Pull vfs file handling updates from Christian Brauner:
 "This contains Amir's work to fix a long-standing problem where an
  unprivileged overlayfs mount can be used to avoid fanotify permission
  events that were requested for an inode or superblock on the
  underlying filesystem.

  Some background about files opened in overlayfs. If a file is opened
  in overlayfs @file->f_path will refer to a "fake" path. What this
  means is that while @file->f_inode will refer to inode of the
  underlying layer, @file->f_path refers to an overlayfs
  {dentry,vfsmount} pair. The reasons for doing this are out of scope
  here but it is the reason why the vfs has been providing the
  open_with_fake_path() helper for overlayfs for very long time now. So
  nothing new here.

  This is for sure not very elegant and everyone including the overlayfs
  maintainers agree. Improving this significantly would involve more
  fragile and potentially rather invasive changes.

  In various codepaths access to the path of the underlying filesystem
  is needed for such hybrid file. The best example is fsnotify where
  this becomes security relevant. Passing the overlayfs
  @file->f_path->dentry will cause fsnotify to skip generating fsnotify
  events registered on the underlying inode or superblock.

  To fix this we extend the vfs provided open_with_fake_path() concept
  for overlayfs to create a backing file container that holds the real
  path and to expose a helper that can be used by relevant callers to
  get access to the path of the underlying filesystem through the new
  file_real_path() helper. This pattern is similar to what we do in
  d_real() and d_real_inode().

  The first beneficiary is fsnotify and fixes the security sensitive
  problem mentioned above.

  There's a couple of nice cleanups included as well.

  Over time, the old open_with_fake_path() helper added specifically for
  overlayfs a long time ago started to get used in other places such as
  cachefiles. Even though cachefiles have nothing to do with hybrid
  files.

  The only reason cachefiles used that concept was that files opened
  with open_with_fake_path() aren't charged against the caller's open
  file limit by raising FMODE_NOACCOUNT. It's just mere coincidence that
  both overlayfs and cachefiles need to ensure to not overcharge the
  caller for their internal open calls.

  So this work disentangles FMODE_NOACCOUNT use cases and backing file
  use-cases by adding the FMODE_BACKING flag which indicates that the
  file can be used to retrieve the backing file of another filesystem.
  (Fyi, Jens will be sending you a really nice cleanup from Christoph
  that gets rid of 3 FMODE_* flags otherwise this would be the last
  fmode_t bit we'd be using.)

  So now overlayfs becomes the sole user of the renamed
  open_with_fake_path() helper which is now named backing_file_open().
  For internal kernel users such as cachefiles that are only interested
  in FMODE_NOACCOUNT but not in FMODE_BACKING we add a new
  kernel_file_open() helper which opens a file without being charged
  against the caller's open file limit. All new helpers are properly
  documented and clearly annotated to mention their special uses.

  We also rename vfs_tmpfile_open() to kernel_tmpfile_open() to clearly
  distinguish it from vfs_tmpfile() and align it the other kernel_*()
  internal helpers"

* tag 'v6.5/vfs.file' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  ovl: enable fsnotify events on underlying real files
  fs: use backing_file container for internal files with "fake" f_path
  fs: move kmem_cache_zalloc() into alloc_empty_file*() helpers
  fs: use a helper for opening kernel internal files
  fs: rename {vfs,kernel}_tmpfile_open()