlink content

[Kirill Tkhai]

Make possible to have link content prefix yyy
different from the link name xxx:

$ readlink /proc/[pid]/ns/xxx
yyy:[4026531838]

This will be used in next patch.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
---
 fs/nsfs.c               |    4 +++-
 include/linux/proc_ns.h |    1 +
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 1656843e87d2..495f12b83a7b 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -195,9 +195,11 @@ int ns_get_name(char *buf, size_t size, struct task_struct *task,
 {
 	struct ns_common *ns;
 	int res = -ENOENT;
+	const char *name;
 	ns = ns_ops->get(task);
 	if (ns) {
-		res = snprintf(buf, size, "%s:[%u]", ns_ops->name, ns->inum);
+		name = ns_ops->real_ns_name ? : ns_ops->name;
+		res = snprintf(buf, size, "%s:[%u]", name, ns->inum);
 		ns_ops->put(ns);
 	}
 	return res;
diff --git a/include/linux/proc_ns.h b/include/linux/proc_ns.h
index 12cb8bd81d2d..88dba3b53375 100644
--- a/include/linux/proc_ns.h
+++ b/include/linux/proc_ns.h
@@ -14,6 +14,7 @@ struct inode;
 
 struct proc_ns_operations {
 	const char *name;
+	const char *real_ns_name;
 	int type;
 	struct ns_common *(*get)(struct task_struct *task);
 	void (*put)(struct ns_common *ns);

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

[Kirill Tkhai]

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]

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrei Vagin <avagin@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 de461aa0bf9a..4dd02ff0b0bd 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -374,6 +374,20 @@ 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));
@@ -443,6 +457,17 @@ 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);

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

[Kirill Tkhai]

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]

Signed-off-by: Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
Reviewed-by: Cyrill Gorcunov <gorcunov-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>
Acked-by: Andrei Vagin <avagin-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
---
 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 de461aa0bf9a..4dd02ff0b0bd 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -374,6 +374,20 @@ 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));
@@ -443,6 +457,17 @@ 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);

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

[Andrew Morton]

On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:

> 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]
> 
> --- 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

This interface should be documented somewhere under Documentation/. 
But I can't immediately find where the /proc/pid/ns/ pseudo-files are
documented...

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

[Andrew Morton]

On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:

> 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]
> 
> --- 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

This interface should be documented somewhere under Documentation/. 
But I can't immediately find where the /proc/pid/ns/ pseudo-files are
documented...

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

[Andrei Vagin]

On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> 
> > 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]
> > 
> > --- 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
> 
> This interface should be documented somewhere under Documentation/. 
> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
> documented...

I know that they are documented in man7/namespaces.7

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187

> 
> 

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

[Andrei Vagin]

On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> 
> > 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]
> > 
> > --- 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
> 
> This interface should be documented somewhere under Documentation/. 
> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
> documented...

I know that they are documented in man7/namespaces.7

https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187

> 
> 

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

[Kirill Tkhai]

On 31.03.2017 04:04, Andrei Vagin wrote:
> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>
>>> 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]
>>>
>>> --- 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
>>
>> This interface should be documented somewhere under Documentation/. 
>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>> documented...
> 
> I know that they are documented in man7/namespaces.7
> 
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187

I suggest the below patch, but it's too early for the man description till
the feature is in mainline, because the man page requires commit id of the feature.

[PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 man7/namespaces.7 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/man7/namespaces.7 b/man7/namespaces.7
index 6dfceaa2a..06041774f 100644
--- a/man7/namespaces.7
+++ b/man7/namespaces.7
@@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
+lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
 .fi
@@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
 .TP
 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
-This file is a handle for the PID namespace of the process.
+This file is a handle for the PID namespace of the process. It's
+permanent during the whole process life.
+.TP
+.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
+.\" commit FIXME
+This file is a handle for the PID namespace of a next born child
+of the process. It's changed after unshare(2) and via setns(2),
+so the file may differ from /proc/[pid]/ns/pid.
 .TP
 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 .\" commit cde1975bc242f3e1072bde623ef378e547b73f91

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

[Kirill Tkhai]

On 31.03.2017 04:04, Andrei Vagin wrote:
> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:
>>
>>> 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]
>>>
>>> --- 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
>>
>> This interface should be documented somewhere under Documentation/. 
>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>> documented...
> 
> I know that they are documented in man7/namespaces.7
> 
> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187

I suggest the below patch, but it's too early for the man description till
the feature is in mainline, because the man page requires commit id of the feature.

[PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file

Signed-off-by: Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
---
 man7/namespaces.7 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/man7/namespaces.7 b/man7/namespaces.7
index 6dfceaa2a..06041774f 100644
--- a/man7/namespaces.7
+++ b/man7/namespaces.7
@@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
+lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
 .fi
@@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
 .TP
 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
-This file is a handle for the PID namespace of the process.
+This file is a handle for the PID namespace of the process. It's
+permanent during the whole process life.
+.TP
+.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
+.\" commit FIXME
+This file is a handle for the PID namespace of a next born child
+of the process. It's changed after unshare(2) and via setns(2),
+so the file may differ from /proc/[pid]/ns/pid.
 .TP
 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 .\" commit cde1975bc242f3e1072bde623ef378e547b73f91

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

[Kees Cook]

On Fri, Mar 31, 2017 at 2:45 AM, Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
> On 31.03.2017 04:04, Andrei Vagin wrote:
>> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>>
>>>> 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]
>>>>
>>>> --- 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
>>>
>>> This interface should be documented somewhere under Documentation/.
>>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>>> documented...
>>
>> I know that they are documented in man7/namespaces.7
>>
>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187
>
> I suggest the below patch, but it's too early for the man description till
> the feature is in mainline, because the man page requires commit id of the feature.
>
> [PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file
>
> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> ---
>  man7/namespaces.7 | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> diff --git a/man7/namespaces.7 b/man7/namespaces.7
> index 6dfceaa2a..06041774f 100644
> --- a/man7/namespaces.7
> +++ b/man7/namespaces.7
> @@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
> +lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]

Minor nit: this needs to be "\-" for the "-"

>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
>  .fi
> @@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
>  .TP
>  .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
>  .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
> -This file is a handle for the PID namespace of the process.
> +This file is a handle for the PID namespace of the process. It's
> +permanent during the whole process life.
> +.TP
> +.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
> +.\" commit FIXME
> +This file is a handle for the PID namespace of a next born child
> +of the process. It's changed after unshare(2) and via setns(2),
> +so the file may differ from /proc/[pid]/ns/pid.
>  .TP
>  .IR /proc/[pid]/ns/user " (since Linux 3.8)"
>  .\" commit cde1975bc242f3e1072bde623ef378e547b73f91
>

-Kees

-- 
Kees Cook
Pixel Security

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

[Kees Cook]

On Fri, Mar 31, 2017 at 2:45 AM, Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:
> On 31.03.2017 04:04, Andrei Vagin wrote:
>> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:
>>>
>>>> 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]
>>>>
>>>> --- 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
>>>
>>> This interface should be documented somewhere under Documentation/.
>>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>>> documented...
>>
>> I know that they are documented in man7/namespaces.7
>>
>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187
>
> I suggest the below patch, but it's too early for the man description till
> the feature is in mainline, because the man page requires commit id of the feature.
>
> [PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file
>
> Signed-off-by: Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
> ---
>  man7/namespaces.7 | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> diff --git a/man7/namespaces.7 b/man7/namespaces.7
> index 6dfceaa2a..06041774f 100644
> --- a/man7/namespaces.7
> +++ b/man7/namespaces.7
> @@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
> +lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]

Minor nit: this needs to be "\-" for the "-"

>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
>  .fi
> @@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
>  .TP
>  .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
>  .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
> -This file is a handle for the PID namespace of the process.
> +This file is a handle for the PID namespace of the process. It's
> +permanent during the whole process life.
> +.TP
> +.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
> +.\" commit FIXME
> +This file is a handle for the PID namespace of a next born child
> +of the process. It's changed after unshare(2) and via setns(2),
> +so the file may differ from /proc/[pid]/ns/pid.
>  .TP
>  .IR /proc/[pid]/ns/user " (since Linux 3.8)"
>  .\" commit cde1975bc242f3e1072bde623ef378e547b73f91
>

-Kees

-- 
Kees Cook
Pixel Security

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

[Kirill Tkhai]

On 31.03.2017 18:06, Kees Cook wrote:
> On Fri, Mar 31, 2017 at 2:45 AM, Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>> On 31.03.2017 04:04, Andrei Vagin wrote:
>>> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>>>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>>>>
>>>>> 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]
>>>>>
>>>>> --- 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
>>>>
>>>> This interface should be documented somewhere under Documentation/.
>>>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>>>> documented...
>>>
>>> I know that they are documented in man7/namespaces.7
>>>
>>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187
>>
>> I suggest the below patch, but it's too early for the man description till
>> the feature is in mainline, because the man page requires commit id of the feature.
>>
>> [PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>>  man7/namespaces.7 | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>> diff --git a/man7/namespaces.7 b/man7/namespaces.7
>> index 6dfceaa2a..06041774f 100644
>> --- a/man7/namespaces.7
>> +++ b/man7/namespaces.7
>> @@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
>> +lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]
> 
> Minor nit: this needs to be "\-" for the "-"

