[Xen-changelog] [xen stable-4.5] tools: pygrub: if partition table is empty, try treating as a whole disk

[patchbot@xxxxxxx]

commit 812406cf2b6731d07f0f840d799fcfa5917dbaf4
Author:     Ian Campbell <ian.campbell@xxxxxxxxxx>
AuthorDate: Thu Nov 5 14:46:12 2015 +0000
Commit:     Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
CommitDate: Mon Feb 29 15:47:42 2016 +0000

    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@xxxxxxxxxx>
    Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>
    Acked-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
    Cc: 745419-forwarded@xxxxxxxxxxxxxxx
    (cherry picked from commit fb31b1475f1bf179f033b8de3f0e173006fd77e9)
    (cherry picked from commit 6c9b1bcce4fcc872edddd44f88390a67d5954069)
---
 tools/pygrub/src/pygrub | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/pygrub/src/pygrub b/tools/pygrub/src/pygrub
index 3ec52fd..00cf77e 100755
--- a/tools/pygrub/src/pygrub
+++ b/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):
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.5

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