Re: [PATCH RFC v5] pidns: introduce syscall translate_pid

[Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com>]

On 04/05/2018 12:02 AM, Konstantin Khlebnikov wrote:
> On 05.04.2018 01:29, Eric W. Biederman wrote:
>> Nagarathnam Muthusamy <nagarathnam.muthusamy@oracle.com> writes:
>>
>>> On 04/04/2018 12:11 PM, Konstantin Khlebnikov wrote:
>>>> Each process have different pids, one for each pid namespace it 
>>>> belongs.
>>>> When interaction happens within single pid-ns translation isn't 
>>>> required.
>>>> More complicated scenarios needs special handling.
>>>>
>>>> For example:
>>>> - reading pid-files or logs written inside container with pid 
>>>> namespace
>>>> - attaching with ptrace to tasks from different pid namespace
>>>> - passing pids across pid namespaces in any kind of API
>>>>
>>>> Currently there are several interfaces that could be used here:
>>>>
>>>> Pid namespaces are identified by inode number of /proc/[pid]/ns/pid.
>>
>> Using the inode number in interfaces is not an option. Especially not
>> withou referencing the device number for the filesystem as well.
>
> This is supposed to be single-instance fs,
> not part of proc but referenced but its magic "symlinks".
>
> Device numbers are not mentioned in "man namespaces".
>
>>
>>>> Pids for nested Pid namespaces are shown in file /proc/[pid]/status.
>>>> In some cases conversion pid -> vpid could be easily done using this
>>>> information, but backward translation requires scanning all tasks.
>>>>
>>>> Unix socket automatically translates pid attached to SCM_CREDENTIALS.
>>>> This requires CAP_SYS_ADMIN for sending arbitrary pids and entering
>>>> into pid namespace, this expose process and could be insecure.
>>>>
>>>> This patch adds new syscall for converting pids between pid 
>>>> namespaces:
>>>>
>>>> pid_t translate_pid(pid_t pid, int source_type, int source,
>>>>                                  int target_type, int target);
>>>>
>>>> @source_type and @target_type defines type of following arguments:
>>>>
>>>> TRANSLATE_PID_CURRENT_PIDNS  - current pid namespace, argument is 
>>>> unused
>>>> TRANSLATE_PID_TASK_PIDNS     - task pid-ns, argument is task pid
>>>
>>> I believe using pid to represent the namespace has been already
>>> discussed in V1 of this patch in https://lkml.org/lkml/2015/9/22/1087
>>> after which we moved on to fd based version of this interface.
>>
>> Or in short why is the case of pids important?
>>
>> You Konstantin you almost said why they were important in your message
>> saying you were going to send this one.  However you don't explain in
>> your description why you want to identify pid namespaces by pid.
>>
>
> Open of /proc/[pid]/ns/pid requires same permissions as ptrace,
> pid based variant doesn't have such restrictions.

Can you provide more information on usecase requiring PID translation 
but not used for tracing related purposes?
On a side note, can we have the types TRANSLATE_PID_CURRENT_PIDNS and 
TRANSLATE_PID_FD_PIDNS integrated first and then possibly extend the 
interface to include TRANSLATE_PID_TASK_PIDNS in future?

Thanks,
Nagarathnam.
> Most pid-based syscalls are racy in some cases but they are
> here for decades and everybody knowns how to deal with it.
> So, I've decided to merge both worlds in one interface which clearly 
> tells what to expect.