[syslinux] Installing syslinux on a purely virtual disk

[Sebastian Herbszt]

Richard W.M. Jones via Syslinux wrote:
> I'm trying to add boot support to
> https://github.com/libguestfs/nbdkit/tree/master/plugins/floppy
>
> This NBD server plugin generates a completely virtual FAT filesystem
> and partition table.  Nothing is ever written to a file, and it must
> run without anything needing to be root, loop mounting etc.  Running
> "syslinux --install <something>" is a non-starter.
>
> So I've looked at how syslinux works and I see that it adds the files
> LDLINUX.SYS and LDLINUX.C32 to the FAT filesystem -- I can easily
> emulate this bit in the plugin since we are already creating a full
> FAT32 filesystem on the fly.

How flexibel do you want this to be? Will users be allowed to supply
their own SYSLINUX files or are those fixed and provided by "nbdkit"?

> However the problem is that syslinux also creates a boot sector [ie.
> core/bios/diskboot.inc in the syslinux source] and I suppose it must
> encode the offset of the LDLINUX.SYS file.  I haven't quite worked out
> the details.

Syslinux provides the following required files:

MBR - "mbr.bin" and "mbr_bin.c"
boot sector - "ldlinux.bss" and "bootsect_bin.c"
ldlinux - "ldlinux.sys" and "ldlinux_bin.c"
ldlinux COM32 - "ldlinux.c32" and ldlinuxc32_bin.c

Those c files can be used in "virtual-floppy.c".

The installer "linux/syslinux.c" uses "libinstaller/syslxmod.c" to
modify the content (boot sector and ldlinux).

> The question, is there a way to simulate the work that syslinux does
> in a reasonably supportable way that won't break on future updates of
> syslinux?
>
> Rich.
>

The Sect1Ptr0_VAL and Sect1Ptr1_VAL magic values didn't change in
almost a decade. So a simple search and replace approach should work
for a while. On the other hand the required offsets are embedded in
"ldlinux.sys" (Sect1Ptr0, Sect1Ptr1 with Sect1Ptr0Ptr and Sect1Ptr1Ptr
in the extended patch area). Since "ldlinux.sys" requires patching
anyway this approach should work even longer since it is also used by
the installer itself.

If you want to use a fixed SYSLINUX version you can use pre-patched
files. I tried that and got "ldlinux.sys" running but it failed to load
ldlinux.c32.

I used "syslinux_mbr" in "create_mbr" and made the partition bootable.
"create_partition_boot_sector" was modified to use "syslinux_bootsect"
with Sect1Ptr0 = 0x80 and Sect1Ptr1 = 0. The value 0x80 was chosen
because the plugin put the first file (ldlinux.sys) there.
"ldlinux.sys" itself was pre-patched by the official installer.
Therefore I created a compatible temporary file system with "mkfs.vfat
-f 1 -F 32 -R 32 -s 32 -v".
"ldlinux.c32" was copied to the directory to be also served by "nbdkit"
but it didn't fully work. Hope this helps anyway.

Sebastian