Re: [PATCH RFC] pidns: introduce syscall getvpid

[Konstantin Khlebnikov]

On 15.09.2015 20:41, Serge Hallyn wrote:
> Quoting Stéphane Graber (stgraber@xxxxxxxxxx):
> > On Tue, Sep 15, 2015 at 06:01:38PM +0300, Konstantin Khlebnikov wrote:
> > > On 15.09.2015 17:27, Eric W. Biederman wrote:
> > > > Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx> writes:
> > > >
> > > > > pid_t getvpid(pid_t pid, pid_t source, pid_t target);
> > > > >
> > > > > This syscall converts pid from one pid-ns into pid in another pid-ns:
> > > > > it takes @pid in namespace of @source task (zero for current) and
> > > > > returns related pid in namespace of @target task (zero for current too).
> > > > > If pid is unreachable from target pid-ns then it returns zero.
> > > >
> > > > This interface as presented is inherently racy.  It would be better
> > > > if source and target were file descriptors referring to the namespaces
> > > > you wish to translate between.
> > >
> > > Yep, it's racy. As well as any operation with non-child pids.
> > > With file descriptors for source/target result will be racy anyway.
> > >
> > > > > Such conversion is required for interaction between processes from
> > > > > different pid-namespaces. For example when system service talks with
> > > > > client from isolated container via socket about task in container:
> > > >
> > > > Sockets are already supported.  At least the metadata of sockets is.
> > > >
> > > > Maybe we need this but I am not convinced of it's utility.
> > > >
> > > > What are you trying to do that motivates this?
> > >
> > > I'm working on hierarchical container management system which
> > > allows to create and control nested sub-containers from containers
> > > ( https://github.com/yandex/porto ). Main server works in host and
> > > have to interact with all levels of nested namespaces. This syscall
> > > makes some operations much easier: server must remember only pid in
> > > host pid namespace and convert it into right vpid on demand.
> >
> > Note that as Eric said earlier, sending a PID inside a ucred through a
> > unix socket will have the pid translated.
> >
> > So while your solution certainly should be faster, you can already achieve
> > what you want today by doing:
> >
> > == Translate PID in container to PID in host
> >   - open a socket
> >   - setns to container's pidns
> >   - send ucred from that container containing the requested container PID
> >   - host sees the host PID
> >
> > == Translate PID on host to PID in container
> >   - open a socket
> >   - setns to container's pidns
> >   - send ucred from the host containing the request host PID
> >     (send will fail if the host PID isn't part of that container)
> >   - container sees the container PID
>
> In addition, since commit e4bc332451 : /proc/PID/status: show all sets of pid according to ns
> we now also have 'NSpid' etc in /proc/$$/status.

As I see this works perfectly only for converting host pid into virtual.

Backward conversion is troublesome: we have to scan all pids in host
procfs and somehow filter tasks from container and its sub-pid-ns.
Or I am missing something trivial?

--
Konstantin
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