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Date: Fri, 29 May 2015 15:21:43 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Kuenhwan Kwak <kh243.kwak@samsung.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>,
        Chen Hanxiao <chenhanxiao@cn.fujitsu.com>, cpgs@samsung.com,
        linux-kernel@vger.kernel.org,
        "Eric W. Biederman" <ebiederm@xmission.com>
Subject: Re: [PATCH 1/1] /proc/$PID/status : show list NSpid data based on
 current process namespace.
Message-Id: <20150529152143.5d3dce7fc4986744e4f64ac8@linux-foundation.org>
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On Fri, 29 May 2015 11:57:21 +0900 Kuenhwan Kwak <kh243.kwak@samsung.com> wrote:

> This patch helps creating a pid mapping data to parent processes.
> 
> Reading 'NSpid' field in '/proc/$PID/status' is currently a simple
> way to getting child pid from parent pid in userspace. But this field
> supplies only single direction mapping('parent pid' to 'child pid').
> If parent process want to translate child pid to current namespace pid,
> there is no way to get except full searching in current procfs.
> 
> This patch will helps in getting current namespace pid by reading child
> procfs file without any side effects.
> 
> For example, Process id is 24771 in level 0, 435 in level 1.
> a) The output of '/proc/24771/status' in level 0 namespace.
> NSpid : 24771 435
> 
> b) The output of '/proc/435/status' in level 1 namespace.
> NSpid : 435
> 
> c) Process in level 0 mount level1 proc to '/var/child/proc'
> after setns(). The output of '/var/child/proc/435/status' is
> NSpid : 24771 435
> 
> ...
>
> --- a/fs/proc/array.c
> +++ b/fs/proc/array.c
> @@ -83,6 +83,7 @@
>  #include <linux/tracehook.h>
>  #include <linux/string_helpers.h>
>  #include <linux/user_namespace.h>
> +#include <linux/sched.h>
>  
>  #include <asm/pgtable.h>
>  #include <asm/processor.h>
> @@ -149,6 +150,9 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>  	const struct cred *cred;
>  	pid_t ppid, tpid = 0, tgid, ngid;
>  	unsigned int max_fds = 0;
> +#ifdef CONFIG_PID_NS
> +	struct pid_namespace *current_pid_ns = task_active_pid_ns(current);
> +#endif
>  
>  	rcu_read_lock();
>  	ppid = pid_alive(p) ?
> @@ -198,19 +202,19 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
>  
>  #ifdef CONFIG_PID_NS
>  	seq_puts(m, "\nNStgid:");
> -	for (g = ns->level; g <= pid->level; g++)
> +	for (g = current_pid_ns->level; g <= pid->level; g++)
>  		seq_printf(m, "\t%d",
>  			task_tgid_nr_ns(p, pid->numbers[g].ns));
>  	seq_puts(m, "\nNSpid:");
> -	for (g = ns->level; g <= pid->level; g++)
> +	for (g = current_pid_ns->level; g <= pid->level; g++)
>  		seq_printf(m, "\t%d",
>  			task_pid_nr_ns(p, pid->numbers[g].ns));
>  	seq_puts(m, "\nNSpgid:");
> -	for (g = ns->level; g <= pid->level; g++)
> +	for (g = current_pid_ns->level; g <= pid->level; g++)
>  		seq_printf(m, "\t%d",
>  			task_pgrp_nr_ns(p, pid->numbers[g].ns));
>  	seq_puts(m, "\nNSsid:");
> -	for (g = ns->level; g <= pid->level; g++)
> +	for (g = current_pid_ns->level; g <= pid->level; g++)
>  		seq_printf(m, "\t%d",
>  			task_session_nr_ns(p, pid->numbers[g].ns));

These changes alter current behaviour, don't they?  How do we know this
won't impact existing userspace code?