On Mon, Sep 21, 2015 at 10:49:39AM +0800, Chen Fan wrote: > > 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 So please implement it, as Eric suggested, using the ns inode fds instead of racy pid_t hints for namespaces. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
