Re: [PATCH v4] pidns: introduce syscall translate_pid

[Prakash Sangappa]

On 10/16/17 5:52 PM, Andy Lutomirski wrote:
> On Mon, Oct 16, 2017 at 3:54 PM, prakash.sangappa
> <prakash.sangappa@xxxxxxxxxx> 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@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.
>
> 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.

-Prakash.