[syslinux] Isohybrid wiki page and UEFI

[Thomas Schmitt]

Hi,

Bruno Cornec wrote:
> With grub.efi (0.99 version from RHEL6) I can have access to the grub
> prompt. 

Most of the various EFI bootable ISOs use FAT images with
GRUB2 equipment. E.g. this 250 MB image:
  http://cdimage.debian.org/debian-cd/8.2.0/amd64/iso-cd/debian-8.2.0-amd64-netinst.iso


> Any other idea wrt debugging that is again welcome.

I dimly remember to have read a sparse staement by hpa
about ISO and EFI, but cannot find it in my mailbox archives
back to 2010, which is surely before that mail conversation.
(I once made some theories about the EFI-in-ISO failure and hpa
 told me that it was about other obstacles.)


Ady wrote:
> The reason genisoimage is highly modified is because... It is 
> unmaintained!

Sad but true. The cdrkit website is gone. Most "original" is
the debian source package.


> The "real" genisoimage (and any reincarnation of 
> it) might never work with Syslinux; who knows?

It works fine for BIOS purposes, assisted by a run of isohybrid,
if desired.
The Debian version has no means to express other El Torito
Platform Ids than number 0 ("80x86" as of El Torito 1.0).
This is done by using option -b.


> I cannot see any good reason to keep testing (Syslinux) with 
> "genisoimage".

Bruno obviously uses genisoimage from Fedora or RedHat, which
has options -e and -efi-boot.
The output from this program suffices for EFI purposes, as
i can tell from the output of xorriso -report_el_torito:

> El Torito images   :   N  Pltf  B   Emul  Ld_seg  Hdpt  Ldsiz         LBA
> El Torito boot img :   1  BIOS  y   none  0x0000  0x00      4       15035
> El Torito boot img :   2  UEFI  y   none  0x0000  0x00  60000          35
> El Torito img path :   1  /isolinux.bin
> El Torito img opts :   1  boot-info-table isohybrid-suitable
> El Torito img path :   2  /1.img

The file /1.img is properly advertised as EFI System Partition,
beginning at byte 35 * 2048 and covering 60000 * 512 bytes. 

Any problem with booting must sit inside file /1.img.


> No, again, the UEFI-required FAT image for booting optical media does 
> not need (nor want, nor accept, nor use) any kind of partitioning 
> scheme.

Although i fully agree to KISS, i believe that a compliant EFI
implementation should be able to stand a MBR/EBR in the system
partition.

UEFI 2.6, 1.3 Goals: 
"The design of the system partition structures also preserves all the
 structures that are currently used in the “PC-AT” boot environment.
 Thus, it is a simple matter to construct a single system that is
 capable of booting a legacy OS or an EFI-aware OS from the
 same disk."

A partition MBR (EBR) is not unusual on PC disks. The topic of
chainloading is associated with them.

UEFI 2,6, 12.3.1 System Partition:
"A System Partition supports backward compatibility with legacyi
 systems by reserving the first block (sector) of the partition
 for compatibility code."

I interpret "reserving" as "not interpreting", which matches the
expressed goal of compatibility.
Not storing own stuff at block 0 is a common property of many
filesystems.


> Now, regarding (additional) partitioning schemes being added by 
> isohybrid,

Bruno's ISO does not contain isohybrid info. It is pure El Torito
suitable for (maybe virtual) optical media only.

An isohybrid run with option --uefi would equip the ISO with
MBR and GPT. Then it could be tested on hard disk or USB stick.
See wiki example:
  isohybrid --uefi output.iso

This test would show whether the problem is with ISO 9660
or with optical media (and their block size 2048).
(I dimly remember it was declared an ISO issue.)


Have a nice day :)

Thomas