Syslinux - Feb 2012 - [PATCH 3/3] syslinux: support isolinux configuration files

This one is the most critical for my app, but also possibly the most 
controversial to integrate.

A few applications that convert isolinux ISOs to bootable USB, including 
mine, copy the ISO content to a VFAT formatted partition and then add 
syslinux to make it bootable.
This would work great... if isolinux and syslinux shared the same 
defaults when it comes to configuration files.

Unfortunately, isolinux requires an isolinux.cfg in /boot/isolinux, 
/isolinux or /, whereas syslinux requires a syslinux.cfg in 
/boot/syslinux, /syslinux or /.

Therefore, my proposal is to add the isolinux config and config dirs to 
the syslinux searcheable locations, so that the process of bootable ISO 
-> bootable USB can be made easier.

My limited testing with recent Linux image seems to indicate that doing 
just that works quite well, and thus I plan to use an ldlinux.sys that 
has been patched in such a way. However, I would prefer if an official 
ldlinux.sys could be used.

I also haven't found a good answer on why isolinux and syslinux require 
the use of isolated config repositories.
I can understand why having separate config directory can be useful for 
the creation of hybrid images, that work for both disk and ISO, and why 
they may require different init. But in that case, I don't exactly see 
why directory precedence rules couldn't be applied: if syslinux then 
look first in syslinux expected dirs and fallback to isolinux ones, if 
isolinux than look first in isolinux expected dirs, then fallback to 
syslinux ones.
The only discussion I was able to locate with regards to cfg location 
was in December 2005, but didn't seem to answer that question.

In this patch, the extra config and directories for isolinux are being 
added after the existing ones (including /) so I believe it should be 
fairly safe for existing syslinux users. Precedence rules should avoid 
issues of a syslinux.cfg being overridden by an existing isolinux.cfg 
whereas that wasn't the case in earlier versions.

The only problem I can remotely see is if someone produced weird image 
with an isolinux.cfg in /syslinux/ but placed their actual syslinux.cfg 
in / for some reason. But I fail to see why anyone would create an image 
like that except by mistake, as it wouldn't be bootable as an ISO.

Regards,

/Pete
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Am 14.02.2012 15:05, schrieb Pete Batard:
> This one is the most critical for my app, but also possibly the most
> controversial to integrate.
> 
> A few applications that convert isolinux ISOs to bootable USB, including
> mine, copy the ISO content to a VFAT formatted partition and then add
> syslinux to make it bootable.
> This would work great... if isolinux and syslinux shared the same
> defaults when it comes to configuration files.
NAK.

If we want to unify configuration file names, we first need to unify
behaviour:

1) "Working directory"
* ISOLINUX: Working directory is /, config file is hardcoded to a
restricted set of paths.
* SYSLINUX: Working directory is the directory the config file lives,
the same where ldlinux.sys lives.
* PXELINUX: Working directory is the "prefix", config file is given as
a
relative path.

2) ".." support:
* ISOLINUX: No ".." support in paths
* SYSLINUX: ".." in paths supported
* PXELINUX: No ".." support in paths

When we unified directory structure in the Arch Linux ISO, this caused
problems. We now have several "loadconfig" tricks to get a unified
configuration:

* ISOLINUX: /isolinux/isolinux.cfg:

DEFAULT loadconfig

LABEL loadconfig
  CONFIG /arch/boot/syslinux/archiso.cfg
  APPEND /arch/

(Note that the path /arch/ is hardcoded).

* SYSLINUX: Install syslinux in /arch/boot/syslinux/ldlinux.sys, config
file /arch/boot/syslinux/syslinux.cfg:

DEFAULT loadconfig

LABEL loadconfig
  CONFIG archiso.cfg
  APPEND ../../

(Note that the path /arch/ is not hardcoded, you can move the toplevel
directory anywhere on the medium).

* PXELINUX: Directly tell syslinux the right prefix ($TFTPDIR/arch/) and
have "boot/syslinux/archiso.cfg" as the configuration file.

This was the ONLY way to unify configuration files across all three
systems and at the same time use _relative paths_ everywhere. It would
have been easier if isolinux and pxelinux had allowed "..".

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On Tue, Feb 14, 2012 at 09:05, Pete Batard <pete at akeo.ie>
wrote:
> This one is the most critical for my app, but also possibly the most
> controversial to integrate.
>
> A few applications that convert isolinux ISOs to bootable USB, including
> mine, copy the ISO content to a VFAT formatted partition and then add
> syslinux to make it bootable.
> This would work great... if isolinux and syslinux shared the same defaults
> when it comes to configuration files.
>
> Unfortunately, isolinux requires an isolinux.cfg in /boot/isolinux,
> /isolinux or /, whereas syslinux requires a syslinux.cfg in /boot/syslinux,
> /syslinux or /.
For building my ISOs, I put a tiny syslinux.cfg in the root directory
that INCLUDEs my main config and use paths relative to that (no
leading "/").  This allows the same config file on the ISO as the USB
and the same LABEL statements in my PXELINUX config files (or I could
just INCLUDE the syslinux.cfg or main config).

-- 
-Gene