HFS+ resource fork lookup breaks opendir() library function.  Since
opendir first calls open() with O_DIRECTORY flag set.  O_DIRECTORY means
"refuse to open if not a directory".  The open system call in the kernel
does a check for inode->i_op->lookup and returns -ENOTDIR.  So if
hfsplus_file_lookup is set it allows opendir() for plain files.

Also resource fork lookup in HFS+ does not work.  Since it is never
invoked after VFS permission checking.  It will always return with
-EACCES.

When we call opendir() on a file, it does not return NULL.  opendir()
library call is based on open with O_DIRECTORY flag passed and then
layered on top of getdents() system call.  O_DIRECTORY means "refuse to
open if not a directory".

The open() system call in the kernel does a check for: do_sys_open()
-->..--> can_lookup() i.e it only checks inode->i_op->lookup and returns
ENOTDIR if this function pointer is not set.

In OSX, we can open "file/rsrc" to get the resource fork of "file".  This
behavior is emulated inside hfsplus on Linux, which means that to some
degree every file acts like a directory.  That is the reason lookup()
inode operations is supported for files, and it is possible to do a lookup
on this specific name.  As a result of this open succeeds without
returning ENOTDIR for HFS+

Please see the LKML discussion thread on this issue:
http://marc.info/?l=linux-fsdevel&m=122823343730412&w=2

I tried to test file/rsrc lookup in HFS+ driver and the feature does not
work.  From OSX:

$ touch test
$ echo "1234" > test/..namedfork/rsrc
$ ls -l test..namedfork/rsrc
--rw-r--r-- 1 tuxera staff 5 10 dec 12:59 test/..namedfork/rsrc

[sougata@ultrabook tmp]$ id
uid=1000(sougata) gid=1000(sougata) groups=1000(sougata),5(tty),18(dialout),1001(vboxusers)

[sougata@ultrabook tmp]$ mount
/dev/sdb1 on /mnt/tmp type hfsplus (rw,relatime,umask=0,uid=1000,gid=1000,nls=utf8)

[sougata@ultrabook tmp]$ ls -l test/rsrc
ls: cannot access test/rsrc: Permission denied

According to this LKML thread it is expected behavior.

http://marc.info/?t=121139033800008&r=1&w=4



I guess now that permission checking happens in vfs generic_permission() ?
 So it turns out that even though the lookup() inode_operation exists for
HFS+ files.  It cannot really get invoked ?.  So if we can disable this
feature to make opendir() work for HFS+.

Signed-off-by: Sougata Santra <sougata@tuxera.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Cc: Vyacheslav Dubeyko <slava@dubeyko.com>
Cc: Anton Altaparmakov <aia21@cam.ac.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>