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Hi Miklos,

One of the popular use cases for Kata Containers is running docker-in-docker.  That is, a container image is run which in turn will make use of a container runtime to do a container build.

When combined with virtio-fs, we end up with a configuration like:
xfs/ext4 -> overlayfs -> virtio-fs -> overlayfs 

As discussed in [1], per overlayfs spec: 
"The upper filesystem will normally be writable and if it is it must support the creation of trusted.* extended attributes, and must provide valid d_type in readdir responses, so NFS is not suitable."

At this point, with virtio-fs this, [2], check fails.  

Vivek mentioned that bypassing this check *may* be feasible, [3].  Can you help identify if this is feasible, and rationale for NFS not being available as an upper (though, more importantly, understanding what needs to be done to add proper support for virtio-fs as upper layer).

Thanks,
Eric 

[1] - https://github.com/kata-containers/runtime/issues/1888
[2] - https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/overlayfs/super.c#n753
[3] - https://github.com/kata-containers/runtime/issues/1888#issuecomment-518259095