[MESOS-3178] Perform a self bind mount of rootfs itself in fs::chroot::enter.

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 0.24.0
• Target Version/s: 0.24.0
• Epic Link: filesystem isolation
• Sprint: Twitter Mesos Q3 Sprint 2
• Story Points: 2
• Assignee: Jie Yu
• Reporter: Jie Yu
• Created: 30/Jul/15 21:59
• Updated: 03/Aug/15 18:01
• Resolved: 03/Aug/15 18:01

Description

Syscall 'pivot_root' requires that the old and the new root are not in the same filesystem. Otherwise, the user will receive a "Device or resource busy" error.

Currently, we rely on the provisioner to prepare the rootfs and do proper bind mount if needed so that pivot_root can succeed. The drawback of this approach is that it potentially pollutes the host mount table which requires cleanup logics.

For instance, in the test, we create a test rootfs by copying the host files. We need to do a self bind mount so that we can pivot_root on it. That pollute the host mount table and it might leak mounts if test crashes before we do the lazy umount:
https://github.com/apache/mesos/blob/master/src/tests/containerizer/launch_tests.cpp#L96-L102

What I propose is that we always perform a recursive self bind mount of rootfs itself in fs::chroot::enter (after enter the new mount namespace). Seems that this is also done in libcontainer:
https://github.com/opencontainers/runc/blob/master/libcontainer/rootfs_linux.go#L402