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List:       grub-devel
Subject:    Re: Several GNU projects wondering about the reason for mformat partition table
From:       Mihai Moldovan <ionic () ionic ! de>
Date:       2019-05-20 22:09:43
Message-ID: 014172be-737a-d755-d77a-af0b3ff3f3c2 () ionic ! de
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* On 5/20/19 6:52 PM, Thomas Schmitt wrote:
> in the course of a problem search between GRUB, Guix, and xorriso we found
> the trigger of a problem with some old Macbook EFI firmware in a gesture
> of mformat.
> Line 1375 of
>   http://svn.savannah.gnu.org/viewvc/mtools/trunk/mformat.c?revision=506&view=markup
> has
> 
>   /* install fake partition table pointing to itself */
> 
> Now we are wondering for which use case there is this MBR partition table
> entry starting with LBA 0 and claiming the whole device.
> 
> The SVN at savannah shows that this gesture was introduced with revision 4
> in may 2002 by "aknaff". My hope is that this was you and that you can tell
> grub-devel (Cc'ed) more about the motivation.

No authoritative answer from me, but if this is coupling GPT + MBR, creating a
MBR partition table from LBA 0 to the end of (reachable) device space (which is
not necessarily the actual device space/end for limiting factors such as disk
sector size [mis-]usage and the like) is a pretty common thing to do and called
"protective MBR". The rationale being to "trick" tools that are only MBR-aware
into thinking that the device is NOT unpartitioned and hence not erroring out or
offering to "correct the situation".



Mihai


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