Syslinux - Dec 2016 - isohybrid boot from logical partition

Hi,
> The ISO will boot successfully (when chainloaded from grub) on 2 and
> 3, but not on 5 and 6.
Did you try with a boot loader other than GRUB ?

Afaik, the MBRs of newly bought USB sticks often contain code which
hops on the MBR code of the partition that is marked by the boot flag.

At least one could exclude that your problem is in a GRUB peculiarity.


Have a nice day :)

Thomas

Hi,

do we have bystanders who can read x86 assembler code or know easy-to-grasp
reference docs?
(I left machine code programming when i gave up using my VIC-20.)

The MBR template isohdppx.bin stems from
  http://git.zytor.com/syslinux/syslinux.git/tree/mbr/isohdpfx.S
with macro PARTITION_SUPPORT defined.

One can see the error message text in there:

bad_signature:
	call	error
	.ascii	"isolinux.bin missing or corrupt.\r\n"

It seems to be reached only if the seen isolinux.bin does not bear the
magic number of isohybrid capable isolinux.bin.

        cmpl	$HYBRID_MAGIC,(isolinux_hybrid_signature)
        jne	bad_signature

Since we know that the isolinux.bin binary is well ok on other partitions,
i'd say that the MBR code does not load the right blocks from disk when
it wants to get isolinux.bin.


What happens if you use a partition editor to delete all MBR partitions
from the ISO image before you put it into the partition on disk ?
(An alternative to using a partition editor is
   dd if=/dev/zero conv=notrunc bs=1 count=64 seek=446 of=...iso
)


Have a nice day :)

Thomas

Thomas Schmitt wrote:
> Since we know that the isolinux.bin binary is well ok on other partitions,
> i'd say that the MBR code does not load the right blocks from disk when
> it wants to get isolinux.bin.
Yes, these are my thoughts as well.  I think the jump from GRUB to the
isohybrid code on the logical partition is working fine, but when the
isohybrid code is trying to jump to isolinux.bin, something about the
partition structure is causing it to look in the wrong spot.

Thomas Schmitt wrote:
> What happens if you use a partition editor to delete all MBR partitions
> from the ISO image before you put it into the partition on disk ?
> (An alternative to using a partition editor is
>    dd if=/dev/zero conv=notrunc bs=1 count=64 seek=446 of=...iso
> )
I followed these steps on two of the test ISO's I am using (debian
netinst and clonezilla).
md5sum confirms the ISO's to be different after the "dd"

I then wrote this ISO's to both primary and logical partitions:
In both cases the image boots fine off the primary partition, but not
the logical (the error message remains the same -> "isolinux.bin
missing or corrupt")

What is the thought process of clearing the ISO MBR?

(it's getting late here, so might not reply to any further replies
until tomorrow)

Thanks for you help / suggestions so far
Duncan



On 29 December 2016 at 21:10, Thomas Schmitt <scdbackup at gmx.net>
wrote:
> Hi,
>
> do we have bystanders who can read x86 assembler code or know easy-to-grasp
> reference docs?
> (I left machine code programming when i gave up using my VIC-20.)
>
> The MBR template isohdppx.bin stems from
>   http://git.zytor.com/syslinux/syslinux.git/tree/mbr/isohdpfx.S
> with macro PARTITION_SUPPORT defined.
>
> One can see the error message text in there:
>
> bad_signature:
>         call    error
>         .ascii  "isolinux.bin missing or corrupt.\r\n"
>
> It seems to be reached only if the seen isolinux.bin does not bear the
> magic number of isohybrid capable isolinux.bin.
>
>         cmpl    $HYBRID_MAGIC,(isolinux_hybrid_signature)
>         jne     bad_signature
>
> Since we know that the isolinux.bin binary is well ok on other partitions,
> i'd say that the MBR code does not load the right blocks from disk when
> it wants to get isolinux.bin.
>
>
> What happens if you use a partition editor to delete all MBR partitions
> from the ISO image before you put it into the partition on disk ?
> (An alternative to using a partition editor is
>    dd if=/dev/zero conv=notrunc bs=1 count=64 seek=446 of=...iso
> )
>
>
> Have a nice day :)
>
> Thomas
>