linux-kernel - Re: [PATCH 0/2] Fix debugfs bind mount regression

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Date:	Wed, 09 Mar 2016 14:57:47 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Seth Forshee <seth.forshee@...onical.com>
Cc:	linux-fsdevel@...r.kernel.org,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Fix debugfs bind mount regression

Seth Forshee <seth.forshee@...onical.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?

Eric