Adopt S20urandom

From buildroot/package/urandom-scripts/S20urandom

overlay/etc/init.d/S20urandom

@@ -0,0 +1,74 @@
+#! /bin/sh
+#
+# Preserve the random seed between reboots. See urandom(4).
+#
+
+# Quietly do nothing if /dev/urandom does not exist
+[ -c /dev/urandom ] || exit 0
+
+URANDOM_SEED="/var/lib/random-seed"
+
+# shellcheck source=/dev/null
+[ -r "/etc/default/urandom" ] && . "/etc/default/urandom"
+
+if pool_bits=$(cat /proc/sys/kernel/random/poolsize 2> /dev/null); then
+	pool_size=$((pool_bits/8))
+else
+	pool_size=512
+fi
+
+check_file_size() {
+	[ -f "$URANDOM_SEED" ] || return 1
+	# Try to read two blocks but exactly one will be read if the file has
+	# the correct size.
+	size=$(dd if="$URANDOM_SEED" bs="$pool_size" count=2 2> /dev/null | wc -c)
+	test "$size" -eq "$pool_size"
+}
+
+init_rng() {
+	if check_file_size; then
+		printf 'Initializing random number generator: '
+		dd if="$URANDOM_SEED" bs="$pool_size" of=/dev/urandom count=1 2> /dev/null
+		status=$?
+		if [ "$status" -eq 0 ]; then
+			echo "OK"
+		else
+			echo "FAIL"
+		fi
+		return "$status"
+	fi
+}
+
+save_random_seed() {
+	printf 'Saving random seed: '
+	if touch "$URANDOM_SEED" 2> /dev/null; then
+		old_umask=$(umask)
+		umask 077
+		dd if=/dev/urandom of="$URANDOM_SEED" bs="$pool_size" count=1 2> /dev/null
+		status=$?
+		umask "$old_umask"
+		if [ "$status" -eq 0 ]; then
+			echo "OK"
+		else
+			echo "FAIL"
+		fi
+	else
+		status=$?
+		echo "SKIP (read-only file system detected)"
+	fi
+	return "$status"
+}
+
+case "$1" in
+	start|restart|reload)
+		# Carry a random seed from start-up to start-up
+		# Load and then save the whole entropy pool
+		init_rng && save_random_seed;;
+	stop)
+		# Carry a random seed from shut-down to start-up
+		# Save the whole entropy pool
+		save_random_seed;;
+	*)
+		echo "Usage: $0 {start|stop|restart|reload}"
+		exit 1
+esac