linux-kernel - Re: Detecting bind-mounts

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Message-ID: <4CD46953.1010902@msgid.tls.msk.ru>
Date:	Fri, 05 Nov 2010 23:30:11 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Pádraig Brady <P@...igBrady.com>
CC:	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Detecting bind-mounts

05.11.2010 21:30, Michael Tokarev wrote:
> 05.11.2010 13:24, Pádraig Brady wrote:
>> On 04/11/10 20:45, Michael Tokarev wrote:
> []
>>> 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 `stat` command recently got support for
>> printing the mount point for a file:
>> http://git.savannah.gnu.org/gitweb/?p=coreutils.git;a=commit;h=ddf6fb86
>>
>> `stat` will output the alias for a bind mounted file
>> while `df` will output the initial mount point of its backing device
>> So you could do something like:
>>
>> file=.
>> df_mnt=$(df -P "$file" | sed -n '2s/.* \([^ ]*$\)/\1/p')
>> stat_mnt=$(stat -c%m "$file")
>> test "$df_mnt" = "$stat_mnt" || echo "bind mount"
> 
> This is incorrect in two ways.
> 
> First of all, stat(1), even after that commit you quote,
> still compares st_dev fields, which are the same for this
> and parent directory in case of bind mount.  So this version
> of stat(1) does _not_ detect a bind mount, unfortunately.

And this statement, in turn, is untrue.  I apologize for the
misinformation, it wasn't intentional.  The mentioned commit
adds the ability to detect bind mounts indeed.  but...

> Second, I asked for a low-level way to detect such a mount.
> I know how to do it not as efficient as stat(2) and not as
> reliable but much simpler than you propose above, in shell
> or in C, and I already provided that way in my original
> email: we just parse /proc/mounts file, this is faster and
> more reliable than the above shell fragment which calls a
> few external commands.

.. the way used by stat(1) is to enumerate /proc/mounts --
which is what I were able to come with initially.  It is slow
and unreliable.  Hence I asked if a faster way exist.

/mjt
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