Re: [PATCH v4] pidns: introduce syscall translate_pid

[prakash.sangappa]

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.

-Prakash
> Thanks,
> Nagarathnam.