Creating hardlinks for directories.
James Wilkinson
fedora at westexe.demon.co.uk
Mon Jan 16 13:03:02 UTC 2006
Vikram Goyal wrote:
> Rummaging through the man pages and pondering over the problem I came
> across ln man page. An excerpt:
>
> -d, -F, --directory
> allow the superuser to attempt to hard link directories
> (note: will probably fail due to system restrictions,
> even for the superuser)
>
> So I tried it as super user but failed miserably. Then I thought if the
> option is there then there must exist a possible way of usage.
Please note that the ln command and associated manpage come from the GNU
project. As such, they are certainly not Linux-specific. I understand
that many of the GNU utilities pre-date the Linux kernel: they were
certainly designed to install on a large number of weird and
less-than-wonderful archaic systems. (And this was common, especially
on systems which saw a lot of interactive shell use).
So the "possible way of usage" could include "using GNU ln on a
different system with a different C library and a different kernel".
To the best of my knowledge, this *is* the only way. The e-mail at
http://lwn.net/Articles/100184/ dates from August 2004, but Linus
Torvalds explicitly says:
The general VFS layer has a lot of rules, and avoids these problems by
simply never having aliases between two directories. If the same directory
shows up multiple times (which can happen with bind mounts), they have the
exact same dentry for the directory, it's just found through two different
vfsmount instances. That's why vfsmounts exist - they allow the same name
cache entry to show up in different places at the same time.
In other words, if a directory is hard linked, it's a kernel bug.
James.
--
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