Re: PROBLEM: hfsplus filesystem allows opendir() on plain files

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Dec 06, 2008 at 10:31:33PM +0000, Al Viro wrote:
> On Fri, Dec 05, 2008 at 11:21:22PM -0800, Brad Boyer wrote:
> 
> > Apple added hard links with osx. Back when I wrote hfsplus originally,
> > there wasn't any such thing on HFS+ either. The support for the hack
> > that added the appearance of hard links was added after I stopped
> > maintaining the code. It's really just a matter of hiding the real
> > file in a special place and putting stubs with special settings where
> > all the links appear in the namespace. In all truth both HFS and HFS+
> > are inherently incompatible with the very concept of a hard link or
> > having a file still exist after removal. It's just a horrible hack
> > that makes it look like it works.
> 
> Well...  That's not too unreasonably way to do them, actually - just
> treat your hidden directory as real inode table.  The tricky part is
> keeping i_ino stable through all that, but if it's done...
> 
> FWIW, fs/qnx4 uses a somewhat similar scheme, IIRC.
> 
> To make things workable you need
> 	* hidden directory that would never be reachable by syscalls
> 	* recognizable files that would contain references to names
> in that directory
> 	* lookup that would normally lead to such file getting inode
> of object in hidden directory
> 	* link(2) with target being *not* one of those would start with
> moving target to hidden directory and creating such "link" file in its
> place.  Then it would create a link file in source.  That, BTW, would have
> interesting implications wrt locking - we'd need to modify target's parent,
> which can get sucky.
> 	* unlink(2) with victim dentry not being a "link" file *and* being
> busy would move the victim to hidden.  Otherwise it'd kill the victim
> outright.
> 	* final iput() of file that'd been taken to hidden does deletion
> of directory entry in hidden if i_nlink is 0.
> 
> The interesting part is locking - you need to deal with lookup vs. eviction
> of link(2) target to hidden and with rename vs. the same thing.

PS: that's for handling link(2) on non-directories with that kind of layout,
*NOT* for file-cross-directory hybrids or links to directories.