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While most people (especially who use tmpfs as top writable branch)
doesn't care, I care and think it can be a security problem.
For example, when the lower readonly branch may contain
/etc/{passwd,shadow} and the permission bits of the upper empty
branch is world-writable, then a malicious user can make these files
manually with by-passing aufs.
Aufs can do nothing but produce a warning.

For details, see aufs manual in aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>