Thanks, Kees. The updated version is below:

[PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
 man7/namespaces.7 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/man7/namespaces.7 b/man7/namespaces.7
index 6dfceaa2a..b4e9b13f0 100644
--- a/man7/namespaces.7
+++ b/man7/namespaces.7
@@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
+lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children \-> pid:[4026531834]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
 .fi
@@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
 .TP
 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
-This file is a handle for the PID namespace of the process.
+This file is a handle for the PID namespace of the process. It's
+permanent during the whole process life.
+.TP
+.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
+.\" commit FIXME
+This file is a handle for the PID namespace of a next born child
+of the process. It's changed after unshare(2) and via setns(2),
+so the file may differ from /proc/[pid]/ns/pid.
 .TP
 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 .\" commit cde1975bc242f3e1072bde623ef378e547b73f91

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

[Kirill Tkhai]

On 31.03.2017 18:06, Kees Cook wrote:
> On Fri, Mar 31, 2017 at 2:45 AM, Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:
>> On 31.03.2017 04:04, Andrei Vagin wrote:
>>> On Thu, Mar 30, 2017 at 03:05:20PM -0700, Andrew Morton wrote:
>>>> On Thu, 30 Mar 2017 13:27:59 +0300 Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> wrote:
>>>>
>>>>> 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]
>>>>>
>>>>> --- 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
>>>>
>>>> This interface should be documented somewhere under Documentation/.
>>>> But I can't immediately find where the /proc/pid/ns/ pseudo-files are
>>>> documented...
>>>
>>> I know that they are documented in man7/namespaces.7
>>>
>>> https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/man7/namespaces.7#n187
>>
>> I suggest the below patch, but it's too early for the man description till
>> the feature is in mainline, because the man page requires commit id of the feature.
>>
>> [PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file
>>
>> Signed-off-by: Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
>> ---
>>  man7/namespaces.7 | 10 +++++++++-
>>  1 file changed, 9 insertions(+), 1 deletion(-)
>> diff --git a/man7/namespaces.7 b/man7/namespaces.7
>> index 6dfceaa2a..06041774f 100644
>> --- a/man7/namespaces.7
>> +++ b/man7/namespaces.7
>> @@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
>>  lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
>> +lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children -> pid:[4026531834]
> 
> Minor nit: this needs to be "\-" for the "-"

Thanks, Kees. The updated version is below:

[PATCH] namespaces.7: Document the /proc/[pid]/ns/pid_for_children file

Signed-off-by: Kirill Tkhai <ktkhai-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
---
 man7/namespaces.7 | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/man7/namespaces.7 b/man7/namespaces.7
index 6dfceaa2a..b4e9b13f0 100644
--- a/man7/namespaces.7
+++ b/man7/namespaces.7
@@ -125,6 +125,7 @@ lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 ipc \-> ipc:[4026531839]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 mnt \-> mnt:[4026531840]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 net \-> net:[4026531969]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid \-> pid:[4026531836]
+lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 pid_for_children \-> pid:[4026531834]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 user \-> user:[4026531837]
 lrwxrwxrwx. 1 mtk mtk 0 Apr 28 12:46 uts \-> uts:[4026531838]
 .fi
@@ -186,7 +187,14 @@ This file is a handle for the network namespace of the process.
 .TP
 .IR /proc/[pid]/ns/pid " (since Linux 3.8)"
 .\" commit 57e8391d327609cbf12d843259c968b9e5c1838f
-This file is a handle for the PID namespace of the process.
+This file is a handle for the PID namespace of the process. It's
+permanent during the whole process life.
+.TP
+.IR /proc/[pid]/ns/pid_for_children " (since Linux 4.12)"
+.\" commit FIXME
+This file is a handle for the PID namespace of a next born child
+of the process. It's changed after unshare(2) and via setns(2),
+so the file may differ from /proc/[pid]/ns/pid.
 .TP
 .IR /proc/[pid]/ns/user " (since Linux 3.8)"
 .\" commit cde1975bc242f3e1072bde623ef378e547b73f91