udev: reset loopback block device ownership and mode on detach

Loopback block devices are agressively reused, without being removed in
between. This means various inode attributes on their device nodes will
– so far – remain in effect between uses of the devices. Since there are
applications which change access mode/ownership of such devices after
attaching files to them, let's undo this again when we detect them to be
unused again.

Fixes: #37745

rules.d/60-block.rules

@@ -11,3 +11,6 @@ ACTION=="change", SUBSYSTEM=="scsi", ENV{DEVTYPE}=="scsi_device", TEST=="block",
 ACTION!="remove", SUBSYSTEM=="block", \
   KERNEL=="loop*|mmcblk*[0-9]|msblk*[0-9]|mspblk*[0-9]|nvme*|sd*|vd*|xvd*|bcache*|cciss*|dasd*|ubd*|ubi*|scm*|pmem*|nbd*|zd*|rbd*|zram*|ublkb*", \
   OPTIONS+="watch"
+
+# Reset access rights to each loopback device once it gets detached.
+SUBSYSTEM=="block", ACTION=="change", ENV{DISK_MEDIA_CHANGE}=="1", TEST!="loop/backing_file", GROUP="disk", MODE="660"

test/units/TEST-17-UDEV.loop-own.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# SPDX-License-Identifier: LGPL-2.1-or-later
+# shellcheck disable=SC2317
+set -ex
+set -o pipefail
+
+# shellcheck source=test/units/util.sh
+. "$(dirname "$0")"/util.sh
+
+at_exit() (
+    set +e
+
+    [[ -d "$TMPDIR" ]] && rm -rf "$TMPDIR"
+
+    udevadm control --log-level=info
+)
+
+trap at_exit EXIT
+
+udevadm control --log-level=debug
+
+TMPDIR="$(mktemp -d)"
+truncate -s 16M "$TMPDIR"/foo.raw
+mkfs.ext4 "$TMPDIR"/foo.raw
+
+D="$(systemd-dissect --attach --loop-ref=schlumpf "$TMPDIR"/foo.raw)"
+
+udevadm wait --timeout=30 --settle /dev/disk/by-loop-ref/schlumpf
+
+SAVED_GROUP="$(stat -c "%g" "$D")"
+SAVED_MODE="$(stat -c "%a" "$D")"
+
+chmod 705 "$D"
+chown root:65534 "$D"
+
+test "$(stat -c "%g %a" "$D")" = "65534 705"
+
+losetup -d "$D"
+
+for _ in {0..4}; do
+    udevadm settle --timeout=5
+
+    if [[ "$(stat -c "%g" "$D")" = "$SAVED_GROUP" && "$(stat -c "%a" "$D")" = "$SAVED_MODE" ]] ; then
+        exit 0
+    fi
+
+    sleep 1
+done
+
+exit 1