current-modules-overlay.sh

#!/bin/bash

function die {
	msg="$1"
	shift
	printf "$msg\n" "$@" 1>&2
	exit 1
}

module_base_dir="$(pkg-config --variable module_directory kmod 2>/dev/null)"
: ${module_base_dir:=/lib/modules}
if ! [[ "$module_base_dir" =~ ^/ ]]; then
	die 'Module directory "%s" was not an absolute path' "$module_base_dir"
fi

if mountpoint -q "$module_base_dir"; then
	# if the overlay already exists, assume it's set up properly and succeed without doing any work
	exit 0
fi

if [[ "$module_base_dir" =~ [=,:] ]]; then
	die 'Module directory "%s" contains a special character, and cannot be overlay mounted' "$module_base_dir"
fi

tmpfs_dir="$module_base_dir/../current-modules"
if ! mountpoint -q "$tmpfs_dir"; then
	die 'The current modules tmpfs is not mounted at "%s"' "$tmpfs_dir"
fi

kernel_version="$(uname -r 2>/dev/null)"
if [[ -n "$kernel_version" ]]; then
	current_modules_dir="$module_base_dir/$kernel_version"
	# Remove any whiteout file so the current modules will show up
	if [[ -c "$current_modules_dir" ]] && [[ "$(stat -c '%r' "$current_modules_dir")" = "0" ]]; then
		rm "$current_modules_dir"
	fi
fi

overlay_workdir="$module_base_dir/../current-modules-workdir"
mkdir -p "$overlay_workdir" || exit $?

# Concatenation is safe because we know module_base_dir starts with a slash
tmpfs_module_base_dir="$tmpfs_dir$module_base_dir"
# The overlayfs module may not be loaded, so attempt to load it from the current module tmpfs. Failure is not fatal; the real test is if mount works.
modprobe -d "$tmpfs_dir" overlay
# The overlayfs documentation says "Offline changes to the lower tree are only allowed if the “metacopy”, “index”, “xino” and “redirect_dir” features have not been used", so disable all of those features
mount -t overlay -o "metacopy=off,index=off,xino=off,redirect_dir=off,lowerdir=$tmpfs_module_base_dir,upperdir=$module_base_dir,workdir=$overlay_workdir" current-modules-overlay "$module_base_dir" || exit $?
exit 0