Re: [PATCH 1/1] /proc/$PID/status : show list NSpid data based on current process namespace.

[Kuenhwan Kwak <kh243.kwak@samsung.com>]

On 05/30/2015 07:21 AM, Andrew Morton wrote:
> 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?

According to proc_mount() function, 'ns' value is came from current process also.
and procfs is accessed by same pid namespace processes usually. So
there is no impact of using current value.

This change is only effective if parent ns process access to child procfs.
In that case, print out more NSpid info for parent ns process.

Kuenhwan Kwak.