From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Serge E. Hallyn" Subject: Re: [PATCH RFC] pidns: introduce syscall getvpid Date: Wed, 16 Sep 2015 11:31:23 -0500 Message-ID: <20150916163123.GA1039@mail.hallyn.com> References: <20150915120924.14818.49490.stgit@buzz> <87h9mvg3kw.fsf@x220.int.ebiederm.org> <55F832D2.1070605@yandex-team.ru> <20150915151729.GA144242@dakara> <20150915174143.GE4699@ubuntumail> <55F91C3D.1040209@yandex-team.ru> <20150916143939.GA32226@mail.hallyn.com> <87twquzag1.fsf@x220.int.ebiederm.org> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Return-path: Content-Disposition: inline In-Reply-To: <87twquzag1.fsf-JOvCrm2gF+uungPnsOpG7nhyD016LWXt@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: "Eric W. Biederman" Cc: Konstantin Khlebnikov , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Serge Hallyn , Oleg Nesterov , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrew Morton , Linus Torvalds List-Id: linux-api@vger.kernel.org On Wed, Sep 16, 2015 at 09:49:02AM -0500, Eric W. Biederman wrote: > "Serge E. Hallyn" writes: > = > > On Wed, Sep 16, 2015 at 10:37:33AM +0300, Konstantin Khlebnikov wrote: > >> On 15.09.2015 20:41, Serge Hallyn wrote: > >> >Quoting St=E9phane Graber (stgraber-GeWIH/nMZzLQT0dZR+AlfA@public.gmane.org): > >> >>On Tue, Sep 15, 2015 at 06:01:38PM +0300, Konstantin Khlebnikov wrot= e: > >> >>>On 15.09.2015 17:27, Eric W. Biederman wrote: > >> >>>>Konstantin Khlebnikov 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 curren= t 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 namesp= aces > >> >>>>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 w= ith > >> >>>>>client from isolated container via socket about task in container: > >> >>>> > >> >>>>Sockets are already supported. At least the metadata of sockets i= s. > >> >>>> > >> >>>>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 a= chieve > >> >>what you want today by doing: > >> >> > >> >>=3D=3D Translate PID in container to PID in host > >> >> - open a socket > >> >> - setns to container's pidns > >> >> - send ucred from that container containing the requested containe= r PID > >> >> - host sees the host PID > >> >> > >> >>=3D=3D 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 set= s 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 virtua= l. > >> = > >> 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 St=E9phane 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. -serge