Message-ID: <4CD31B6A.7040902@msgid.tls.msk.ru>
Date: Thu, 04 Nov 2010 23:45:30 +0300
From: Michael Tokarev <mjt@tls.msk.ru>
Organization: Telecom Service, JSC
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To: Linux-kernel <linux-kernel@vger.kernel.org>
Subject: Detecting bind-mounts
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Hello.

There are quite some talks on the 'net - questions, not
answers - about detecting bind mounts - be it a directory
or a file.

There are 2 (mostly) different kinds of applications.  One
is cp/tar/find with --same-filesystem option (or equivalent),
that should not cross mountpoints.  And one more, apps like
mountpoint(1) from sysvinit - a utility to determine if a
given path is a mountpoint.

Neither of the two work when two directores on the same
filesystem are bind-mounted.

The usual idiom is to compare st_dev of current directory and
the parent - if they're different that's a mount point.  But
in this case, two st_devs will be the same, so such a mount
point will not be detected.

It is even worse for bind-mounted files (as opposed to dirs):
there's no path/file/.. entry to stat(2), and cutting the
last component from the pathname does not work reliable due
to symlinks (it may be a symlink from different filesystem).

So far I know only one way to detect a bind mount like this,
and it is unreliable anyway.  It is to parse /proc/mounts
and try to find the object(s) in question.  Unreliable because
of, again, symlinks, and possible complex mounts and bind-
mounts.  And this is also very slow - imagine using this way
for find/tar/cp --one-file-system.

Is there some simpler and more reliable way? Maybe use mount
syscall, like we use kill($pid, 0) to check existance of a
process?

And as far as I understand, the same applies to multiple
mounts of the same filesystem.

Thanks!

/mjt
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