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Re: How important is the MBR partition offset of grub-mkrescue ?

From: Vladimir 'φ-coder/phcoder' Serbinenko
Subject: Re: How important is the MBR partition offset of grub-mkrescue ?
Date: Sun, 03 Nov 2013 14:05:50 +0100
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> This yields an MBR (copied from $old_iso) with partition start
> LBA 64 (16 blocks of 2048)
Hm, that would mean that some crapware like adobe might write between
MBR and first partition.
> My test machine boots via BIOS. The ISO image is only equipped
> for BIOS, anyway. I have no means to test UEFI with partition
> offset 16.
> Does contemporary grub-mkrescue cause xorriso to produce GPT
> for UEFI ? (That would be a new adventure with -partition_offset.)
Yes, and we add HFS+ as well. Isn't this HFS+ catalog sufficient for the
problem at hand?
> If anybody has opportunity and curiosity:
> It should be possible to append option -partition_offset 16
> to the options of grub-mkrescue, so that it reaches xorriso
> as one of the ${source} arguments.
> There was a bug with -partition_offset with older versions
> of xorriso, which caused Debian to stop using this option
> in its amd64 UEFI capable images. So better get newest stable
> xorriso for such a test. Currently this is:
Can we detect presence of this bug?

> Theoretical problems:
Additional problem you don't mention: consumption of space by additional
headers. We use xorriso for making floppies as well (and it works).
> El Torito booting from CD/DVD should not be influenced by
> this unusual layout.  The ISO 9660 image beginning at LBA 0
> is quite the same as without that option.
> Nevertheless, there was a report that Apple "Snow Leopard"
> refused to mount an ISO image with partition offset 16.
> Note well that this was not about booting.
But it will most likely swallow HFS+-hybrid disk without any problem.
Perhaps we should generate HFS+-hybrid even for BIOS-only CDs.
Another solution is to have a hybrid ISO + FAT or ISO + HFS+ + FAT layouts.
> Further, one never knows what the booting operating system
> expects as layout of USB sticks. I consider the current layout
> with LBA 1 to be more confusing. But that's only me ...
The reason for the specified partition is twofold:
- Some computers check for presence of an active partition.
- To avoid some software overwriting the ISO
If user desires to have a partition for data he can always add a second
partition but it doesn't solve the problem of accessing ISO files.

> Have a nice day :)
> Thomas
> _______________________________________________
> Grub-devel mailing list
> address@hidden

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