storage: implement two-layers of dm-snapshot in block-snapshot script

Have dm-snapshot of dm-snapshot. The first layer is to "cache" changes
done by base volume holder (TemplateVM in case of root.img), the second
layer is to hold changes do by snapshot volume holder (AppVM in case of
root.img). In case of Linux VMs the second layer is normally done inside
of VM (original volume is exposed read-only). But this does not work for
non-Linux VMs, orr even Linux but without qubes-specific startup
scripts.

This is first part of the change - actual construction of two layers of
dm-snapshot, not plugged in to core scripts yet.

QubesOS/qubes-issues#2256

linux/system-config/block-snapshot

@@ -53,14 +53,21 @@ get_dev() {
 }
 
 get_dm_snapshot_name() {
+  local base cow cow2
+
   base=$1
   cow=$2
+  cow2=$3
 
-  echo snapshot-$(stat -c '%D:%i' "$base")-$(stat -c '%D:%i' "$cow")
+  name="snapshot-$(stat -c '%D:%i' "$base")-$(stat -c '%D:%i' "$cow")"
+  if [ -n "$cow2" ]; then
+    name="$name-$(stat -c '%D:%i' "$cow2")"
+  fi
+  echo "$name"
 }
 
 create_dm_snapshot() {
-  local base_dev cow_dev base_sz
+  local base_dev cow_dev base_sz base cow dm_devname
 
   dm_devname=$1
   base=$2
@@ -77,7 +84,7 @@ create_dm_snapshot() {
 }
 
 create_dm_snapshot_origin() {
-  local base_dev base_sz
+  local base_dev base_sz dm_devname base
 
   dm_devname=$1
   base=$2
@@ -103,8 +110,14 @@ case "$command" in
         fi
         echo $p > "$HOTPLUG_STORE-params"
         echo $t > "$HOTPLUG_STORE-type"
-        base=${p/:*/}
+        base=${p%%:*}
-        cow=${p/*:/}
+        cow=${p#*:}
+        cow2=${p##*:}
+        if [ "$cow" != "$cow2" ]; then
+          cow=${cow%:*}
+        else
+          cow2=""
+        fi
 
         if [ -L "$base" ]; then
           base=$(readlink -f "$base") || fatal "$base link does not exist."
@@ -114,6 +127,10 @@ case "$command" in
           cow=$(readlink -f "$cow") || fatal "$cow link does not exist."
         fi
 
+        if [ -L "$cow2" ]; then
+          cow2=$(readlink -f "$cow2") || fatal "$cow2 link does not exist."
+        fi
+
         # first ensure that snapshot device exists (to write somewhere changes from snapshot-origin)
         dm_devname=$(get_dm_snapshot_name "$base" "$cow")
 
@@ -122,6 +139,12 @@ case "$command" in
         # prepare snapshot device
         create_dm_snapshot $dm_devname "$base" "$cow"
 
+        if [ -n "$cow2" ]; then
+          dm_devname_full=$(get_dm_snapshot_name "$base" "$cow" "$cow2")
+          create_dm_snapshot "$dm_devname_full" "/dev/mapper/$dm_devname" "$cow2"
+          dm_devname="$dm_devname_full"
+        fi
+
         if [ "$t" == "snapshot" ]; then
           #that's all for snapshot, store name of prepared device
           xenstore_write "$XENBUS_PATH/node" "/dev/mapper/$dm_devname"
@@ -152,8 +175,14 @@ case "$command" in
     case $t in
       snapshot|origin)
         p=$3
-        base=${p/:*/}
+        base=${p%%:*}
-        cow=${p/*:/}
+        cow=${p#*:}
+        cow2=${p##*:}
+        if [ "$cow" != "$cow2" ]; then
+          cow=${cow%:*}
+        else
+          cow2=""
+        fi
 
         if [ -L "$base" ]; then
           base=$(readlink -f "$base") || fatal "$base link does not exist."
@@ -163,6 +192,10 @@ case "$command" in
           cow=$(readlink -f "$cow") || fatal "$cow link does not exist."
         fi
 
+        if [ -L "$cow2" ]; then
+          cow2=$(readlink -f "$cow2") || fatal "$cow2 link does not exist."
+        fi
+
         # first ensure that snapshot device exists (to write somewhere changes from snapshot-origin)
         dm_devname=$(get_dm_snapshot_name "$base" "$cow")
 
@@ -171,6 +204,12 @@ case "$command" in
         # prepare snapshot device
         create_dm_snapshot $dm_devname "$base" "$cow"
 
+        if [ -n "$cow2" ]; then
+          dm_devname_full=$(get_dm_snapshot_name "$base" "$cow" "$cow2")
+          create_dm_snapshot "$dm_devname_full" "/dev/mapper/$dm_devname" "$cow2"
+          dm_devname="$dm_devname_full"
+        fi
+
         if [ "$t" == "snapshot" ]; then
           #that's all for snapshot, store name of prepared device
           echo "/dev/mapper/$dm_devname"
@@ -232,7 +271,7 @@ case "$command" in
         fi
 
         # get list of used (loop) devices
-        deps="$(dmsetup deps $node | cut -d: -f2 | sed -e 's#(7, \([0-9]\+\))#/dev/loop\1#g')"
+        deps="$(dmsetup deps $node -o blkdevname | cut -d: -f2 | sed -e 's#(\([a-z0-9-]\+\))#/dev/\1#g')"
 
         # if this is origin
         if [ "${node/origin/}" != "$node" ]; then
@@ -241,7 +280,8 @@ case "$command" in
             use_count=$(dmsetup info $snap|grep Open|awk '{print $3}')
             if [ "$use_count" -eq 0 ]; then
               # unused snapshot - remove it
-              deps="$deps $(dmsetup deps $snap | cut -d: -f2 | sed -e 's#(7, \([0-9]\+\))#/dev/loop\1#g')"
+              deps="$deps $(dmsetup deps $snap -o blkdevname | cut -d: -f2 |\
+                   sed -e 's#(\([a-z0-9-]\+\))#/dev/\1#g')"
               log debug "Removing $snap"
               dmsetup remove $snap
             fi
@@ -265,11 +305,18 @@ case "$command" in
           dmsetup remove $node
         fi
 
-        # try to free loop devices
+        # try to free unused devices
         for dev in $deps; do
           if [ -b "$dev" ]; then
             log debug "Removing $dev"
-            losetup -d $dev 2> /dev/null || true
+            case $dev in
+              /dev/loop*)
+                losetup -d $dev 2> /dev/null || true
+                ;;
+              /dev/dm-*)
+                dmsetup remove $dev 2> /dev/null || true
+                ;;
+            esac
           fi
         done