Cannot mount proc in unprivileged containers if /proc/xen is mounted

[Seth Forshee]

SRU Justification

Impact: The xenfs filesystem is traditionally mounted at /proc/xen in xen guests. This directory doesn't use the special "create proc mountpoint" interface and thus fails the permanently empty test in fs_fully_visible(). This causes mounting of proc to fail in user namespace containers.

Fix: Use the special proc interface to make this a "permanently empty" directory.

Regression potential: This change will make it impossible to create files within /proc/xen, but since the directory is only ever used as a mount point this should not cause any problems.

Original bug report and testing results can be found at https://github.com/lxc/lxd/issues/2238.

[Stefan Bader (smb)]

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-xenial' to 'verification-done-xenial'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Rahul Chincholkar (rchincholkar)]

I have verified the proposed kernel for xenial (4.4.0-36-generic)
It is working without any issue while launching/starting/stopping the containers.

Thanks!
Rahul C

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 4.4.0-36.55

---------------
linux (4.4.0-36.55) xenial; urgency=low

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