linux-kernel - Re: Detecting bind-mounts

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Message-ID: <4CD44D60.5050105@msgid.tls.msk.ru>
Date:	Fri, 05 Nov 2010 21:30:56 +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 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.

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.

In the above example, both stat(1) (even the one with the
commit you refers to) and df(1) reports the same for the
case I'm referring to, the both fails to detect a bind-
mount.

Thanks!

/mjt
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