linux-kernel - Re: [PATCH 2/2] pidns: Expose task pid_ns_for

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Date:   Mon, 23 Jan 2017 22:49:14 +0100
From:   Alban Crequy <alban.crequy@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     Andrey Vagin <avagin@...nvz.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH 2/2] pidns: Expose task pid_ns_for_children to userspace

On 14 January 2017 at 15:15, Kirill Tkhai <ktkhai@...tuozzo.com> 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]
>
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>

What's happening if a process, after unsharing CLONE_NEWPID, does not
fork but instead let another process open the new "pid_for_children"
and then setns()+fork()? Is that other process allowed to create the
"pid 1" in the new pid namespaces? Is that also allowed if the other
process lives in a sibling pid namespace? If so, that would break what
pid_namespaces(7) says:

    "the parental relationship between processes mirrors the parental
     relationship between PID namespaces: the parent of a process is
     either in the same namespace or resides in the immediate parent
     PID namespace."