Date: Wed, 9 Mar 2016 21:18:20 +0000
From: Serge Hallyn <serge.hallyn@ubuntu.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Seth Forshee <seth.forshee@canonical.com>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/2] Fix debugfs bind mount regression
Message-ID: <20160309211820.GA29848@ubuntumail>
References: <1457536687-32794-1-git-send-email-seth.forshee@canonical.com>
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Quoting Eric W. Biederman (ebiederm@xmission.com):
> Seth Forshee <seth.forshee@canonical.com> writes:
> 
> > Some full-OS container software bind mounts debugfs into containers to
> > satisfy the assumptions of older userspaces which expect to be able to
> > mount debugfs. This regressed in 4.1 due to the addition of tracefs,
> > which gets automounted in the tracing subdirectory of debugfs. In a
> > cloned mount namespace the bind mount now fails because the tracefs
> > mount is a locked child of the debugfs mount.
> >
> > For new mounts we already make an exception to the "locked child mount"
> > rule. Directories in psuedo filesystems created for the sole purpose of
> > being mountpoints are created as permanently empty directories which can
> > never contain any entries, therefore the kernel can know than any mounts
> > on these directories are not for security purposes. These mounts are
> > then excluded from locked mount tests in some circumstances.
> >
> > The same logic clearly applies to directories created in
> > debugfs_create_automount(). The following patches update this function
> > to create permanently empty directories for mountpoints and adds an
> > exclusion to the tests for bind mounts to exclude child mounts on
> > permanently empty directories.
> 
> So I don't know that this approach is bad.  However in reading through
> your patch descriptions I do not see any consideration of using
> "mount --rbind"  instead of "mount --bind".  AKA adding the MS_REC flag
> to your bind mount.
> 
> I would think simply using MS_REC would solve this problem, without
> needing any additional kernel support.  Am I missing something?

That's what we're doing to work around it fwiw, but it would be nice to
not have to.