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

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Jul 09, 2020 at 10:02:20AM -0400, Vivek Goyal wrote:
> 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

I probably should add "nfs_export" to the list as well. Though it is
implicitly there as enabling nfs export requires to enable index. But
saying it explicitly is even better.

Vivek

>  
>  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
>