Ship /etc from /usr through an overlay mount

The existing tmpfile logic took care of folders that the ebuild keepdir
directive wanted to exist on the OS. However, files and symlinks were
not created, causing them to be missing if we didn't explicitly modify
the ebuild files in coreos-overlay to use tmpfiles or patching of
paths to be in /usr. We need a logic to provide /etc files from the
current /usr partition without getting stale. This can be done best
with an overlay mount which requires to keep the original /etc files
under /usr.
Move the final /etc folder of the image build to /usr/share/flatcar/etc
to serve as lower layer in the overlay. Also remove any state from the
rootfs to make sure that we don't rely on it when testing our images
before the release. What we get with an overlay mount is essentially a
similar behavior to a 3-way merge because as long as the user didn't
change the files, the old version is replaced with the new version and
as soon as the user did changes, that file is frozen and wins over the
provided old (in case of a rollback) or new versions from /usr. It does
not work on file lines but on whole file contents, yet that is also
what rpm-ostree does to my knowledge. Also, run tmpfiles once and do
the SELinux labeling to prevent files being created in the upperdir
because they were missing in the lowerdir, or because they had missing
SELinux labels.

build_library/build_image_util.sh

@@ -752,9 +752,6 @@ finish_image() {
   sudo "${BUILD_LIBRARY_DIR}/gen_tmpfiles.py" --root="${root_fs_dir}" \
       --output="${root_fs_dir}/usr/lib/tmpfiles.d/base_image_var.conf" \
       ${tmp_ignore} "${root_fs_dir}/var"
-  sudo "${BUILD_LIBRARY_DIR}/gen_tmpfiles.py" --root="${root_fs_dir}" \
-      --output="${root_fs_dir}/usr/lib/tmpfiles.d/base_image_etc.conf" \
-      ${tmp_ignore} "${root_fs_dir}/etc"
 
   # Only configure bootloaders if there is a boot partition
   if mountpoint -q "${root_fs_dir}"/boot; then
@@ -782,18 +779,72 @@ EOF
         "${BUILD_DIR}/${image_kconfig}"
   fi
 
+  # Build the selinux policy
+  if pkg_use_enabled coreos-base/coreos selinux; then
+      sudo chroot "${root_fs_dir}" bash -c "cd /usr/share/selinux/mcs && semodule -s mcs -i *.pp"
+  fi
+
+  # Run tmpfiles once to make sure that /etc has everything in place before
+  # we freeze it in /usr/share/flatcar/etc as lowerdir in the overlayfs.
+
+  # But first, to successfully run tmpfiles, we need to have all users/groups
+  # in /etc/passwd, and afterwards we can recreate the files for the dev
+  # container with flatcar-tmpfiles (not really needed but maybe nice to have
+  # as it also lands as reference in /usr/share/flatcar/etc).
+  local dbfile
+  for dbfile in passwd shadow group gshadow; do
+    sudo cp -f "${root_fs_dir}"/usr/share/baselayout/"${dbfile}" "${root_fs_dir}"/etc/
+  done
+  sudo systemd-tmpfiles --create --remove --boot --exclude-prefix=/dev --root="${root_fs_dir}"
+  for dbfile in passwd shadow group gshadow; do
+    sudo rm -f "${root_fs_dir}"/etc/"${dbfile}"
+  done
+  sudo "${root_fs_dir}"/usr/sbin/flatcar-tmpfiles "${root_fs_dir}"
+  # Now that we used the tmpfiles for creating /etc we delete them because
+  # the L, d, and C entries cause upcopies
+  sudo sed -i '/^[CLd] *\/etc\//d' "${root_fs_dir}"/usr/lib/tmpfiles.d/*
+
+  # SELinux: Label the root filesystem for using 'file_contexts'.
+  # The labeling has to be done before moving /etc to /usr/share/flatcar/etc to prevent wrong labels for these files and as
+  # the relabeling on boot would cause upcopies in the overlay.
+  if pkg_use_enabled coreos-base/coreos selinux; then
+    # TODO: Breaks the system:
+    # sudo setfiles -Dv -r "${root_fs_dir}" "${root_fs_dir}"/etc/selinux/mcs/contexts/files/file_contexts "${root_fs_dir}"
+    # sudo setfiles -Dv -r "${root_fs_dir}" "${root_fs_dir}"/etc/selinux/mcs/contexts/files/file_contexts "${root_fs_dir}"/usr
+    # For now we only try it with /etc
+    sudo setfiles -Dv -r "${root_fs_dir}" "${root_fs_dir}"/etc/selinux/mcs/contexts/files/file_contexts "${root_fs_dir}"/etc
+  fi
+
+  # Backup the /etc contents to /usr/share/flatcar/etc to serve as source
+  # for creating missing files
+  sudo cp -a "${root_fs_dir}/etc" "${root_fs_dir}/usr/share/flatcar/etc"
+  # Remove the rootfs state as it should be recreated through the
+  # tmpfiles and may not be present on updating machines. This
+  # makes sure our tests cover the case of missing files in the
+  # rootfs and don't rely on the new image. Not done for the developer
+  # container.
+  if [[ -n "${image_kernel}" ]]; then
+    local folder
+    # Everything except /boot and /usr because they are mountpoints and /lost+found because e2fsck expects it
+    for folder in "${root_fs_dir}/"*; do
+      if [ "${folder}" = "${root_fs_dir}/boot" ] || [ "${folder}" = "${root_fs_dir}/usr" ] || [ "${folder}" = "${root_fs_dir}/lost+found" ]; then
+        continue
+      fi
+      sudo rm --one-file-system -rf "${folder}"
+    done
+  else
+    # For the developer container we still need to remove the resolv.conf symlink to /run
+    # because the resolved-managed file is not present there
+    sudo rm "${root_fs_dir}/etc/resolv.conf"
+  fi
+
   # Zero all fs free space to make it more compressible so auto-update
   # payloads become smaller, not fatal since it won't work on linux < 3.2
   sudo fstrim "${root_fs_dir}" || true
   if mountpoint -q "${root_fs_dir}/usr"; then
     sudo fstrim "${root_fs_dir}/usr" || true
   fi
 
-  # Build the selinux policy
-  if pkg_use_enabled coreos-base/coreos selinux; then
-      sudo chroot "${root_fs_dir}" bash -c "cd /usr/share/selinux/mcs && semodule -s mcs -i *.pp"
-  fi
-
   # Make the filesystem un-mountable as read-write and setup verity.
   if [[ ${disable_read_write} -eq ${FLAGS_TRUE} ]]; then
     # Unmount /usr partition