mount --bind

Most Linux and Unix file systems don't allow hard links to directories (except for the . and .. entries that mkdir creates itself). The reasons are are pretty obvious: you could really confuse programs like ls (ls -R), find and of course fsck if you created links that recursed back to themselves. If there was a compelling reason to allow directory hard links, you'd need to rewrite any program that wants to walk a file system tree to be aware of the possible problems..

So instead we have symlinks. You've probably used them for things like shifting around disk space or to give more convenient access to a directory. For example, Mac OS X creates /tmp as a symbolic link to "private/tmp". We use symlinks to make other directories visible under Apache's htdocs directory (though the same thing can be accomplished with Apache's configuration files).

One problem with symbolic links is that really they are just files. A special kind of file, yes, but a symlink only points at a directory - it doesn't act like one. So, for example, if you put a symlink to /xyz in a users home directory, and the user has write permission to his home (as he ordinarily would), he can remove your symlink. Nothing you can do with ordinary permissions can prevent that. You can do a "chattr +i" on your symlink, but because it is a symlink, that passes through to the actual directory, making it unusable. If you use "+u" (undeletable), that again passes through, and the user still can delete your symlink.

This can be extremely annoying, especially when users accidentally delete a symlink they need to have. Of course your real directory is still safe, but you need to recreate the symlink. In the mean time, your user is confused or maybe even broken.

There is at least one way around this. If the thing you want to link to is a mounted file system, you can use the "--bind" option of mount to create an unbreakable link.

If the thing to link to it isn't a separate fs, you can almost always make it be one.

Here's how it works. Let's say we have /dev/foo mounted at /foo and I want a "link" to that under /home/fred. All I have to do is:

Fred can have full write permissions on /foo if he needs it, but he will not be able to remove /home/fred/foo. Not even root can:

# rm -rf /home/fred/foo
rm: cannot remove directory '/home/fred/foo': Device or resource busy

Some related links you might find interesting:

http://lwn.net/Articles/159077/
http://linuxgazette.net/issue93/tag/2.html

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Sun Apr 30 06:08:08 2006: Subject:   drag


I use mount to bind directories when using a Knoppix cdrom to recover from mucked up linux bootloaders or resetting root passwords and whatnot.

Assuming that /dev/hda1 is your OS's on your harddrive's main partition...

Boot up with knoppix.
mount /mnt/hda1
(should set that up automaticly, could be in /media though)
cd /mnt/hda1
mount -t proc proc ./proc
mount -t sysfs sys ./sys
mount -o bind /KNOPPIX/dev ./dev
(or is that /mnt/KNOPPIX/dev? I forget, been a while)
chroot ./ su -

Or something similar to that. Should end up having full access to the hardware from inside the chroot environment for setting up grub or whatnot.

I also use that in conjunction with debootstrap for installing Debian on machines for for whatever reason have issues with the installer cdrom (such as only have knoppix disk handy, or normal installer cdrom doesn't have drivers for the network or ide controller.



Tue May 2 17:53:13 2006: Subject: --bind in mac os x?   anonymous


is there a way to do a --bind in Mac OS X (10.4.5) ????

please???



Tue May 2 20:10:50 2006: Subject:   TonyLawrence


Not that I'm aware of.



Wed Nov 12 04:46:01 2008: Subject: cool   Gonzalo


cool!

I had a problem with my ftp client and now it is solved. Thanks.



Sat Nov 22 09:19:56 2008: Subject:   seb


Hi,

there is something I can't do with "--bind".
In fact, I try to make some hardlinks between files in the newly binded directory and I always have the cross-device error (even if it's on the same device !).
Do you know if it's possible to do this ? Do I miss something ?

thx



Mon Feb 2 11:26:05 2009: Subject: So er...   Roger


"Fred can have full write permissions on /foo if he needs it, but he will not be able to remove /home/fred/foo. Not even root can:"

So how would one go about removing one of these then?



Mon Feb 2 12:03:28 2009: Subject:   TonyLawrence


Unmount it.

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Fri Apr 3 16:33:08 2009: Subject: mount --bind 1 directory to 3 mount points   NelsonDelgado


I have a ftp server with 3 local users chrooted to their home directories, and 1 annonymous directory for public downloads available to anyone (/home/ftp). I already bound /home/ftp to /home/user1/public-ftp so user1 can upload files to the public directory, but how can I bind the same public directory to all three users so they can upload there too... one directory to 3 mount points? Is it possible?
Thanks beforehand... Nelson

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