Re: "Too many levels of symbolic links"

[Ian Kent <raven@themaw.net>]

On Fri, 2016-05-20 at 16:12 +0200, Donald Buczek wrote:
> On 03/16/16 03:10, Ian Kent wrote:
> > On Wed, 2016-03-09 at 18:44 +0100, Donald Buczek wrote:
> > > As a reminder, here is the script we used to demonstrate the problem (
> > > assuming /project/mariux32 is served by autofs) :
> > > 
> > > ===
> > > #! /bin/sh
> > > 
> > > ls -ld /project/mariux32/.
> > > unshare -m -- bash -c "sleep 6;ls -ld /project/mariux32/.;echo
> > > exit..." &
> > > kill -USR1  `cat /var/run/autofs-running`
> > > sleep 3
> > > ls -ld /project/mariux32/.
> > > wait
> > > ===
> > > 
> > > In your mail quoted below you wrote, the error would be avoided if "/"
> > > was set to shared before automount is started, but I can't confirm
> > > this.
> > > 
> > > ===
> > > root:nsa:/scratch/local/# systemctl stop automount.service
> > > root:nsa:/scratch/local/# mount --make-rshared /
> > > root:nsa:/scratch/local/# systemctl start automount.service
> > > root:nsa:/scratch/local/# ./test.sh
> > > drwxrwsr-x 6 mx32prj mx32grp 56 Feb 24  2011 /project/mariux32/.
> > > ls: cannot access /project/mariux32/.: Too many levels of symbolic
> > > links
> > > drwxrwsr-x 6 mx32prj mx32grp 56 Feb 24  2011 /project/mariux32/.
> > > exit...
> > > root:nsa:/scratch/local/#
> > > ===
> > Actually, assuming /project is an indirect mount, I think that should
> > have been OK.
> 
> My fault. It does work. I've overlooked
> 
>  > unshare  since  util-linux  version  2.27  automatically  sets 
> propagation to private in a
>  > new mount namespace to make sure that the new namespace is really
>  > unshared.  It's possible to disable this feature with option 
> --propagation unchanged.
> 
> So in fact it does
> 
>  > unshare(CLONE_NEWNS)                    = 0
>  > mount("none", "/", NULL, MS_REC|MS_PRIVATE, NULL) = 0
> 
> If I add "--propagation unchanged" to the test script, it works. Might 
> be a solution for our environment.
> 
> I understand, that the combined semantics of automount and mount 
> namespaces are yet to be defined and there is no clear,unique way to do 
> that.
> 
> But independent of that, I think it might be better if autofs would 
> continue to try to mount on top of a dentry with DCACHE_MOUNTED set and 
> this might be a requirement for most of the thinkable solutions.
> 
> > Again, there's more going on, probably unshare not cloning file handles
> > (the -m clones only the mount namespace I think). Without a way of
> > sending mount requests to the automount daemon, which needs to be done
> > via a file handle (the current pipe implementation or even if it was
> > socket based) the symptom will be the same as when / is propagation
> > private.
> 
> Well, with "mount --make-rshared /" and "unshare -m --propagation 
> unchanged" it looks like everything seems to be working. And the mounts 
> can be triggered from the new namespace as well. There is only one 
> cosmetic thing: When a automount expires, the unmount fails when the 
> (shared mount) in the other namespace still is busy. This generates some 
> log.
> 
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: expiring path 
> /project/mariux32
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: >> umount.nfs: 
> /project/mariux32: device is busy
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: >> umount.nfs: 
> /project/mariux32: device is busy
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: >> umount.nfs: 
> /project/mariux32: device is busy
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: Unable to update the 
> mtab file, /proc/mounts and /etc/mtab will differ
> 2016-05-20T15:43:39+02:00 deadbird automount[938]: expired /project/mariux32

I've looked at the mount propagation code many times and it always ends in
confusion.

For example, in this case, it looks like the reference count on all the mounts
"of the parent mount" are checked which seems wrong to me since a specific mount
contained in the parent mount is being umounted.

But I have to assume the actual outcome is that the mount would be umounted if
it (itself) isn't busy in any branch of the propagation tree.

On the face of it that appears to conflict with the notion of independent
namespaces where mount and umount need to be able to be done independently
within namespaces.

It looks as though the changes that decoupled the kernel mount itself from the
vfsmount struture are in 3.8 and those are needed for mount/umount independence,
at a minimum.

Maybe they weren't complete or maybe the propagation code is conflicting with
the usage (it came some time before the namespace implementation) and this case
hasn't been covered.

This is the type of side effect that worries me most.

> 
> Regards
>    Donald
> 
> 
> > 
> > This is just a guess though.
> > 
> > > Thank you!
> > > 
> > >     Donald
> > > 
> > > 
> > > On 03/04/14 07:06, Ian Kent wrote:
> > > > On Mon, 2014-03-03 at 10:40 +0800, Ian Kent wrote:
> > > > > > That doesn't solve the problem, however, that mounts cloned by a
> > > > > > unshare(CLONE_NEWNS) would never expire. Also there is another
> > > > > > bug
> > > > > > somewhere, because I see, that the mount, visible to the
> > > > > > /usr/lib/colord/colord process was logged as "unmounted" in the
> > > > > > nfs
> > > > > > server when it expired in the global namespace. So I doubt it
> > > > > > would be
> > > > > > working even for that process. So possibly automounted mounts
> > > > > > shouldn't
> > > > > > be cloned at all? Together with chroot or pivot_root the
> > > > > > sematics would
> > > > > > be more than unclear anyway. Your problem now :-)
> > > > > Hehe, like I said some people are going to be disappointed.
> > > > > 
> > > > > There's just one question about this that remains.
> > > > > 
> > > > > Assuming systemd is setting "/" shared what happens if "mount
> > > > > --make-rprivate /" is run before autofs is started?
> > > > > 
> > > > > So if you can spend a little more time on this an answer to this
> > > > > would
> > > > > be helpful.
> > > > No need for this, thanks to your reproducer.
> > > > 
> > > > In fact the problem doesn't appear happen if "/" is set shared so in
> > > > your case "/" must be set either slave or private.
> > > > 
> > > > And expanding the reproducer a bit I see another failure case too,
> > > > and
> > > > it doesn't appear to be the unreliable d_mountpoint() check, not
> > > > sure
> > > > yet exactly what it is.
> > > > 
> > > > Thanks
> > > > Ian
> > > > 
> > > 
> 
> 
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