Re: Booting El Torito image from a partition table

[Emilio Lazo Zaia]

I did it.

On 01/12/15 11:57, Andrei Borzenkov wrote:
> 30.11.2015 23:16, Emilio Lazo Zaia пишет:
> > > 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...
> Sorry, it is memdisk
>
> http://www.syslinux.org/wiki/index.php/MEMDISK
>
> > 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.
> Yes. GRUB modules must match GRUB run-time (which is compiled into
> single kernel.img).
>
> > 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.
> No, you misunderstand. The problem is not boot block, but core.img
> itself - it lacks partition driver(s) and so is not able to access
> partitions. It simply does not see them.
>
> It is theoretically possible to decompose core.img (actually,
> eltorito.img in this case) and add more drivers and adjust prefix as
> needed and pack back into single executable. But I would try memdisk
> first :)
Yes, I mean eltorito.img code, which is cdboot.img + core.img 
concatenated; right? So the code that lacks partition drivers is the 
core.img part... I've extracted core.img from Kaspersky CD:

- In Kaspersky CD (kav_rescue_10.iso) I've extracted the boot code + 
core.img, located at the embedding area, just after the MBR of the 
hybrid image:

# dd if=kav_rescue_10.iso bs=512 skip=1 count=54 of=eltorito.img

- Dropped the cdboot.img part:

# dd if=eltorito.img bs=2048 skip=1 of=core.img

- I created a helper partition (20MB, on /dev/sdf31) in btrfs and 
mounted it on /x.
- Extracted all contents of kav_rescue_10.iso on my home, cd to 
boot/grub and copy extracted core.img to i386-pc to be used with 
grub-install:

# cp [/path/to/extracted/core.img] i386-pc/core.img
# grub-install /dev/sdf31 --boot-directory=/x -d i386-pc

- Now we have a GRUB installation in /dev/sdf31. This GRUB is the same 
as the Kaspersky, so the modules on CD will match the compilation.
- Copy all files from /boot/grub on Kaspersky to /x
- Edited a new grub.cfg on helper partition (/x) to search for a file on 
the real Kaspersky CD, set root, prefix and load the config file, with 
something like:

search -f /rescue/KRD.VERSION --set=root
set prefix=($root)/boot/grub
configfile=/boot/grub/grub.cfg

Now, in the master syslinux partition, simply chainloading to the 
helper's partition boot sector will execute GRUB boot code and load 
config file on the real Kaspersky CD. The result: Kaspersky splash 
screen is shown from within syslinux. :-)

As a side question: which filesystems supports embeeding other than btrfs?

Thanks a lot.
Emilio

PS: I'll try also with memdisk, but I guess the whole kav_rescue_10.iso 
must lie on syslinux master partition.