tools: pygrub: if partition table is empty, try treating as a whole disk

pygrub (in identify_disk_image()) detects a DOS style partition table
via the presence of the 0xaa55 signature at the end of the first
sector of the disk.

However this signature is also present in whole-disk configurations
when there is an MBR on the disk. Many filesystems (e.g. ext[234])
include leading padding in their on disk format specifically to enable
this.

So if we think we have a DOS partition table but do not find any
actual partition table entries we may as well try looking at it as a
whole disk image. Worst case is we probe and find there isn't anything
there.

This was reported by Sjors Gielen in Debian bug #745419. The fix was
inspired by a patch by Adi Kriegisch in
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745419#27

Tested by genext2fs'ing my /boot into a new raw image (works) and
then:
   dd if=/usr/lib/grub/i386-pc/g2ldr.mbr of=img conv=notrunc bs=512 count=1

to add an MBR (with 0xaa55 signature) to it, which after this patch
also works.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: 745419-forwarded@bugs.debian.org
(cherry picked from commit fb31b1475f)
(cherry picked from commit 6c9b1bcce4)

tools/pygrub/src/pygrub

@@ -157,6 +157,11 @@ def get_partition_offsets(file):
         else:
             part_offs.append(offset)
 
+    # We thought we had a DOS partition table, but didn't find any
+    # actual valid partition entries. This can happen because an MBR
+    # (e.g. grubs) may contain the same signature.
+    if not part_offs: part_offs = [0]
+
     return part_offs
 
 class GrubLineEditor(curses.textpad.Textbox):