tree ef2b73dd5e04d5b97a0bb10e8a163811ce9a3845
parent 4ce5d2b1a8fde84c0eebe70652cf28b9beda6b4e
author Eric W. Biederman <ebiederm@xmission.com> 1364698661 -0700
committer Eric W. Biederman <ebiederm@xmission.com> 1377569823 -0700

userns: Better restrictions on when proc and sysfs can be mounted

Rely on the fact that another flavor of the filesystem is already
mounted and do not rely on state in the user namespace.

Verify that the mounted filesystem is not covered in any significant
way.  I would love to verify that the previously mounted filesystem
has no mounts on top but there are at least the directories
/proc/sys/fs/binfmt_misc and /sys/fs/cgroup/ that exist explicitly
for other filesystems to mount on top of.

Refactor the test into a function named fs_fully_visible and call that
function from the mount routines of proc and sysfs.  This makes this
test local to the filesystems involved and the results current of when
the mounts take place, removing a weird threading of the user
namespace, the mount namespace and the filesystems themselves.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>