qubes-prepare-vm-kernel: More robust u2mfn version detection

Prior to this commit, if the u2mfn module had never been installed on
the system *via DKMS* before, then the "u2mfn_ver" variable would not be
populated properly, due to the reliance on "dkms status" which can only
detect currently installed DKMS modules.

Fix this issue by looking for u2mfn's DKMS configuration file in
/usr/src and and grep'ing for the module version.

Signed-off-by: M. Vefa Bicakci <m.v.b@runbox.com>

kernel-modules/qubes-prepare-vm-kernel

@@ -26,11 +26,28 @@ set -e
 basedir=/var/lib/qubes/vm-kernels
 
 function recompile_u2mfn() {
-    kver=$1
-    u2mfn_ver=`dkms status u2mfn|tail -n 1|cut -f 2 -d ' '|tr -d ':,'`
-    if ! modinfo -k "$kver" -n u2mfn >/dev/null; then
-        dkms install u2mfn/$u2mfn_ver -k $kver --no-initrd
+    kver="${1}"
+
+    if modinfo -k "$kver" -n u2mfn >/dev/null 2>&1; then
+        echo "---> The u2mfn kernel module is already installed for ${kver}."
+        return
     fi
+
+    echo "---> Compiling the u2mfn kernel module"
+
+    # ls's -v option sorts the u2mfn directories according to
+    # their version numbers, which lets "tail -n1" capture the
+    # latest version.
+    u2mfn_ver="$(ls -1v /usr/src/u2mfn-*/dkms.conf 2>/dev/null | tail -n1 | \
+        xargs --no-run-if-empty grep -F PACKAGE_VERSION | \
+        cut -d= -f2 | tr -d "\"'")"
+
+    if test -z "${u2mfn_ver}"; then
+        echo "ERROR: Could not detect the location of the DKMS tree of the u2mfn module."
+        exit 1
+    fi
+
+    dkms install --no-initrd "u2mfn/${u2mfn_ver}" -k "${kver}"
 }
 
 function build_modules_img() {
@@ -78,7 +95,6 @@ fi
 
 echo "--> Building files for $kernel_version in $output_dir"
 
-echo "---> Recompiling kernel module (u2mfn)"
 recompile_u2mfn "$kernel_version"
 mkdir -p "$output_dir"
 cp "/boot/vmlinuz-$kernel_version" "$output_dir/vmlinuz"