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    SubjectRe: [PATCH RFC] pidns: introduce syscall getvpid
    Quoting Stéphane Graber (stgraber@ubuntu.com):
    > 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@yandex-team.ru> 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.

    -serge


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