[syslinux] isohybrid and ISO images whose size is not a multiple of 2048 bytes vs. VirtualBox

[intrigeri]

Hi,

at [Tails] we're in the process of shipping hybrid ISO images by
default again. We're using `isohybrid -h 255 -s 63' to do that (thanks
to the advice we got on this mailing-list a few months ago!).

And then, we've discovered that sometimes, isohybrid produces ISO
images whose size is not a multiple of 2048 bytes.

Who cares? ... may you ask. Well, apparently, VirtualBox does: it
refuses to add such an ISO image as a virtual DVD. The [details] have
been researched by my team-mate anonym (cc'd), for the curious.
According to this research, it might even be that VirtualBox is
correct in this respect.

Now, anonym also discovered that one can trivially "fix" such an ISO
image by padding it with zeroes until its size is a multiple of 2048
bytes (`truncate -s %2048 $ISO'). And then, VirtualBox is happy again.
That's what we're going to do on the short term on our side, but we
like it better when things are improved upstream as a result of our
work, hence this email. So:

What do you think shall be done on the isohybrid side? Shall it do
this round-up-to-multiple-of-2048-bytes operation itself?

[Tails] https://tails.boum.org/
[details] https://labs.riseup.net/code/issues/8891#note-9

Cheers,
-- 
intrigeri