diff --git a/lib/tiny-cloud/init-early b/lib/tiny-cloud/init-early
index 652aac2..ca34252 100644
--- a/lib/tiny-cloud/init-early
+++ b/lib/tiny-cloud/init-early
@@ -7,22 +7,19 @@
 expand_root() {
     skip_action expand_root && return
 
-    # explicitly use busybox, in case util-linux is also installed
-    local mountpoint=$(busybox mountpoint -n / | cut -d' ' -f1)
-    local volume=$(echo "$mountpoint" |
-        sed -Ee "s/(nvme\d+n\d|(xv|s)d[a-z])p?\d?$/\1/"
-    )
-    local partition
+    local dev=$(awk '$2 == "/" {print $1}' "$ROOT"/proc/mounts)
+    local partition=$(cat "$ROOT/sys/class/block/${dev#/dev/}/partition" 2>/dev/null)
 
-    if [ "$mountpoint" != "$volume" ]; then
+    if [ -n "$partition" ]; then
         # it's a partition, resize it
-        partition=$(echo "$mountpoint" | sed -Ee "s/.*(\d+)$/\1/")
-        echo ", +" | sfdisk -q --no-reread -N "$partition" "$volume"
-        partx -u "$volume"
+        local volume=$(readlink -f "$ROOT/sys/class/block/${dev#/dev/}/..")
+        volume="/dev/${volume##*/}"
+        echo ", +" | $MOCK sfdisk -q --no-reread -N "$partition" "$volume"
+        $MOCK partx -u "$volume"
     fi
     # resize filesystem
-    mount -orw,remount /
-    resize2fs "$mountpoint"
+    $MOCK mount -orw,remount /
+    $MOCK resize2fs "$dev"
 }
 
 has_cloud_hotplugs() { [ -n "$HOTPLUG_MODULES" ]; }
diff --git a/tests/init-early.test b/tests/init-early.test
new file mode 100755
index 0000000..2d3539d
--- /dev/null
+++ b/tests/init-early.test
@@ -0,0 +1,51 @@
+#!/usr/bin/env atf-sh
+
+. $(atf_get_srcdir)/test_env.sh
+
+export PREFIX="$srcdir"
+export MOCK=echo
+
+init_tests \
+	expand_root \
+	expand_root_partition
+
+PROVIDERS="aws azure gcp nocloud oci"
+
+expand_root_body() {
+	fake_bin test-expand-root <<-EOF
+		#!/bin/sh
+		. "$srcdir"/lib/tiny-cloud/init-early
+		expand_root
+	EOF
+	mkdir proc
+	echo "/dev/xvda / ext4 rw,noatime 0 0" > proc/mounts
+	for provider in $PROVIDERS; do
+		CLOUD="$provider" atf_check \
+			-o match:"resize2fs /dev/xvda" \
+			test-expand-root
+	done
+}
+
+expand_root_partition_body() {
+	fake_bin test-expand-root <<-EOF
+		#!/bin/sh
+		. "$srcdir"/lib/tiny-cloud/init-early
+		expand_root
+	EOF
+	mkdir -p proc sys/class/block \
+		sys/devices/pci0000:00/0000:00:1b.0/0000:01:00.0/nvme/nvme0/nvme0n1/nvme0n1p2 \
+		sys/devices/pci0000:00/0000:00:1b.0/0000:01:00.0/nvme/nvme0/nvme0n1/device
+	ln -s ../../devices/pci0000:00/0000:00:1b.0/0000:01:00.0/nvme/nvme0/nvme0n1 sys/class/block/nvme0n1
+	ln -s ../../devices/pci0000:00/0000:00:1b.0/0000:01:00.0/nvme/nvme0/nvme0n1/nvme0n1p2 sys/class/block/nvme0n1p2
+	echo 2 > sys/class/block/nvme0n1p2/partition
+
+	echo "/dev/nvme0n1p2 / ext4 rw,noatime 0 0" > proc/mounts
+	for provider in $PROVIDERS; do
+		CLOUD="$provider" atf_check \
+			-o match:"sfdisk .*/dev/nvme0n1" \
+			-o match:"partx .*/dev/nvme0n1" \
+			-o match:"resize2fs /dev/nvme0n1p2" \
+			test-expand-root
+	done
+}
+