Merge tag 'v4.13-rc1' into patchwork

Linux v4.13-rc1

* tag 'v4.13-rc1': (11136 commits)
  Linux v4.13-rc1
  random: reorder READ_ONCE() in get_random_uXX
  random: suppress spammy warnings about unseeded randomness
  replace incorrect strscpy use in FORTIFY_SOURCE
  kmod: throttle kmod thread limit
  kmod: add test driver to stress test the module loader
  MAINTAINERS: give kmod some maintainer love
  xtensa: use generic fb.h
  fault-inject: add /proc/<pid>/fail-nth
  fault-inject: simplify access check for fail-nth
  fault-inject: make fail-nth read/write interface symmetric
  fault-inject: parse as natural 1-based value for fail-nth write interface
  fault-inject: automatically detect the number base for fail-nth write interface
  kernel/watchdog.c: use better pr_fmt prefix
  MAINTAINERS: move the befs tree to kernel.org
  lib/atomic64_test.c: add a test that atomic64_inc_not_zero() returns an int
  mm: fix overflow check in expand_upwards()
  ubifs: Set double hash cookie also for RENAME_EXCHANGE
  ubifs: Massage assert in ubifs_xattr_set() wrt. init_xattrs
  ubifs: Don't leak kernel memory to the MTD
  ...

1 files changed, 34 insertions, 0 deletions

diff --git a/Documentation/filesystems/overlayfs.txt b/Documentation/filesystems/overlayfs.txt
index c9e884b52698..36f528a7fdd6 100644
--- a/Documentation/filesystems/overlayfs.txt
+++ b/Documentation/filesystems/overlayfs.txt
@@ -201,6 +201,40 @@ rightmost one and going left.  In the above example lower1 will be the
 top, lower2 the middle and lower3 the bottom layer.
 
 
+Sharing and copying layers
+--------------------------
+
+Lower layers may be shared among several overlay mounts and that is indeed
+a very common practice.  An overlay mount may use the same lower layer
+path as another overlay mount and it may use a lower layer path that is
+beneath or above the path of another overlay lower layer path.
+
+Using an upper layer path and/or a workdir path that are already used by
+another overlay mount is not allowed and will fail with EBUSY.  Using
+partially overlapping paths is not allowed but will not fail with EBUSY.
+
+Mounting an overlay using an upper layer path, where the upper layer path
+was previously used by another mounted overlay in combination with a
+different lower layer path, is allowed, unless the "inodes index" feature
+is enabled.
+
+With the "inodes index" feature, on the first time mount, an NFS file
+handle of the lower layer root directory, along with the UUID of the lower
+filesystem, are encoded and stored in the "trusted.overlay.origin" extended
+attribute on the upper layer root directory.  On subsequent mount attempts,
+the lower root directory file handle and lower filesystem UUID are compared
+to the stored origin in upper root directory.  On failure to verify the
+lower root origin, mount will fail with ESTALE.  An overlayfs mount with
+"inodes index" enabled will fail with EOPNOTSUPP if the lower filesystem
+does not support NFS export, lower filesystem does not have a valid UUID or
+if the upper filesystem does not support extended attributes.
+
+It is quite a common practice to copy overlay layers to a different
+directory tree on the same or different underlying filesystem, and even
+to a different machine.  With the "inodes index" feature, trying to mount
+the copied layers will fail the verification of the lower root file handle.
+
+
 Non-standard behavior
 ---------------------