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

[Vladimir 'φ-coder/phcoder' Serbinenko]

On 03.11.2013 16:21, Thomas Schmitt wrote:
> Hi,
> 
>> Isn't this HFS+ catalog sufficient for the problem at hand?
> 
> I see the misleading link to /dev/sdb1 with Debian 6
> and with a Debian 7 based distro. Dunno which udev rule
> first looks at /dev/sdb for an ISO PVD and then links
> ./by-label to the first partition of that device.
> 
> The filesystem itself is not necessarily mounted.
> Actually, the /dev/sdb1 with start at LBA 1 cannot be mounted.
> Neither as ISO nor as HFS+ (if that was present). An offset
> of 512 bytes would spoil any address that works in an ISO image.
> 
If GPT is present it takes precedence over MBR contents. And GPT would
point nicely to HFS+
> 
>> Can we detect presence of this bug?
> 
> It is a bug of the perceiving system. Linux specific. Lifetime
> at least for Debian 6 and 7, i'd say. 
> 
> Put a grub-mkrescue generated ISO on a USB stick (e.g. /dev/sdb),
> replug, and see what your Linux has in /dev/disk/by-label.
> (If it has such a directory, at all.)
> 
So it's bug in Debian, not in xorriso?
> 
>> Additional problem you don't mention: consumption of space by additional
>> headers. We use xorriso for making floppies as well (and it works).
> 
> The feature is easy to add by user arguments of grub-mkrescue
> anyway. No need for action inside grub-mkrescue.
> I am mainly fishing for experience with pure GRUB2 boot setups
> and Linux. Most distros let BIOS boot ISOLINUX.
> 
> Is there a distro which produces its ISO images with grub-mkrescue
> and uses all or most of the BIOS, UEFI, and Apple features ?
> 
> As soon as i have a working ISO image, i can play with it.
> I never learned to build operating systems or boot loaders myself.
> It would be helpful to have a copy of the grub-mkrescue script
> which was used for image production. I understand it evolves.
> 
It's in our git repo in util/ . There are 2 versions currently: sh one
and C one. It wasn't decided yet as to whether we'll switch to C one.
http://git.savannah.gnu.org/cgit/grub.git/tree/util?h=phcoder/install_c
grub-mkrescue.in is sh one and grub-mkrescue.c is C one.
> 
>> Another solution is to have a hybrid ISO + FAT or ISO + HFS+ + FAT layouts.
> 
> But will this help against udev madness ?
>
FAT would make it look like most common USB stick.

> Whatever. If i remember correctly, we still have open plans
> about adding FAT to the super-hybrid layout.
> See the very end of
>   
> http://bazaar.launchpad.net/~libburnia-team/libisofs/scdbackup/view/head:/doc/boot_sectors.txt
> 
> (I see that i did not yet document the --grub2-boot-info
> patching.)
> 
> 
> Have a nice day :)
> 
> Thomas
> 
> 
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