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Comment by smw | Hacker News Reader

I had someone do this on an important production Solaris machine once, many years ago. Luckily they just moved /lib instead of deleting it -- and on Solaris (some of?) the binaries in /sbin were statically linked, including ln. Hard linking /lib back to the correct path was enough to recover.

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Do you mean moving it? You cannot make a hard link for a directory. That is disallowed on Unix to prevent the creation of orphaned subtrees (i.e. ln a directory into itself and then rmdir the original link).

GNU ln has this ( https://man.archlinux.org/man/ln.1#d ):

  -d, -F, --directory
    allow the superuser to attempt to hard link directories (this will probably fail due to system restrictions, even for the superuser)

which implies that that's not quite an absolute limit. I don't see any comment either way on https://illumos.org/man/1/ln , but it's plausible that some version of Solaris had wiggle room; it's a terrible idea for obvious reasons, but there's really no hard technical reason why a system couldn't allow you to create hard links to directories.

The Solaris VFS is flexible enough for a filesystem to support this, but any filesystem that does violates POSIX according to a comment in ZFS. A check of the illumos UFS driver source code reveals that it violates POSIX by permitting root to do this. I wrote more about this with links to relevant illumos source code here:

https://news.ycombinator.com/item?id=43447288

There are two checks prohibiting it:

• Non-root users aren't allowed to make directory hard links.

• Many versions of the userspace program `ln` don't let you do it.

But the `link` system call can, at least on Solaris, if called by user 0. (Not sure about Linux: I tried it once and it didn't work, but I was doing weird things with FUSE and also trying to name the link `..`, so I don't know why it failed.)

It is more nuanced than that. First, the Solaris 11.4 documentation says that this is not allowed:

https://docs.oracle.com/cd/E88353_01/html/E72487/link-8.html

The illumos man page is less clear:

https://illumos.org/man/8/link

The illumos ZFS driver makes it very clear that this is not allowed under POSIX in a comment and explicitly disallows it in zfs_link():

https://github.com/illumos/illumos-gate/blob/master/usr/src/...

However, It appears that the illumos UFS driver supports this:

https://github.com/illumos/illumos-gate/blob/master/usr/src/...

Presumably, the Solaris 10 UFS driver also supports it (or supported it in older versions of Solaris 10). Given that someone at Oracle likely modified the Solaris man page to differ from the older OpenSolaris man page in illumos, I would expect recent versions of Solaris to disallow this on UFS, but someone would need to check.

That said, I have to recant my previous comment. smw likely linked the directory, which is insane, but would have worked on older Solaris versions if we assume the modern illumos UFS driver is unchanged in this regard.

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