Re: zero-length symlinks

[David Holland <dholland-tech%netbsd.org@localhost>]

On Sun, Nov 03, 2013 at 07:48:54PM -0500, Mouse wrote:
 > >> On Unix System V, the link command would allow hard-linking
 > >> directories when used as root.
 > 
 > Also, recently enough that at least some versions of NetBSD do it,
 > unlink(2) performed by root on the last non-. link to a directory would
 > silently orphan the directory, requiring fsck to fix.
 > 
 > I've long thought this deserved to be considered a bug.

That is a bug, yes.

 > > The reason being that hard links to directories means that the tree
 > > of directories is no longer a DAG and that causes serious problems
 > > for the tree traversing code.
 > 
 > Well, might no longer be a DAG.  I don't see why this couldn't be
 > checked at link(2) time; it's checked within rename(2).

It is checked, at substantial expense and, historically, mostly
incorrectly. (Don't run concurrent renames on 1.4T; you can easily
corrupt the filesystem.)

The check also depends on the filesystem being a tree, so it's quite
likely that if you tried to implement this generalization that you'd
get into trouble.

-- 
David A. Holland
dholland%netbsd.org@localhost