Why mounting overlayfs with root as lowerdir produce an empty /proc in mergedir?

Question

Here's the test case:

#!/bin/bash

more="$@"
saved="$EUID"

if [ "$EUID" = "0" ]; then
    echo "don't run as root"
    exit
fi

m="/tmp/mount-point"

exec sudo unshare -m /bin/bash <<UNSHARED
mkdir -p "$m"
mount -t tmpfs tmpfs "$m"
cd "$m"
mkdir top merge work

mount -t overlay overlay -o lowerdir=/,upperdir=top,workdir=work merge

#bash < /dev/tty
chroot merge sh -c 'sudo -u "#$saved" sh -c "id; ls /proc -lR"'
UNSHARED

Here's what it prints:

uid=1001(username) gid=1001(username) groups=1001(username),7(lp),10(wheel)
/proc:
total 0

My /proc, as you can probably guess, is mounted and not empty in the big fs.

Answer 1

An answer from the maintainer Miklos Szeredi:

It's just how overlayfs works. Mount a new instance of proc under /merge

So I'm guessing overlayfs is unioning filesystems starting from the specified directories not the general mount-composited image the userland is seeing. Furthermore:

There are userspace variants which make this possible; e.g. unionfs-fuse.

Thanks,
Miklos