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

Ady ady-sf at hotmail.com
Tue Feb 17 11:16:09 PST 2015

> 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
Any (common) ISO image size should be a multiple of 2048 bytes, as this 
is the normal bytes_per_sector value.

For isohybrid images, the result should be a multiple of 1MiB (1048576 
bytes), which is itself a multiple of 2048 bytes.

So, in addition to adding the corresponding MBR and partition table to 
the original ISO image, the isohybrid tool should also check that the 
ISO image is a multiple of 1MiB. If it is not, then the isohybrid tool 
should first add a "tail" of zeroes to the ISO image up to the next 
multiple of 1MiB, and then add the MBR and partition table.

I do not know the exact complete procedure that the TAILS team is using 
to build the original ISO image, and how it is transformed into an 
isohybrid image (there are multiple tools for each, sometimes 

If the isohybrid method (whichever tool you are using) is not applied 
to your original ISO image, is such ISO image' size a multiple of 2048 

What's the difference, in bytes, between the non-hybrid ISO image and 
the isohybrid one? Could you please provide both exact sizes? It is 
possible that some "limit" (e.g. around the 1GiB) might be triggering 
some condition (I am not saying that this is a bug in the isohybrid 
tools, or that such case might trigger a problem; just assuming that 
there might be some potential chance that some case might have not been 
considered in the source code).

I don't know whether there is some bug in any one of the isohybrid 
tools included in Syslinux (there might be). Which one are you using? 
Is it part of the same exact version of the ISOLINUX bootloader? Or, 
perhaps, are you using an external tool (i.e. not using any of the ones 
included in Syslinux)?

Have you tried not using the Syslinux-related packages, and instead 
using the upstream official *pre-built* binaries (downloaded from 
kernel.org) to build a new ISO image and then to make it isohybrid 
(both pre-built binaries, the isolinux.bin bootloader and the isohybrid 
tool, from the upstream official archives)?

Additionally, IIRC, the "isolinux-debug.bin" file cannot be used for 
isohybrid images.


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