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

On 04/24/2018 10:36 PM, Konstantin Khlebnikov wrote:

On 23.04.2018 20:37, Nagarathnam Muthusamy wrote:

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On 04/05/2018 12:02 AM, Konstantin Khlebnikov wrote:

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On 05.04.2018 01:29, Eric W. Biederman wrote:

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Nagarathnam Muthusamy [off-list ref] writes:

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On 04/04/2018 12:11 PM, Konstantin Khlebnikov wrote:

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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".

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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?

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 https://github.com/yandex/porto 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.

I still dont get the exact reason why PID based approach to identify the 
namespace during pid translation process is absolutely required compared 
to fd based approach. From your version of TranslatePid in

https://github.com/yandex/porto/blob/0d7e6e7e1830dcd0038a057b2ab9964cec5b8fab/src/util/unix.cpp

I see that you are going through the trouble of forking a process and 
sending SMC_CREDENTIALS for pid translation. Even your existing API 
could be extremely simplified if translate_pid based on file descriptors 
make it to the gate and I believe from the last discussion it was almost 
there https://patchwork.kernel.org/patch/10305439/

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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.

If you are talking about the translate_pid API proposed, I believe the 
V4 proposed under https://patchwork.kernel.org/patch/10003935/ had only 
fd based API before a mix of PID and fd based is proposed in V5. Again, 
I was just wondering if we can get the FD based approach in first and 
then extend the API to include PID based approach later as fd based 
approach could provide a lot of immediate benefits?

Thanks,
Nagarathnam.

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Thanks,
Nagarathnam.

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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.