udev: better support device-mapper/LVM

Do not ignore them completely.

udev/udev-block-add-change

@@ -23,6 +23,11 @@ if [ -n "`ls -A /sys/$DEVPATH/holders 2> /dev/null`" ]; then
     xs_remove
     exit 0
 fi
+# ... and used device-mapper devices
+if [ -n "$DM_NAME" ] && /usr/sbin/dmsetup info "$DM_NAME" | grep -q "^Open count:.*[1-9]"; then
+    xs_remove
+    exit 0
+fi
 # ... and "empty" loop devices
 if [ "$MAJOR" -eq 7 -a ! -d /sys/$DEVPATH/loop ]; then
     xs_remove
@@ -48,6 +53,10 @@ fi
 if [ -d /sys/$DEVPATH/loop ]; then
     DESC=$(cat /sys/$DEVPATH/loop/backing_file)
 fi
+# and for device-mapper
+if [ -n "$DM_NAME" ]; then
+    DESC="$DM_NAME"
+fi
 
 # Get lock only in dom0 - there are so many block devices so it causes xenstore
 # deadlocks sometimes.

udev/udev-qubes-block.rules

@@ -11,7 +11,7 @@ KERNEL=="xvda|xvdb|xvdc*|xvdd", ENV{UDISKS_IGNORE}="1"
 ENV{MAJOR}=="202", GOTO="qubes_block_end"
 
 # Skip device-mapper devices
-ENV{MAJOR}=="253", GOTO="qubes_block_end"
+ENV{MAJOR}=="253", ENV{DM_NAME}=="snapshot-*", GOTO="qubes_block_end"
 
 IMPORT{db}="QUBES_EXPOSED"
 ACTION=="add", IMPORT{program}="/usr/bin/flock /var/run/qubes-udev.lock -c /usr/libexec/qubes/udev-block-add-change"