Messages in this thread | | | Date | Thu, 23 May 2013 15:38:17 +0400 | From | Stanislav Kinsbursky <> | Subject | Re: [RFC PATCH] fs: call_usermodehelper_root helper introduced |
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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? 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.
-- 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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