Make file->f_path always point to the overlay dentry so that the path in
/proc/pid/fd is correct and to ensure that label-based LSMs have access to the
overlay as well as the underlay (path-based LSMs probably don't need it).

Using my union testsuite to set things up, before the patch I see:

	[root@andromeda union-testsuite]# bash 5</mnt/a/foo107
	[root@andromeda union-testsuite]# ls -l /proc/$$/fd/
	...
	lr-x------. 1 root root 64 Jun  5 14:38 5 -> /a/foo107
	[root@andromeda union-testsuite]# stat /mnt/a/foo107
	...
	Device: 23h/35d Inode: 13381       Links: 1
	...
	[root@andromeda union-testsuite]# stat -L /proc/$$/fd/5
	...
	Device: 23h/35d Inode: 13381       Links: 1
	...

After the patch:

	[root@andromeda union-testsuite]# bash 5</mnt/a/foo107
	[root@andromeda union-testsuite]# ls -l /proc/$$/fd/
	...
	lr-x------. 1 root root 64 Jun  5 14:22 5 -> /mnt/a/foo107
	[root@andromeda union-testsuite]# stat /mnt/a/foo107
	...
	Device: 23h/35d Inode: 40346       Links: 1
	...
	[root@andromeda union-testsuite]# stat -L /proc/$$/fd/5
	...
	Device: 23h/35d Inode: 40346       Links: 1
	...

Note the change in where /proc/$$/fd/5 points to in the ls command.  It was
pointing to /a/foo107 (which doesn't exist) and now points to /mnt/a/foo107
(which is correct).

The inode accessed, however, is the lower layer.  The union layer is on device
25h/37d and the upper layer on 24h/36d.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

This patch adds two macros:

OVL_XATTR_PRE_NAME and OVL_XATTR_PRE_LEN

to present ovl_xattr name prefix and its length. Also, a
new macro OVL_XATTR_OPAQUE is introduced to replace old
*ovl_opaque_xattr*.

Fix the length of "trusted.overlay." to *16*.
Signed-off-by: Nhujianyang <hujianyang@huawei.com>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

Add helper to iterate through all the layers, starting from the upper layer
(if exists) and continuing down through the lower layers.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

OVL_PATH_PURE_UPPER -> __OVL_PATH_UPPER | __OVL_PATH_PURE
OVL_PATH_UPPER      -> __OVL_PATH_UPPER
OVL_PATH_MERGE      -> __OVL_PATH_UPPER | __OVL_PATH_MERGE
OVL_PATH_LOWER      -> 0

Multiple R/O layers will allow __OVL_PATH_MERGE without __OVL_PATH_UPPER.
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>

Overlayfs allows one, usually read-write, directory tree to be
overlaid onto another, read-only directory tree.  All modifications
go to the upper, writable layer.

This type of mechanism is most often used for live CDs but there's a
wide variety of other uses.

The implementation differs from other "union filesystem"
implementations in that after a file is opened all operations go
directly to the underlying, lower or upper, filesystems.  This
simplifies the implementation and allows native performance in these
cases.

The dentry tree is duplicated from the underlying filesystems, this
enables fast cached lookups without adding special support into the
VFS.  This uses slightly more memory than union mounts, but dentries
are relatively small.

Currently inodes are duplicated as well, but it is a possible
optimization to share inodes for non-directories.

Opening non directories results in the open forwarded to the
underlying filesystem.  This makes the behavior very similar to union
mounts (with the same limitations vs. fchmod/fchown on O_RDONLY file
descriptors).

Usage:

  mount -t overlayfs overlayfs -olowerdir=/lower,upperdir=/upper/upper,workdir=/upper/work /overlay

The following cotributions have been folded into this patch:

Neil Brown <neilb@suse.de>:
 - minimal remount support
 - use correct seek function for directories
 - initialise is_real before use
 - rename ovl_fill_cache to ovl_dir_read

Felix Fietkau <nbd@openwrt.org>:
 - fix a deadlock in ovl_dir_read_merged
 - fix a deadlock in ovl_remove_whiteouts

Erez Zadok <ezk@fsl.cs.sunysb.edu>
 - fix cleanup after WARN_ON

Sedat Dilek <sedat.dilek@googlemail.com>
 - fix up permission to confirm to new API

Robin Dong <hao.bigrat@gmail.com>
 - fix possible leak in ovl_new_inode
 - create new inode in ovl_link

Andy Whitcroft <apw@canonical.com>
 - switch to __inode_permission()
 - copy up i_uid/i_gid from the underlying inode

AV:
 - ovl_copy_up_locked() - dput(ERR_PTR(...)) on two failure exits
 - ovl_clear_empty() - one failure exit forgetting to do unlock_rename(),
   lack of check for udir being the parent of upper, dropping and regaining
   the lock on udir (which would require _another_ check for parent being
   right).
 - bogus d_drop() in copyup and rename [fix from your mail]
 - copyup/remove and copyup/rename races [fix from your mail]
 - ovl_dir_fsync() leaving ERR_PTR() in ->realfile
 - ovl_entry_free() is pointless - it's just a kfree_rcu()
 - fold ovl_do_lookup() into ovl_lookup()
 - manually assigning ->d_op is wrong.  Just use ->s_d_op.
 [patches picked from Miklos]:
 * copyup/remove and copyup/rename races
 * bogus d_drop() in copyup and rename

Also thanks to the following people for testing and reporting bugs:

  Jordi Pujol <jordipujolp@gmail.com>
  Andy Whitcroft <apw@canonical.com>
  Michal Suchanek <hramrach@centrum.cz>
  Felix Fietkau <nbd@openwrt.org>
  Erez Zadok <ezk@fsl.cs.sunysb.edu>
  Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NMiklos Szeredi <mszeredi@suse.cz>