[PATCH] overlayfs, doc: Do not allow lower layer recreation with redirect_dir enabled

[Vivek Goyal <vgoyal@redhat.com>]

Currently we seem to support lower layer recreation and re-use with existing
upper until and unless "index" or "metadata only copy up" feature is
enabled.

If redirect_dir feature is enabled then re-creating/modifying lower layers
will break things. For example.

- mkdir lower lower/foo upper work merged
- touch lower/foo/foo-child
- mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=work,redirect_dir=on none merged
- mv merged/foo merged/bar
- ls merged/bar/ (this should list foo-child)

- umount merged
- mv lower/foo lower/baz
- mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=work,redirect_dir=on none merged
- ls merged/bar/  (Now foo-child has disappeared)

IOW, modifying lower layers did not crash overlay but it resulted in
directory contents being lost and that can be unexpected. So don't
support lower layer recreation/modification when redirect_dir is enabled
at any point of time.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 Documentation/filesystems/overlayfs.rst | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 660dbaf0b9b8..1d1a8da7fdbc 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -371,8 +371,8 @@ conflict with metacopy=on, and will result in an error.
 [*] redirect_dir=follow only conflicts with metacopy=on if upperdir=... is
 given.
 
-Sharing and copying layers
---------------------------
+Sharing, copying and recreating lower 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
@@ -388,8 +388,12 @@ though it will not result in a crash or deadlock.
 
 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
-or "metadata only copy up" feature is enabled.
+different lower layer path, is allowed, unless any of the following features
+is enabled at any point of time.
+
+- inode index
+- metadata only copy up
+- redirect_dir
 
 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
-- 
2.25.4