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To: freebsd-hackers@FreeBSD.ORG
Subject: How to safely remove a hard link to a directory?
Date: Fri, 13 Sep 1996 20:52:18 -0700
From: Pius Fischer <pius@iago.ienet.com>
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Oops, I know, I know, I shouldn't have done this,
but maybe it's not too late. I think I need some help
or advice on what to do now before I do anything that
could really screw up the filesystem.

On a pre-2.1.5 version of FreeBSD-stable, I wrote a little
C program that just passes its first two command line
arguments to link(2). That way, when I'm superuser, I can
create a hard link to a directory (I won't get into why
I wanted to do this). The program is called 'lndir' and
here's basically what I did:

/home/pius/web>mkdir testdir
/home/pius/web>touch testdir/testfile

Then I become superuser and do this:

/home/pius/web>./lndir testdir testdir2

Now I want to remove both directories:

/home/pius/web>rm testdir/testfile
/home/pius/web>rm testdir2/testfile
rm: testdir2/testfile: No such file or directory
/home/pius/web>rmdir testdir2
rmdir: testdir2: Directory not empty
/home/pius/web>rmdir testdir
rmdir: testdir: Directory not empty
/home/pius/web>ls -al testdir2
total 8
drwx------  3 pius  web  512 Sep 13 19:52 .
drwx--x---  6 pius  web  512 Sep 13 19:57 ..
/home/pius/web>ls -al testdir
total 8
drwx------  3 pius  web  512 Sep 13 19:52 .
drwx--x---  6 pius  web  512 Sep 13 19:57 ..
/home/pius/web>

Uh, so now I can't remove either directory and I'm a little
afraid of doing an rm -rf or writing a C program that calls
unlink(2) before I really know what is going to happen.

Of course only after creating this hard link do I read the man page
for link(2) on my FreeBSD 2.2-960612-SNAP system and the last couple
lines are a little scary ("The link system call traditionally allows the
super-user to link directories which corrupts the filesystem coherency.").
The man page on the older system had no such warning.

I was aware of what Richard Stevens says on p. 101 in his _Advanced
Programming in the UNIX environment_ (again, the footnote is kind
of scary and so I tried to be careful), but I figured he corrupted
his filesystem because he created a hard link to the parent directory,
creating a loop, and then tried to unlink the hard link.

So, anyways, how can I safely remove these two directories? Maybe I'm
a little too paranoid right now, but I'm going to go ahead and backup
the important data from this filesystem onto another machine ...

Thanks very much for any help,
Pius