Hard links to directories: why not?

[Warner Losh]

In article <12877 at yunexus.YorkU.CA> shields at yunexus.YorkU.CA (Paul
Shields) writes: 
>P.S: on VAX/VMS 3.7 the above (with a different command set of course)
>is possible. I don't know about old versions of UNIX.

The commands that existed in VMS 3.7 still existed in VMS 5.3.  I
don't know if they will still let you create cycles in the directory
structure.  I do know they were used to share files in a VAXcluster
because VMS didn't have symbolic links....

This hard linking on VMS has caused lots of trouble since it normally
isn't done.  BACKUP assumes that all files have one link (basically)
so restoring a disk that was backed up that had odd directory entries
like this caused the files to be duplicated, rather than re-linked.
This was only a problem for one type of backup (used to do
incrementals), not the full image (level 0) backups.

Don't know if they fixed it, but it sounds like a "denial of service"
security hole when the original disks are tight on space.  In
addition, the way that it was implemented caused a hole whereby
certain programs could read files that a user couldn't normally read.
Plan files with finger springs to mind......

Warner

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Warner Losh		imp at Solbourne.COM
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