From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chen Fan Subject: Re: Re: [PATCH RFC] pidns: introduce syscall getvpid Date: Mon, 21 Sep 2015 10:49:39 +0800 Message-ID: <55FF7043.5020701@cn.fujitsu.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> <20150916163123.GA1039@mail.hallyn.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20150916163123.GA1039-7LNsyQBKDXoIagZqoN9o3w@public.gmane.org> Sender: linux-api-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Serge E. Hallyn" , "Eric W. Biederman" Cc: Konstantin Khlebnikov , Serge Hallyn , =?windows-1252?Q?St=E9phane_Graber?= , linux-api-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Oleg Nesterov , linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Andrew Morton , Linus Torvalds List-Id: linux-api@vger.kernel.org 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" writes: >> >>> On Wed, Sep 16, 2015 at 10:37:33AM +0300, Konstantin Khlebnikov wro= te: >>>> 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 = wrote: >>>>>>> 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 cu= rrent too). >>>>>>>>> If pid is unreachable from target pid-ns then it returns zero= =2E >>>>>>>> This interface as presented is inherently racy. It would be b= etter >>>>>>>> if source and target were file descriptors referring to the na= mespaces >>>>>>>> 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 any= way. >>>>>>> >>>>>>>>> Such conversion is required for interaction between processes= from >>>>>>>>> different pid-namespaces. For example when system service tal= ks with >>>>>>>>> client from isolated container via socket about task in conta= iner: >>>>>>>> Sockets are already supported. At least the metadata of socke= ts 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 contain= ers >>>>>>> ( https://github.com/yandex/porto ). Main server works in host = and >>>>>>> have to interact with all levels of nested namespaces. This sys= call >>>>>>> makes some operations much easier: server must remember only pi= d in >>>>>>> host pid namespace and convert it into right vpid on demand. >>>>>> Note that as Eric said earlier, sending a PID inside a ucred thr= ough a >>>>>> unix socket will have the pid translated. >>>>>> >>>>>> So while your solution certainly should be faster, you can alrea= dy achieve >>>>>> 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 cont= ainer 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= 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 vi= rtual. >>>> >>>> Backward conversion is troublesome: we have to scan all pids in ho= st >>>> 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 t= o 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 pr= ocfs: http://www.gossamer-threads.com/lists/linux/kernel/1971723?search_strin= g=3Dchen%20hanxiao;#1971723 but it seems complicated implemented by procfs, the original discussion= at: http://www.gossamer-threads.com/lists/linux/kernel/2076440?search_strin= g=3Dchen%20hanxiao;#2076440 Thanks, Chen > > -serge > -- > To unsubscribe from this list: send the line "unsubscribe linux-kerne= l" in > the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > . >