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Re: Booting El Torito image from a partition table

From: Andrei Borzenkov
Subject: Re: Booting El Torito image from a partition table
Date: Wed, 2 Dec 2015 21:47:29 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

02.12.2015 08:20, Emilio Lazo Zaia пишет:
> I did it.

Congratulations :)

> 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:

No, you did not :)

> - 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

Both steps are completely redundant, see below.

> - 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:

You could simply mount partition.

> # cp [/path/to/extracted/core.img] i386-pc/core.img

Redundant as well.

> # grub-install /dev/sdf31 --boot-directory=/x -d i386-pc

grub-install always creates core.img from scratch, so none of previous
steps was needed.

But that is quite different. You mentioned that ISO was created by
grub-mkrescue, and grub-mkrescue does not copy source files necessary to
create core.img. But if these files are present, everything becomes far
more simple.

> - 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. :-)

You can also spare jumping through indirection hoops and simply create
image file that will point directly to your partition, similar to

address@hidden:~$ lsblk
sda                8:0    0 465,8G  0 disk
├─sda1             8:1    0   500M  0 part /boot/efi
├─sda2             8:2    0    40M  0 part
├─sda3             8:3    0     3G  0 part
├─sda4             8:4    0 446,5G  0 part /
└─sda5             8:5    0  15,8G  0 part [SWAP]
loop0              7:0    0   512M  0 loop
├─loop0p1        259:0    0   100M  0 loop
├─loop0p2        259:1    0   411M  0 loop
├─loop0p1 (dm-0) 252:0    0   100M  0 part /mnt
└─loop0p2 (dm-1) 252:1    0   411M  0 part /tmp/kasp
address@hidden:~$ cat /mnt/syslinux.cfg
label Kaspersky
        menu Kaspersky
        com32 chain.c32
        append file=kaspersky.img
address@hidden:~$ grub-mkimage -O i386-pc -o /tmp/temp.img -p
'(,msdos2)/boot/grub' -d /tmp/kasp/boot/grub/i386-pc  biosdisk
part_msdos iso9660
address@hidden:~$ cat /tmp/kasp/boot/grub/i386-pc/lnxboot.img
/tmp/temp.img > /tmp/kaspersky.img
address@hidden:~$ sudo mv /tmp/kaspersky.img /mnt

Now selecting Kaspersky in syslinux boots directly into partition2.
Replace "msdos" with "gpt" if needed and adjust partition number.

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


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