[PATCH v3 0/2] Expose task pid_ns_for_children to userspace

From: Kirill Tkhai
Date: Wed May 03 2017 - 06:32:33 EST

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]

v3: Check child_reaper without tasklist_lock as we are only interested
in the fact it's not zero, and not in a specific value.

v2: Do not allow to take a pid namespace, if there is no child reaper
created. This prevents race between creation of the child reaper and
other tasks.


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 | 28 ++++++++++++++++++++++++++++
4 files changed, 34 insertions(+), 1 deletion(-)

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>