I am trying to mount an overlayfs inside an archivemount (as a follow-up to Layered or Virtual filesystem on Linux).

I am doing this:

mkdir -p upper,work,mount
tar zcf somefile upper/ work/ mount/
mkdir tmp
archivemount -o allow_root somefile tmp
sudo mount -t overlay -o lowerdir=/,upperdir=tmp/upper,workdir=tmp/work overlayfs tmp/mount

Note that I allow root to access the mounted archive (had to update /etc/fuse.conf for that).
It fails with:

mount: tmp/mount: wrong fs type, bad option, bad superblock on overlayfs, missing codepage or helper program, or other error.

It works with the original folders. I checked and by default, archivemount is mounting in read/write by default. I also can write a file in every folder. I also checked the access rights and they seem to be correct. Root as access to mount and can write to it.

What am I doing wrong?

unionfs does a better job unionfs-fuse $(pwd)/tmp/upper:$(pwd) $(pwd)/mount. However the mount-point seems to have to be outside of the archivemount file-system.

I see no advantage of nesting the mount-point, and has the advantage of not needing root privileges (except to install), so this may be workable.

Why

I have no idea why mount-point needs to be outside of the archivemount. If that is what the error is; I have little evidence to make a conclusion. I suspect that the mount point is the only file-access into the archivemount that is done as root, when using unionfs. Other file-access is probably done as you.

In the mount -t overlay case, where the upper-layer and work-area, can not be in the archivemount, it maybe because root has no access to this mount. Try:

archivemount somefile tmp
ls tmp/
sudo ls tmp/ #gets permission denied

So the actual problem was that, by default, archivemount does not allow root to access the mounted filesystem. You have to add the -o allow_root option for that.

Note that you will have to update /etc/fuse.conf to allow that option.