Re: FS: hardlinks on directories

[jw schultz <jw@pegasys.ws>]

On Mon, Aug 04, 2003 at 09:00:53PM +0400, Hans Reiser wrote:
> If you want hard linked directories, send us a patch for v4.  Should be 
> VERY easy to write.   If there is some reason it is not simple, let me 
> know.  Discuss it with Vitaly though, it might affect fsck.

I don't recommend it but if you do make sure those links can
only be made by root. 

SVR3 and earlier allowed manual hardlinking of directories
by root only.  They were a real source of problems.   It
also confused the dickens out of fsck so it would have to be
restricted or allowed by the filesystem code, not the VFS
layer.  I remember playing with it and it was a guarantee
that fsck would have to be run manually.

	$mkdir A
	$mkdir B
	$mkdir C
	$mkdir A/A1
	$ln A/A1 B/B1
	$ln A/A1 C/C1
	$rmdir A/A1

Assuming we can do this.  A1 is an empty directory after all.
Now B1 has a link count of 1, but i'll assume that is OK

	$rmdir A

It is after all empty even though the link count is 3.

	$cd B/B1
	$/bin/pwd
	cannot stat .

Remember B/B1/.. is it A with a nlinks==1

	$cd ..

Now where are you? It used to be called A but now it has no
normal path but can be reached through B/B1/..  It still has
.. so what directory is it linked to that doesn't have an
entry pointing back to it.
	
Lets some fun with it.

	$mkdir A2

Ah, now we have a directory the path to which is B/B1/../A2
We can hide all sorts of stuff here and find will never see
it.  It won't get backed up but maybe that doesn't matter.

What a lovely way to hide a rootkit.

If on the other hand you removed A/A1/.. when removing A/A1
you have a B/B1 and C/C1 without ..

Now umount the filesystem and run fsck.  B/B1 and C/C1 each
refer to the same directory with no .. or a .. that points
to a third directory with nlinks==1 and no directory entries
except a ..

You can put in a slew of logic to reduce the risks somewhat.
Things like only allowing rmdir somedir when
somedir->nlinks <= 2 || somedir/.. != .
but that still leaves the issue of where .. is linked and
the potential to bypass parent directory based access
controls such as Linus likes in a way that the admin will
have a hard time identifying.

The issues on a filesystem where .. is simulated would i
imagine be different.



-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

		Remember Cernan and Schmitt