Syslinux - Dec 2016 - isohybrid boot from logical partition

Hi,

Duncan Elliot wrote:
> As far as I can see, this essentially trims off the first 3 bytes of the
> custom isohybrid MBR image before applying it to the ISO.
You probably mean this line:

        memcpy(mbr, &isohdpfx[hd0 + 3 * partok], MBRSIZE);

The indize do not apply to bytes but to byte arrays of size MBRSIZE.

    extern unsigned char isohdpfx[][MBRSIZE];

(hd0 + 3 * partok) selects a particular MBR template from an array of
six MBRs. See also
  http://www.syslinux.org/wiki/index.php?title=Isohybrid#MBR_selection

It tells which isohd*.bin file is selected by which combination of options.

> does anyone have suggestions for what modification I'd need to look at
> to support this on a logical parttion?
Sorry, not me. One will probably have to analyze the code of MBR and/or
isolinux.bin to find out why it is not working with your situation.

(Did you already tell which partitions exist and in which one do you want
 to put your ISO ?)


Have a nice day :)

Thomas

Thanks for the clarification Thomas.

Thomas Schmitt wrote:
> (Did you already tell which partitions exist and in which one do you want
>  to put your ISO ?)
I've tried a number of different structures, but the results are
consistent (boots fine from primary partitions, not from logical
partitions).

For example for a disk partitioned like below:

Disk /dev/sdc: 3.8 GiB, 4007657472 bytes, 7827456 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xfc25117c

Device     Boot   Start     End Sectors  Size Id Type
/dev/sdc1          2048  616447  614400  300M 83 Linux
/dev/sdc2        616448 1230847  614400  300M 83 Linux
/dev/sdc3       1230848 1845247  614400  300M 83 Linux
/dev/sdc4       1845248 7827455 5982208  2.9G  5 Extended
/dev/sdc5       1847296 2461695  614400  300M 83 Linux
/dev/sdc6       2463744 3078143  614400  300M 83 Linux


I 'dd' the ISO to partitions 2, 3, 5 and 6.
The ISO will boot successfully (when chainloaded from grub) on 2 and
3, but not on 5 and 6.

Thanks,
Duncan



On 29 December 2016 at 19:35, Thomas Schmitt <scdbackup at gmx.net>
wrote:
> Hi,
>
> Duncan Elliot wrote:
>> As far as I can see, this essentially trims off the first 3 bytes of
the
>> custom isohybrid MBR image before applying it to the ISO.
>
> You probably mean this line:
>
>         memcpy(mbr, &isohdpfx[hd0 + 3 * partok], MBRSIZE);
>
> The indize do not apply to bytes but to byte arrays of size MBRSIZE.
>
>     extern unsigned char isohdpfx[][MBRSIZE];
>
> (hd0 + 3 * partok) selects a particular MBR template from an array of
> six MBRs. See also
>   http://www.syslinux.org/wiki/index.php?title=Isohybrid#MBR_selection
>
> It tells which isohd*.bin file is selected by which combination of options.
>
>
>> does anyone have suggestions for what modification I'd need to look
at
>> to support this on a logical parttion?
>
> Sorry, not me. One will probably have to analyze the code of MBR and/or
> isolinux.bin to find out why it is not working with your situation.
>
> (Did you already tell which partitions exist and in which one do you want
>  to put your ISO ?)
>
>
> Have a nice day :)
>
> Thomas
>

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

> For example for a disk partitioned like below:
> 
> Disk /dev/sdc: 3.8 GiB, 4007657472 bytes, 7827456 sectors
> Units: sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disklabel type: dos
> Disk identifier: 0xfc25117c
> 
> Device     Boot   Start     End Sectors  Size Id Type
> /dev/sdc1          2048  616447  614400  300M 83 Linux
> /dev/sdc2        616448 1230847  614400  300M 83 Linux
> /dev/sdc3       1230848 1845247  614400  300M 83 Linux
> /dev/sdc4       1845248 7827455 5982208  2.9G  5 Extended
> /dev/sdc5       1847296 2461695  614400  300M 83 Linux
> /dev/sdc6       2463744 3078143  614400  300M 83 Linux
> 
> 
> I 'dd' the ISO to partitions 2, 3, 5 and 6.
> The ISO will boot successfully (when chainloaded from grub) on 2 and
> 3, but not on 5 and 6.
> 
> Thanks,
> Duncan
> 
 
Since I am not a developer, I do not really know at this time whether 
the problem is really in the "MBR" being used in the isohybrid tool.

As mentioned at:
 
en.wikipedia.org/w/index.php?title=Master_boot_record&action=edit&sectio
n=3 

(and in the tables following that section)


The typical (modern) "MBR" supports 4 primary bootable partitions. The
meaning of "bootable" in that context refers to the possibility of 
setting the "boot" flag on (one of) them.

The classic MBR knows nothing about having a boot flag on a logical 
partition. You can set the boot flag to the Extended partition in the 
MBR's partition table, but Logical partitions are relevant in EBRs, not 
in the MBR.

In addition to the examples shown in that Wikipedia page / section, 
IIRC also Ranish Partition Manager version 2.44 beta includes a 
workaround for the limitation of 4 primary partitions within 512 bytes.

I think GAG has some workaround for the 4 partitions limitation too. 
For completeness, I should add SmartBootManager and XOSL as boot 
managers that somehow are able to boot more than 4 primary partitions, 
although I think that they use more than 512 bytes to achieve the goal.

Summing up: if the problem is indeed in the MBR being used in the 
isohybrid tool, then the workaround might be in using a "customized" 
one (such as those special ones described in the tables in the 
Wikipedia article), supporting more than 4 partitions within 512 bytes.

Regards,
Ady.