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Re: [bug #46716] Protective MBR partition is not marked as bootable

From: Thomas Schmitt
Subject: Re: [bug #46716] Protective MBR partition is not marked as bootable
Date: Sat, 19 Dec 2015 20:37:04 +0100


Andrei Borzenkov wrote:
> a) are we sure every EFI system out there accepts MBR (Apple?)

At least my favorite layout (see my recent mail
address@hidden) is explicitely
promised by UEFI 2.4, "5.2.1 Legacy Master Boot Record (MBR)".

The Apple problems in the blog of mjg, who invented the
ISOLINUX+GRUB2 layout as of Fedora or Debian amd64, seemed
rather related to the need for HFS(+). The Macs made him use
MBR partition type 0x00, IIRC.
(and older ones from within his endeavor)

Since Vladimir provided HFS+ for libisofs, he might have some
Apple knowledge in reserve.

> b) GPT has nice feature of self-identifying block size.

How this ?
There are no block size fields in the GPT storage format.
UEFI 2.4, 5.3.1 frightens me by

  "The device may present a logical block size that is not 512 bytes long."

which would not play well with our habit to create image files
which only later get onto some device.

i wrote:
> > Nevertheless, your overlapping layout would have the appeal of
> > giving a mountable partition:
> > [...]
> > It would travel on the ticket that EFI shall ignore MBR partition
> > type 0x00.

> I'm not sure where this assumptions comes from. EFI does not say
> anything about other partition types,

UEFI 2.4, 5.2.1 Legacy Master Boot Record (MBR)

  "A Partition Record that contains an OSType value of zero or a
   SizeInLBA value of zero may be ignored."

Table 14 shows OSType as byte at offset 4 in the MBR partition
entry. Aka "partition type".

The question is how far this ignoring goes.
ishoybrid+GRUB2 as of mjg hopes for effective non-existence
as far as EFI and its do-not-overlap demand is concerned.

Have a nice day :)


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