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Re: Booting El Torito image from a partition table

From: Emilio Lazo Zaia
Subject: Re: Booting El Torito image from a partition table
Date: Mon, 30 Nov 2015 15:46:04 -0430
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Hello Andrei,

On 30/11/15 12:15, Andrei Borzenkov wrote:
30.11.2015 00:46, Emilio Lazo Zaia пишет:

On 29/11/15 01:36, Andrei Borzenkov wrote:
28.11.2015 23:05, Emilio Lazo Zaia пишет:
Only ms-sys -s worked, and also -t with the GPT version of my test...
Now everything becomes more clear. Thank you. Having all this in mind:
Do I have some kind of solution
to the scenario previously described?

Without GRUB modification the simplest solution is to create core.img
that will in turn load core.img from this ISO image. This first stage
can then be placed on your "master" partition and syslinux can load it.

You mean generate a core.img that points to the core.img contained in
these ISO images? How can a core.img created that way? I tried different
things using your approach, without success.
Yes, you are right. core.img for i386-pc platform that is built by
grub-mkrescue does not include partition support; so it has no chance to
find its files on partition even if you were able to load it via
syslinux chain.

And ISO also lacks kernel.img so we cannot build own image using it.

Sorry, I do not see easy way to do it. And even if we settle on better
support in the future, it does not help you with existing ISOs.

Did you consider using syslinux loopback driver? This way ISO would
appear as ISO (or at least non-partitioned device) to GRUB. It may work ...

I don't know how loopback on syslinux works. I guess is like the GRUB loopback module, i.e. make a device from a file and boot from it. I'll try it...

I have another idea: Make another -small and auxiliary- GRUB partition in the same USB drive and chainload it from syslinux's master partition. It worked in the sense of that I have the GRUB prompt/menu from the "auxiliary" new partition.

What I thought is changing root=, prefix= to the Kaspersky partition (or SG2D if applies) and execute GRUB2's configfile directive to the main grub.cfg on that media. It didn't worked because of some 'unknown symbols' in the case of Kaspersky... Kaspersky includes other self-made GRUB modules (e.g. a module for displaying the EULA text) and these files were compiled against the version of GRUB included in their CD, naturally.

The core.img on that CD can be extracted to be concatenated with boot.img to be able to boot the same version of GRUB included in Kaspersky disk but from the helper partition? I mean installing on the 32MB helper partition the version of GRUB2 included on the Kaspersky CD. In that case I'll be able to change root and prefix variables pointing to the Kaspersky partition and loading the correct grub.cfg with 'configfile'.

That way we can get rid of El Torito code, which don't understand partitioned devices, if I am right, and we'll not have unknown symbols because both GRUBs will be the same.

Thanks Andrei.

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