* [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk
@ 2015-11-05 14:46 Ian Campbell
2015-11-11 14:57 ` Wei Liu
0 siblings, 1 reply; 4+ messages in thread
From: Ian Campbell @ 2015-11-05 14:46 UTC (permalink / raw)
To: ian.jackson, wei.liu2, xen-devel; +Cc: 745419-forwarded, Ian Campbell
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>
Cc: 745419-forwarded@bugs.debian.org
---
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
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk
2015-11-05 14:46 [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk Ian Campbell
@ 2015-11-11 14:57 ` Wei Liu
2015-11-11 15:39 ` Ian Jackson
0 siblings, 1 reply; 4+ messages in thread
From: Wei Liu @ 2015-11-11 14:57 UTC (permalink / raw)
To: Ian Campbell; +Cc: wei.liu2, ian.jackson, 745419-forwarded, xen-devel
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@citrix.com>
> Cc: 745419-forwarded@bugs.debian.org
Acked-by: Wei Liu <wei.liu2@citrix.com>
> ---
> 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
>
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk
2015-11-11 14:57 ` Wei Liu
@ 2015-11-11 15:39 ` Ian Jackson
2015-11-16 11:07 ` Ian Campbell
0 siblings, 1 reply; 4+ messages in thread
From: Ian Jackson @ 2015-11-11 15:39 UTC (permalink / raw)
To: Wei Liu; +Cc: 745419-forwarded, Ian Campbell, xen-devel
Wei Liu writes ("Re: [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk"):
> 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.
...
> Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
In line with my usual policy about pygrub compatibility problems, I
intend to backport this to stable trees.
Ian.
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk
2015-11-11 15:39 ` Ian Jackson
@ 2015-11-16 11:07 ` Ian Campbell
0 siblings, 0 replies; 4+ messages in thread
From: Ian Campbell @ 2015-11-16 11:07 UTC (permalink / raw)
To: Ian Jackson, Wei Liu; +Cc: 745419, xen-devel
Control: tag -1 +fixed-upstream
On Wed, 2015-11-11 at 15:39 +0000, Ian Jackson wrote:
> Wei Liu writes ("Re: [PATCH] tools: pygrub: if partition table is empty,
> try treating as a whole disk"):
> > 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.
> ...
> > Acked-by: Wei Liu <wei.liu2@citrix.com>
>
> Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
applied, thanks.
> In line with my usual policy about pygrub compatibility problems, I
> intend to backport this to stable trees.
Good idea.
Ian.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2015-11-16 11:07 UTC | newest]
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2015-11-05 14:46 [PATCH] tools: pygrub: if partition table is empty, try treating as a whole disk Ian Campbell
2015-11-11 14:57 ` Wei Liu
2015-11-11 15:39 ` Ian Jackson
2015-11-16 11:07 ` Ian Campbell
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