Messages in this thread | | | Date | Thu, 23 May 2013 15:58:44 +0400 | From | Stanislav Kinsbursky <> | Subject | Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced |
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23.05.2013 15:56, Jeff Layton пишет: > On Thu, 23 May 2013 15:38:17 +0400 > Stanislav Kinsbursky <skinsbursky@parallels.com> wrote: > >> 23.05.2013 15:31, Jeff Layton пишет: >>> On Thu, 23 May 2013 14:35:53 +0400 >>> Stanislav Kinsbursky <skinsbursky@parallels.com> wrote: >>> >>>> 23.05.2013 14:00, Eric W. Biederman пишет: >>>>> Stanislav Kinsbursky <skinsbursky@parallels.com> writes: >>>>> >>>>>> 22.05.2013 21:33, Eric W. Biederman пишет: >>>>>>> Stanislav Kinsbursky <skinsbursky@parallels.com> writes: >>>>>>> >>>>>>>> Usermode helper executes all binaries in global "init" root context. This >>>>>>>> doesn't allow to call a binary from other root context (for example in a >>>>>>>> container). >>>>>>>> Currently, both containerized NFS client and NFS server requires an ability to >>>>>>>> execute a binary in a container's root context. Root swap can be done in >>>>>>>> "init" callback, passed by UMH caller. >>>>>>>> But since we have 2 callers already (and more of them are expected to appear >>>>>>>> in future) and because set_fs_root() in not exported, it looks reasonable to >>>>>>>> add one more generic UMH helper to generic fs code. >>>>>>>> Root path reference must be hold by the caller, since it will be put on UMH >>>>>>>> thread exit. >>>>>>> >>>>>>> Awesome. With this patch as an uprivilieged user I get to pick which >>>>>>> binary the kernel will execute. At least if nfs and nfsd ever runs in a >>>>>>> user namespace (something that looks like only matter of time). >>>>>>> >>>>>> >>>>>> Not really. Only by using a kernel module to call the UMH. >>>>>> And an unprivileged can't load a module as far a I know. >>>>>> I.e. NFSd, for example, will use unprivileged user's root to perform this call. >>>>> >>>>> To help me understand the context which instances of call user mode >>>>> helper are you expecting to use this facility? >>>>> >>>> >>>> Ok. Here is how the NFSd uses UMH: >>>> UMH is used on NFSd service to start user-space client tracker daemon >>>> ("/sbin/nfsdcltarck"), which is used to store some per-client locks data on >>>> persistent storage. >>>> >>>>>>> I think this is a seriously bad idea. >>>>>>> >>>>>>> Why can't we do this in userspace with setns as we do with the core dump >>>>>>> helper? >>>>>>> >>>>>> >>>>>> Could you, please, clarify, how setns can help here? >>>>> >>>>> setns can change the mount namespace, and chroot can change to root >>>>> directory in the specified mount namespace. Essentially you can enter >>>>> into a containers complete context (pid, mnt, root, etc) comming from >>>>> the outside. >>>>> >>>> >>>> So, you are actually suggesting to move the binary start from the kernel to user-space. >>>> IOW, you are suggesting to do not using UMH at all. >>>> Am I right? >>>> I don't know the reasons, why it was done by using UMH and not in userspace. >>>> Could you clarify this, Jeff? >>>> >>> >>> nfsdcltrack is a "one-shot" program for managing and querying the nfsd >>> client tracking database. When knfsd needs to query or modify the >>> db, it uses the UMH infrastructure to call this program that does >>> what's requested and then exits. >>> >>> So, I'm not sure I really understand your question. It wasn't done in >>> userspace since the whole purpose of this program is to handle upcalls >>> from the kernel. >>> >> >> The question is what was the reason to start this binary from kernel by UMH? > > Manipulating and querying the client tracking database is an infrequent > event, so having a continuously running daemon is wasteful and means > that the admin has to ensure that it's running. A UMH upcall is much > simpler and generally "just works" if the program is present. > >> I.e. why it can't be started by some user-space process before or after NFSd start? >> I don't familiar with this client tracking facility and that's the only reason why I'm asking. >> > > This program is not a daemon that runs continuously. It's only called > when the kernel needs to manipulate the database. Are you asking > whether we could turn this into a continuously running daemon? If so > then the answer is "yes", but that's not really a good idea either. > > In fact, we had that with the nfsdcld program, but no one liked it > (including me) for the reasons I detailed above. >
No, I'm just asking to understand. Eric was, actually, asking the same. I.e. how does NFSd uses UMH and why this can't be done in userspace? Thanks you for your answer.
-- Best regards, Stanislav Kinsbursky -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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