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

From: Konstantin Khlebnikov
Date: Wed Apr 25 2018 - 01:37:00 EST

On 23.04.2018 20:37, Nagarathnam Muthusamy wrote:

On 04/05/2018 12:02 AM, Konstantin Khlebnikov wrote:
On 05.04.2018 01:29, Eric W. Biederman wrote:
Nagarathnam Muthusamy <nagarathnam.muthusamy@xxxxxxxxxx> 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,

@source_type and @target_type defines type of following arguments:

TRANSLATE_PID_CURRENT_PIDNSÂ - current pid namespace, argument is unused
TRANSLATE_PID_TASK_PIDNSÂÂÂÂ - 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
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?

Any introspection for [nested] containers. It's easier to work when you have all information when you don't have any.
For example our CMS allows to start nested sub-container (or even deeper) by request from any container and have to tell back which pid task is have. And it could translate any pid inside into accessible by client and vice versa.

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?

I don't see reason for this separation.
Pids and pid namespaces are part of the API for a long time.

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.