[syslinux] isohybrid.c, problem reports and questions

[Thomas Schmitt]

Hi,

i am currently exploring the boot data of Matthew Garret's Fedora LiveCD.
For that i read utils/isohybrid.c from git resp. 4.05.
(I assume Matthew used options  --efi --mac --type 0 .)

Some stumblestones showed up:
-----------------------------------------------------------------------

In initialise_gpt() i see unhealthy usage of string constants. Like:

  memcpy(part->name, "ISOHybrid ISO", 28);

This seems to assume dealing with 16 bit characters. (Wikipedia states about
GPT that the character set shall be UTF-16LE.)
The result in Fedora-LiveCD.iso is:

  $ dd if=Fedora-LiveCD.iso bs=1 skip=8248 count=28 | od -c
  0000000   I   S   O   H   y   b   r   i   d       I   S   O  \0   I   S
  0000020   O   H   y   b   r   i   d  \0   A   p   p   l

which is probably a patchwork from constants "ISOHybrid ISO", "ISOHybrid",
and "Apple" that can be found in the code.

FWIW:
  $ echo "ISOHybrid ISO" | iconv -f UTF-8 -t UTF-16LE | od -c
  0000000   I  \0   S  \0   O  \0   H  \0   y  \0   b  \0   r  \0   i  \0
  0000020   d  \0      \0   I  \0   S  \0   O  \0  \n  \0

-----------------------------------------------------------------------

The block counts in the Apple partitions obviously assume 512 bytes per
block, whereas the start block numbers obviously assume 2048 as announced
in the APM header.
In initialise_apm() i see two occasions where block counts are handled:

- This should probably be 4 rather than 0x10:
    part->block_count = bendian_int(0x10);

- These _count variables should probably be divided by 4 before being
  submitted to bendian_int():
    part->block_count = bendian_int(efi_count);
    part->data_count = bendian_int(efi_count);
    part->block_count = bendian_int(mac_count);
    part->data_count = bendian_int(mac_count);
  The numbers stem from read_efi_catalogue() which deals with EFI blocks
  of 512 bytes.

-----------------------------------------------------------------------

The "Last LBA" of the first GPT entry is 1329448, the size of the ISO image
in blocks of 512. But Wikipedia states that this number is inclusive.

Thus in initialise_gpt()
    part->lastLBA = lendian_64(psize);
one should probably subtract 1 from psize before submitting it to
lendian_64().

-----------------------------------------------------------------------

The second Apple partition map entry points to the VFAT image file
/isolinux/efiboot.img but announces as partition type "Apple_HFS".
Is this ok ? 

-----------------------------------------------------------------------

What is the reason for calling function reverse_uuid() ?
Is it intentional that it only byteswaps the first 8 of 16 bytes ?

-----------------------------------------------------------------------


The explored ISO image stems from
  http://mjg59.fedorapeople.org/Fedora-LiveCD.iso
Matthew advised it to me at his article "Anatomy of a Fedora 17 ISO image"
  http://mjg59.dreamwidth.org/11285.html

The current state of my emerging description is at
  http://bazaar.launchpad.net/~libburnia-team/libisofs/scdbackup/view/head:/doc/boot_sectors.txt
Newly emerging is the chapter "SYSLINUX isohybrid for UEFI and x86-Mac".
Comments, hints, bug reports are welcome.


Have a nice day :)

Thomas