MNT_DETACH and mount namespace issue (was: Re: [PATCH] vfs: Fix RCU usage in __propagate_umount())

[Richard Weinberger <richard@nod.at>]

Am 30.07.2014 15:59, schrieb Richard Weinberger:
> If we use the plain list_empty() we might not see the
> hlist_del_init_rcu() and therefore miss one member of the
> list.
> 
> It fixes the following issue:
> $ unshare -m /usr/bin/sleep 10000 &
> $ mkdir -p foo/proc
> $ mount -t proc none foo/proc
> $ mount -t binfmt_misc none foo/proc/sys/fs/binfmt_misc
> $ umount -l foo/proc
> $ rmdir foo/proc
> rmdir: failed to remove ‘foo/proc’: Device or resource busy

Although my fix was wrong, the issue is real, it seems to exist for a very long
time. Just was able to reproduce it on 2.6.32.
Please note that you need a shared root subtree to trigger the issue.
i.e. mount --shared /
Maybe this is why nobody noticed it so far as only systemd distros
have the root subtree shared by default.

I hit the issue on openSUSE 13.1 where an application creates a chroot environment
and then lazy umounts /proc.
It happened on very few machines. An analysis showed that only boxes with an OpenVPN tunnel
were affected. This did not make any sense until I discovered that the OpenVPN systemd
service file has set "PrivateTmp=true". This setting creates
a mount namespace for the said service...

In __propagate_umount() the following piece of code is interesting:

 /*
 * umount the child only if the child has no
 * other children
 */
if (child && list_empty(&child->mnt_mounts)) {
        hlist_del_init_rcu(&child->mnt_hash);
        hlist_add_before_rcu(&child->mnt_hash, &mnt->mnt_hash);
}

child->mnt_mounts is non-empty for the "proc" although the "binfmt_misc"
subtree was removed.
I'm not sure whether this is only one more symptom or the main culprit.

Any ideas?

Thanks,
//richard