OVL_FS() is used to get a struct ovl_fs from a sturct super_block, but
we don't have any check to determine if the superblock is valid or not.

This can lead to unexpected behaviors or bugs that are pretty hard to
track down.

Add an explicit WARN_ON_ONCE() check to OVL_FS() to make sure it's
always used with a valid overlayfs superblock.

To avoid enabling this additional pendatic check everywhere, introduce
the new config option CONFIG_OVERLAY_FS_DEBUG, that can be used in the
future also for other additional debugging checks.

Maybe a nicer solution could be to return an error from OVL_FS() when
it's used with an invalid superblock and propagate the error in the rest
of overlayfs code, but for now having at least the possibility to
trigger a warning can help to catch potential bugs in advance.

Changelog (v1 -> v2):
 - replace BUG_ON() with WARN_ON_ONCE()
 - introduce CONFIG_OVERLAY_FS_DEBUG

Andrea Righi (2):
      ovl: Kconfig: introduce CONFIG_OVERLAY_FS_DEBUG
      ovl: make consistent use of OVL_FS()

 fs/overlayfs/Kconfig     |  9 +++++++++
 fs/overlayfs/copy_up.c   |  2 +-
 fs/overlayfs/export.c    | 10 +++++-----
 fs/overlayfs/inode.c     |  8 ++++----
 fs/overlayfs/namei.c     |  2 +-
 fs/overlayfs/ovl_entry.h | 16 ++++++++++++++++
 fs/overlayfs/super.c     | 12 ++++++------
 fs/overlayfs/util.c      | 18 +++++++++---------
 8 files changed, 51 insertions(+), 26 deletions(-)