[syslinux] Announcing a patch series for isohybrid.c

Thomas Schmitt
Mon, 23 Jun 2014 06:50:15 -0000

Hi,

> There was a seventh patch.

Yes. A late one because of more testing.
Not only my --mbr, but also --mac and --uefi promised short options
in their help text, but did not prepare the options string for
getopt(3) to recognize these options.


> It uses also reformatting.
> Don't wait for me do it.

If you explain to me what is necessary (and maybe also why),
then i could try to do it myself.


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About the pondered adjustment of ISO image size to the isohybrid
padding:

isohybrid pads up the image file to a full cylinder size, but leaves
the ISO filesystem size unchanged. This puts the backup GPT in
danger not to be copied onto medium, if the copy program uses
the ISO filesystem size. It also does not match the behavior
of mkisofs which includes its own padding in the filesystem size.

When preparing for writing the backup GPT, isohybrid.c checks
whether there is enough room for 33*512 bytes between filesystem end
and image file end. If not, then it applies padding even if the
cylinder alignment is already fulfilled.
Adjusting the image size to protect the backup GPT will therefore
cause new padding of one cylinder size.

One could check whether the last 33*512 bytes of the ISO filesystem
are occupied by a backup GPT. There are id texts and CRC-32 sums.
So it would be quite unambigous.
Nevertheless, this will stay guesswork and potentially risky for
the content of the ISO filesystem.
So i gave up the idea to unconditionally adjust filesystem size to
image file size.

It might neverthelss be a valuable option. E.g. --adjust_fs_size.


Have a nice day :)

Thomas