initramfs: use symlink for rw root.img, instead of dm-linear

Do not add 1:1 dm-linear when root.img (/dev/xvda) is read-write. This
was to always have root device at /dev/mapper/dmroot, but unfortunately
grub can't parse it properly (fails at looking for partition number of
/dev/xvda1). There was a hack specifically for this, but it caused other
partitions on xvda unavailable.
On the other hand, symlink is enough to be able to mount always the same
device. Now, grub detect that root fs is on /dev/xvda1, which is
enough to have grub2-install working. Unfortunate effect is that it puts
root=/dev/xvda1 at kernel command line, which breaks AppVMs based on
this TemplateVM. But it's easier to fix kernel command line, than grub
device detection logic.

Fixes "initramfs: add support for root.img with partition table"

QubesOS/qubes-issues#2557

dracut/full-dmroot/qubes_cow_setup.sh

@@ -87,6 +87,7 @@ EOF
 
     echo "0 `cat /sys/class/block/$ROOT_DEV/size` snapshot /dev/$ROOT_DEV /dev/xvdc2 N 16" | \
         dmsetup --noudevsync create dmroot || die "Qubes: FATAL: cannot create dmroot!"
+    dmsetup mknodes dmroot
     log_end
 else
     log_begin "Qubes: Doing R/W setup for TemplateVM..."
@@ -99,17 +100,7 @@ EOF
     fi
     while ! [ -e /dev/xvdc1 ]; do sleep 0.1; done
     mkswap /dev/xvdc1
-    if [ "$ROOT_DEV" = "xvda1" ]; then
-        # grub2-probe for some reason can't parse dm-linear pointing at
-        # partition device directly, but can when it points at it indirectly
-        # through base device and offset; lets make life easier for users of
-        # grub2-install
-        DM_ROOT="/dev/xvda $(cat /sys/class/block/$ROOT_DEV/start)"
-    else
-        DM_ROOT="/dev/$ROOT_DEV 0"
-    fi
-    echo "0 `cat /sys/class/block/$ROOT_DEV/size` linear $DM_ROOT" | \
-        dmsetup create dmroot || { echo "Qubes: FATAL: cannot create dmroot!"; exit 1; }
+    printf 'KERNEL=="%s", SYMLINK+="mapper/dmroot"' "$ROOT_DEV" >> \
+        /etc/udev/rules.d/99-root.rules
     log_end
 fi
-dmsetup mknodes dmroot

dracut/simple/init.sh

@@ -52,6 +52,7 @@ EOF
 
     echo "0 `cat /sys/class/block/$ROOT_DEV/size` snapshot /dev/$ROOT_DEV /dev/xvdc2 N 16" | \
         dmsetup create dmroot || { echo "Qubes: FATAL: cannot create dmroot!"; exit 1; }
+    dmsetup mknodes dmroot
     echo Qubes: done.
 else
     echo "Qubes: Doing R/W setup for TemplateVM..."
@@ -64,20 +65,9 @@ EOF
     fi
     while ! [ -e /dev/xvdc1 ]; do sleep 0.1; done
     mkswap /dev/xvdc1
-    if [ "$ROOT_DEV" = "xvda1" ]; then
-        # grub2-probe for some reason can't parse dm-linear pointing at
-        # partition device directly, but can when it points at it indirectly
-        # through base device and offset; lets make life easier for users of
-        # grub2-install
-        DM_ROOT="/dev/xvda $(cat /sys/class/block/$ROOT_DEV/start)"
-    else
-        DM_ROOT="/dev/$ROOT_DEV 0"
-    fi
-    echo "0 `cat /sys/class/block/$ROOT_DEV/size` linear $DM_ROOT" | \
-        dmsetup create dmroot || { echo "Qubes: FATAL: cannot create dmroot!"; exit 1; }
+    ln -s ../$ROOT_DEV /dev/mapper/dmroot
     echo Qubes: done.
 fi
-dmsetup mknodes dmroot
 
 modprobe ext4