Hard links to directories: why not?

[Alan J Rosenthal]

In article <10527 at odin.corp.sgi.com> schuman at sgi.com (Aaron Schuman) writes:
>>A foolish user could create loops in the directory structure.

mvadh at cbnews.att.com (andrew.d.hay) writes:
>it would be easy to have ln disallow this:
>1)	resolve argv[1] and argv[2] to absolute paths
>2)	determine which path is shorter
>3)	strncmp() both paths for the shorter length
>4)	if you have a match, you're trying to create a loop

It's true that if they match, you're creating a loop.  However, it's not true
that if they don't match you're not creating a loop.
Here's a counterexample:

Suppose your filesystem is on /mnt.
Do this:

	mkdir /mnt/a
	ln /mnt/a /mnt/b
	mkdir /mnt/a/c
	ln /mnt/a /mnt/b/c/d

Now /mnt/a/c/d and /mnt/a are the same.  So you can refer to
/mnt/a/c/d/c/d/c/d/c/d/c/d, etc.

ajr