Re: Re: [PATCH RFC] pidns: introduce syscall getvpid

[Chen Fan]

On 09/17/2015 12:31 AM, Serge E. Hallyn wrote:
> On Wed, Sep 16, 2015 at 09:49:02AM -0500, Eric W. Biederman wrote:
> > "Serge E. Hallyn" <serge@xxxxxxxxxx> writes:
> >
> > > On Wed, Sep 16, 2015 at 10:37:33AM +0300, Konstantin Khlebnikov wrote:
> > > > 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?
> > > Ah, no that doesn't help with this.
> > >
> > > What Stéphane describes is what I've done in several projects.
> > > Getting it right is however actually quite tricky.  I'm not
> > > convinced it's at the level of "since you can do (sweep hands)
> > > all this, we don't need a simple syscall to do it."
> > >
> > > So I'd encourage you to resend using namespace inode fds for
> > > source and target as Eric suggested.  We still may decide that
> > > the syscall isn't needed, but it's a trivial change to your
> > > patch and removes that race.  And I'm not convinced it's not
> > > needed.
> > At this point my primary concern is that a pattern that would need to be
> > convering to and from pids quickly is potentially fundamentally racy to
> > the point of broken.
> The cgmanager GetTasks and GetTasksRecursive, and reading of the
> lxcfs cgroup /tasks files, require converting every pid from the
> cgmanager's namespace to the reading task's namespace.
>
> > Especially with unix domain sockets passing and converting pids in a way
> > that covers the common case.
> >
> > I am clearly missing some nuance of this use case.
> lxcfs and cgmanager are imo proof that we *can* do without the new
> syscall.  However, the git history will show that there are some
> complications, and the system load when a few systemds are starting
> will show that it does take a performance toll on the host at some
> point.  Still as I say it's doable.  The syscall implementation was
> very simple, though.

Yes, previous email discussed about the implementation of syscall or procfs:
http://www.gossamer-threads.com/lists/linux/kernel/1971723?search_string=chen%20hanxiao;#1971723

but it seems complicated implemented by procfs, the original discussion at:
http://www.gossamer-threads.com/lists/linux/kernel/2076440?search_string=chen%20hanxiao;#2076440

Thanks,
Chen

> -serge
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