[U-Boot] [PATCH] EFI: find EFI system partition by legacy MBR partition type

[Andre Przywara]

Hi,

On 06/07/17 11:19, Thomas Schmitt wrote:
> Hi,
> 
> i am the upstream developer of program xorriso which packs up Debian arm64
> ISOs.
> 
> Here is my minority opinion from a discussion with Andre Przywara:
> 
> To my opinion, if U-boot is used as EFI implementation, then it should
> not consider as bootable any "active" MBR partitions or "Legacy BIOS
> Bootable" GPT partitions (see is_bootable() in disk/part_efi.c).

First thing to note here is that U-Boot does not really have an
understanding yet of whether it is acting as an EFI implementation or
not. At this stage it simply looks for boot partition *candidates*,
which will then later be examined more closely to find boot scripts or
EFI apps. Adding one more partition to that list should not cause much
harm, I think.

> While the proposed change of behavior is an undisputable improvement,
> my objection is that the main boot loaders in distro ISOs are GRUB and
> SYSLINUX. Both do not expect that the "active" partition gets booted by
> the firmware but rather that their own MBR at the start of the ISO gets
> started by BIOS or the ESP is brought up by EFI.
> The MBR programs in the ISOs do not go on with booting the "active"
> partition but rather hop onto the El Torito boot image programs in the ISO.

A second thing to note is that there is some fundamental difference here
between the ARM world and x86.
For ARM U-Boot was so far just piggy-backing on the bootable MBR flag to
find /boot partition candidates. I am not sure if there is actually some
spec or standard covering this behaviour, it was just convenient and
worked quite well in the (mostly embedded) ARM world.
And on ARM U-Boot never considered the "boot code" in a boot sector
(neither on the MBR or on an active partition) - which is probably x86
code anyway.

Now I am not sure how this maps to the combination of U-Boot and x86 - I
am not very familiar with the combination of those two.
Does U-Boot actually support chain-loading boot sectors on x86? Or does
it entirely focus on loading either EFI apps or Linux kernels / U-Boot
boot scripts? Maybe Simon could shed some light on this?

Cheers,
Andre.



> 
> The Legacy BIOS Bootable bit of GPT is explicitely not an EFI boot
> indicator. UEFI 2.4 says in table 20 : "UEFI boot manager (see chapter 3)
> must ignore this bit when selecting a UEFI-compliant application".
> The BootIndicator byte of MBR partitions is explicitely not for EFI.
> Table 14 says: "This field shall not be used by UEFI firmware."
> 
> So if "active" partitions are present in GRUB or SYSLINUX equipped ISOs
> they are under no circumstances intended for being booted.
> 
> 
> Currently debian ISOs for arm64 have no "active" partition. But that's
> an inner implementation detail. E.g. HDD bootable ISOs for x86 do have
> the "active"/bootable flag on the ISO 9660 partition out of tradition to
> appease mad BIOS implementations.
> It is well possible to combine x86 BIOS and arm64 EFI boot equipment
> in the same ISO image. So the need for an "active" partition might arise.
> 
> 
> Have a nice day :)
> 
> Thomas
>