Re: [Xen-devel] [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Nov 05, 2015 at 02:46:12PM +0000, Ian Campbell wrote:
> 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>
> Cc: 745419-forwarded@xxxxxxxxxxxxxxx

Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx>

> ---
>  tools/pygrub/src/pygrub | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tools/pygrub/src/pygrub b/tools/pygrub/src/pygrub
> index e4aedda..40f9584 100755
> --- a/tools/pygrub/src/pygrub
> +++ b/tools/pygrub/src/pygrub
> @@ -156,6 +156,11 @@ 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):
> -- 
> 2.1.4
> 

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