Re: [PATCH v4] pidns: introduce syscall translate_pid

[Andy Lutomirski]

On Mon, Oct 16, 2017 at 3:54 PM, prakash.sangappa
<prakash.sangappa@xxxxxxxxxx> wrote:
>
>
> On 10/16/2017 03:07 PM, Nagarathnam Muthusamy wrote:
>>
>>
>>
>> On 10/16/2017 02:36 PM, Andrew Morton wrote:
>>>
>>> On Sat, 14 Oct 2017 11:17:47 +0300 Konstantin Khlebnikov
>>> <khlebnikov@xxxxxxxxxxxxxx> wrote:
>>>
>>>>>>> pid_t translate_pid(pid_t pid, int source, int target);
>>>>>>>
>>>>>>> This syscall converts pid from source pid-ns into pid in target
>>>>>>> pid-ns.
>>>>>>> If pid is unreachable from target pid-ns it returns zero.
>>>>>>>
>>>>>>> Pid-namespaces are referred file descriptors opened to proc files
>>>>>>> /proc/[pid]/ns/pid or /proc/[pid]/ns/pid_for_children. Negative
>>>>>>> argument
>>>>>>> refers to current pid namespace, same as file /proc/self/ns/pid.
>>>>>>>
>>>>>>> Kernel expose virtual pids in /proc/[pid]/status:NSpid, but backward
>>>>>>> translation requires scanning all tasks. Also pids could be
>>>>>>> translated
>>>>>>> by sending them through unix socket between namespaces, this method
>>>>>>> is
>>>>>>> slow and insecure because other side is exposed inside pid namespace.
>>>>
>>>> Andrew asked why we might need this.
>>>>
>>>> Such conversion is required for interaction between processes across
>>>> pid-namespaces.
>>>> For example to identify process in container by pid file looking from
>>>> outside.
>>>>
>>>> Two years ago I've solved this in project of mine with monstrous code
>>>> which
>>>> forks couple times just to convert pid, lucky for me performance wasn't
>>>> important.
>>>
>>> That's a single user who needed this a single time, and found a
>>> userspace-based solution anyway.  This is not exactly compelling!
>>>
>>> Is there a stronger case to be made?  How does this change benefit our
>>> users?  Sell it to us!
>>
>> Oracle database is planning to use pid namespace for sandboxing database
>> instances and they need an API similar to translate_pid to effectively
>> translate process IDs from other pid namespaces. Prakash (cced in mail) can
>> provide more details on this usecase.
>
>
> As Nagarathnam indicated, Oracle Database will be using pid namespaces and
> needs a direct method of converting pids of processes in the pid namespace
> hierarchy. In this use case multiple
> nested PID namespaces will be used.  The currently available mechanism are
> not very efficient for this use case. For ex. as Konstantin described, using
> /proc/<pid>/status would require the application to scan all the pid's
> status files to determine the pid of given process in a child namespace.
>
> Use of SCM_CREDENTIALS's socket message is another way, which would require
> every process starting inside a pid namespace to send this message and the
> receiving process in the target namespace would have to save the converted
> pid and reference it. This mechanism becomes cumbersome especially if the
> application has to deal with multiple nested pid namespaces. Also, the
> Database needs to be able to convert a thread's global pid(gettid()).
> Passing the thread's pid(gettid()) in SCM_CREDENTIALS message requires
> CAP_SYS_ADMIN, which is an issue.
>
> So having a direct method, like the API that Konstantin is proposing, will
> work best for the Database
> since pid of a process in any of the nested pid namespaces can be converted
> as and when required. I think with the proposed API, the application should
> be able to convert pid of a process or tid(gettid()) of a thread as well.
>


Can you explain what Oracle's database is planning to do with this information?