Re: [libvirt] [RFC PATCH 2/2] LXC: Create ro overlay mounts only if we're not within a user namespace

On Mon, Jul 01, 2013 at 01:25:28PM +0200, Richard Weinberger wrote: > Am 01.07.2013 13:22, schrieb Daniel P. Berrange: >> On Mon, Jul 01, 2013 at 01:05:23PM +0200, Richard Weinberger wrote: >>> Am 01.07.2013 12:33, schrieb Daniel P. Berrange: >>>> On Mon, Jul 01, 2013 at 08:29:14AM +0200, Richard Weinberger wrote: >>>>> Any ideas what's going on here? >>>> >>>> No, it is very odd. It smells like a kernel issue to me. What >>>> version are you running ? >>> >>> I see this issue on all kernels. >>> Currently I'm using vanilla v3.9.x and v3.10. >>> >>>> I've also tried running the demo programs shown on the LWN.net >>>> article >>>> >>>> https://lwn.net/Articles/532593/ >>>> >>>> and they don't operate in the way described by the article - the demo >>>> programs continue to ru as 'nfsnobody' even after the mappings are >>>> setup. >>>> >>>> I'm just using the Fedora 3.9.4-303 kernel, rebuilt with userns enabled >>>> in KConfig. I'm wondering if there is still stuff missing in 3.9.x >>>> that prevents this from working properly, or if the kernel behaviour >>>> changed after those LWN articles were written. >>> >>> To me it looks like the capability system behaves odd. >>> The mappings in /proc are fine as long I do not call capng_updatev(). >>> Also calling capng_updatev() with parameters that do not change the current cap set >>> triggers the odd behavior too. >>> >>> So we see two (related?) issues: >>> 1. If we try updating the capabilities of pid1 /proc/1/ has unmapped files till we exec(). >>> 2. Dropping capabilities does not work we always gain a fresh and full capability set. >>> >>> BTW: I'm sure the issues are not caused by Gau Feng's userns patches. >> >> Yeah, I've reproduced this problem with standalone code outside of >> libvirt. >> >> Take the attached code and run > > -ENOATTACHMENT :-( Now really attached. I think I might know what is happening now though. When you start a new namespace, you must mount a new instance of 'proc' filesystem. We are not synchronizing this wrt setup of the uid/gid mappings though, so we are racy. So I have a feeling we're creating the proc filesystem before the mappings are setup. I'm going to add some synchronization in to see if it makes a difference in this respect.