Re: [PATCH v4] pidns: introduce syscall translate_pid

[Jann Horn <jannh@google.com>]

On Tue, Oct 17, 2017 at 5:38 PM, Prakash Sangappa
<prakash.sangappa@oracle.com> wrote:
>
>
> On 10/16/17 5:52 PM, Andy Lutomirski wrote:
>>
>> On Mon, Oct 16, 2017 at 3:54 PM, prakash.sangappa
>> <prakash.sangappa@oracle.com> 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@yandex-team.ru> 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?
>
>
> Database uses the PID to programmatically find out if the process/thread is
> alive(kill 0) also send signals to the processes requesting it to dump
> status/debug information and kill the processes in case of a shutdown abort
> of the instance.

But if kill(pid, 0) returns 0, that doesn't tell you anything, right?
It could be that
the process you're trying to check is still alive, but it could also
be that it has
died, ns_last_pid has wrapped around, and the PID is now being reused by
another process, right?

Wouldn't it be more reliable to open("/proc/self", O_RDONLY)
(or /proc/thread-self) in the process you want to monitor, then send
the resulting file descriptor to the monitoring process with SCM_RIGHTS?
Then something like this should work for checking whether the process
is still alive without relying on PIDs at all:

    int retval = faccessat(child_proc_self_fd, "stat", F_OK, 0);
    if (retval == 0) {
      /* process still exists */
    } else if (retval == -1 && errno == ESRCH) {
      /* process is gone */
    } else {
      err(1, "unexpected fstatat result");
    }