[Devel] Re: Mapping PIDs from parent->child namespaces

[Cedric Le Goater]

On 01/04/2011 05:04 PM, Daniel Lezcano wrote:
> On 01/04/2011 12:02 AM, Mike Heffner wrote:
>> Hi,
>>
>> Is it possible for a process running in a parent PID namespace to map
>> the PID of a process running in a child's namespace from the
>> parent->child's namespace? For example, if I span the process "myproc"
>> with CLONE_NEWPID then a call to getpid() inside myproc will return "1"
>> whereas in the parent's namespace that process could actually be PID
>> "23495". I'd like to be able to know that 23495 maps to 1 in the new NS.
>> Obviously, just mapping the first PID is straightforward since I can
>> just look at the result of clone(). However, mapping the PIDs of
>> processes subsequently forked from "myproc" -- in this example -- I
>> haven't been able to figure out.
> 
> AFAIK, it is not possible.
> 
> That would be very nice to show the pid <-> vpid association.
> 
> The procfs is a good candidate to show these informations.
> 
> That would makes sense to show the content of /proc/<pid>/status with 
> the pid relatively to the namespace.
> 
> Let me give an example:
> 
> Assuming the process '1234' creates a new pid namespace, and the child 
> which is '1' in the new namespace has the real pid '4321'. This one 
> mounts its /proc.
> 
> If the process '1234' looks at /proc/4321/root/proc/1/status, it sees:
> 
> ...
> Tgid:	1
> Pid:	1
> PPid:	0
> ...
> 
> 
> It could be:
> 
> ...
> Tgid:	4321
> Pid:	4321
> PPid:	1234
> ...
> 
> as the file is inspected from the parent namespace. Of course, if the 
> file is looked from the child namespace context, we will see '1', '1' 
> and '0'.
> 
> I suppose the patch in the kernel should very small also.
> 
> Thoughts ?

we use the following patch to get the pid of a task as seen from its
pid namespace. It can be useful to identify tasks writing pids in files. 

Cheers,

C.

diff --git a/fs/proc/array.c b/fs/proc/array.c
index fff6572..9a7bfde 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -337,6 +337,12 @@ static void task_cpus_allowed(struct seq_file *m, struct task
        seq_printf(m, "\n");
 }
 
+static void task_vpid(struct seq_file *m, struct task_struct *task)
+{
+       struct pid_namespace *ns = task_active_pid_ns(task);
+       seq_printf(m, "Vpid:\t%d\n", ns ? task_pid_nr_ns(task, ns) : 0);
+}
+
 int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
                        struct pid *pid, struct task_struct *task)
 {
@@ -357,6 +363,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *
        task_show_regs(m, task);
 #endif
        task_context_switch_counts(m, task);
+       task_vpid(m, task);
        return 0;
 }
 
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