Booting FreeBSD with Syslinux

[Teske, Devin]

On Jul 31, 2013, at 6:38 AM, Daniel O'Connor wrote:

> Hi,
> I am trying to make a FreeBSD 9.2 hybrid image (ie ISO & USB from the same file) and as part of that I need to use syslinux. Unfortunately I can't get Syslinux's mboot.c32 to run the kernel or loader as suggested at http://www.syslinux.org/wiki/index.php/Mboot.c32 - it reports "Invalid Multiboot image: neither ELF header nor a.out kludge found".
> 
> I suspect I would be able to use memdisk as I have used that in the past with syslinux (for 7.x) however this was seems a lot cleaner and easier to generate.
> 
> Has anyone had any success with this?
> 

Absolutely.

You can download and dissect the following to show you how it's done...

http://druidbsd.sourceforge.net/download.shtml#FreeBSD_Druid

It uses syslinux, as you can see here:

http://druidbsd.cvs.sourceforge.net/viewvc/druidbsd/druidbsd/druid/src/boot/freebsd/isolinux.cfg?revision=1.1&view=markup

As you can see, I use the memdisk.c32 module.

Notice that I append "iso raw" as options to memdisk.c32.

You may then be asking yourself... if the thing to do is to use memdisk with an ISO... what's in said ISO?

http://druidbsd.cvs.sourceforge.net/viewvc/druidbsd/druidbsd/druid/mdroot/

Basically...

1. Kernel
2. Boot Forth
3. mfsroot

All that goes into an ISO. When memdisk initiates the ISO, "/boot/cdboot" then gets invoked.

>From there, /boot/loader gets invoked.
>From there, the /boot/loader.rc is loaded.
>From there, loader.4th is loaded.
>From there, loader.conf is loaded.

In a normal FreeBSD boot process, then the kernel gets loaded (I've modified that to not load the kernel until later -- because my Forth boot menu presents a kernel selection option)

>From there, beastie.4th is loaded.
>From there, beastie-start is called and then the beastie menu is drawn.

NOTE: I've skipped a whole bunch of other Forth modules that were loaded "at-once" indirectly

>From there, the user makes any boot option choices, and presses ENTER to boot.
>From there, mfsroot.gz is loaded.
>From there, /stand/sysinstall gets invoked.
>From there, /install.cfg gets invoked.
>From there, /stand/fis gets invoked.
>From there, /dev/iso9660/druid gets mounted onto /cdrom (this ISO9660 volume is actually _not_ the ISO that memdisk booted, but rather this is the actual CDROM (or DVD) that you booted from (which contains both the syslinux boot loader *and* the ISO it booted *and* anything else you want to access).

At this point, /cdrom is your ticket to freedom, busting out of the double-encapsulation (first encapsulation is wrapping the kernel+forth+mfsroot into an ISO, second level of encapsulation is from within the mfsroot; from within the mfsroot, the GEOM provided /dev/iso9660/<VOLID> is an escape hatch to the level *above* the ISO the mfsroot was embedded within).
-- 
Devin

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