give the ext4 image a disk label

 - this allows the kernel cmdline and fstab to refer to it by label
   instead of by /dev/sda or /dev/vda (depending on whether the disk is
   attached to qemu via virtio or not)
 - it allows other consumers of the image to detect that this is an
   image created by debvm and then treat it accordingly

3 files changed, 15 insertions, 6 deletions

diff --git a/README.md b/README.md
index 6331652..fd1c8cc 100644
--- a/README.md
+++ b/README.md
@@ -40,9 +40,10 @@ installed:
 What is this image format precisely?
 ====================================
 
-The image is a sparse ext4 file system image. It contains the root filesystem
-of a (Debian) installation including an init system and a kernel. There is no
-partition table or bootloader. The following paths are assumed inside:
+The image is a sparse ext4 file system image with a disk label "debvm". It
+contains the root filesystem of a (Debian) installation including an init
+system and a kernel. There is no partition table or bootloader. The following
+paths are assumed inside:
  * `/bin/true` is used to detect the architecture of an image
  * `/vmlinuz` must be a symbolic link pointing to a regular file containing
    the kernel.
diff --git a/debvm-create b/debvm-create
index 11e145c..c44b67d 100755
--- a/debvm-create
+++ b/debvm-create
@@ -122,7 +122,7 @@ set -- \
 	"$@"
 
 # unless we set up a fstab, / will be read-only
-set -- "--customize-hook=echo '/dev/vda / ext4 defaults 0 1' >"'"$1/etc/fstab"' "$@"
+set -- "--customize-hook=echo 'LABEL=debvm / ext4 defaults 0 1' >"'"$1/etc/fstab"' "$@"
 
 # set up a hostname
 set -- \
@@ -176,4 +176,4 @@ mmdebstrap "$@"
 
 truncate -s "$SIZE" "$IMAGE"
 /sbin/resize2fs "$IMAGE"
-/sbin/tune2fs -i 0 -O extents,uninit_bg,dir_index,has_journal "$IMAGE"
+/sbin/tune2fs -L debvm -i 0 -O extents,uninit_bg,dir_index,has_journal "$IMAGE"
diff --git a/debvm-run b/debvm-run
index 4ec9122..9b26a76 100755
--- a/debvm-run
+++ b/debvm-run
@@ -46,6 +46,14 @@ done
 test -f "$IMAGE" || die "image '$IMAGE' not found"
 test -s "$IMAGE" || die "image '$IMAGE' is empty"
 
+if ! printf '\123\357' | cmp --bytes=2 "$IMAGE" - 1080; then
+	die "image '$IMAGE' is not in ext4 format"
+fi
+
+if ! printf 'debvm\000' | cmp --bytes=6 "$IMAGE" - 1144; then
+	die "image '$IMAGE' was not created by debvm-create (wrong disk label)"
+fi
+
 cleanup() {
 	set +x
 	test -n "$KERNELTMP" && rm -f "$KERNELTMP"
@@ -78,7 +86,7 @@ INITRDNAME=$(/sbin/debugfs "$IMAGE" -R "stat initrd.img" | sed 's/Fast link dest
 test -n "$KERNELNAME" || die "failed to discover kernel image"
 test -n "$INITRDNAME" || die "failed to discover initrd image"
 
-KERNEL_CMDLINE="root=/dev/vda rw"
+KERNEL_CMDLINE="root=LABEL=debvm rw"
 NETDEV="user,id=net0"
 
 set -- \