fixup! test(nix): add basic functionality testing infra

checks/default.nix

@@ -29,6 +29,7 @@ let
 
           # Add any packages you need inside test scripts here
           environment.systemPackages = with pkgs; [
+            f3
             genpass
             keyutils
           ];

checks/nested.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+set -euxo pipefail
+
+blkdev="/dev/vdb"
+mnt1=$(mktemp -d)
+mnt2=$(mktemp -d)
+pw=$(genpass)
+uuid=$(uuidgen)
+
+# link user and session keyrings so that the key can be found by the kernel
+keyctl link @u @s
+
+echo "$pw" | bcachefs format \
+    --verbose \
+    --encrypted \
+    --uuid "$uuid" \
+    --fs_label test-fs \
+    "$blkdev"
+
+udevadm settle
+
+echo "$pw" | bcachefs mount -v "UUID=$uuid" "$mnt1"
+
+fallocate --length 2G "$mnt1/fs.img"
+
+bcachefs format \
+    --verbose \
+    "$mnt1/fs.img"
+
+loopdev=$(losetup --find --show "$mnt1/fs.img")
+
+udevadm settle
+
+mount "$loopdev" "$mnt2"
+
+f3write "$mnt1"
+f3write "$mnt2"
+
+f3read "$mnt1"
+f3read "$mnt2"