Re: GPT and UEFI booting

[Martin Husemann <martin%duskware.de@localhost>]

On Tue, Apr 05, 2022 at 12:28:16PM +0200, tlaronde%polynum.com@localhost wrote:
> No. This affects only a GPT partition not the "whole" pseudo-partition
> put as a MBR first slot. One can set these flags without flagging the
> PMBR as active, so...

This is a grey area - the UEFI spec requires the "active" byte in the PMBR
to be 0x00 (section 5.2.3 Protective MBR, table 5-4 BootIndicator), to
make sure a non-UEFI BIOS does not consider this a bootable partition
and then says "Must be ignored by UEFI implementations".

However, buggy UEFI seems to exist, so the gpt "create" command grew the -A
option to mark the PMBR entry covering the whole GPT disk as "active".

This is meant for expert use firmware bugs workarounds, and there seems
to be no official way to toggle it off again. You can play tricks with dd
to write a 0 byte to the proper offset ;-}

Martin