RE: [Patch] Do not clear BSS region in x86 stub

[Shao, Marshall]

Hi Ard,

Many thanks for your reply.

> systemd-boot does not use the EFI handover protocol. Please try to
> determine where this confusion comes from: are you using a UKI image
> perhaps?

I can confirm that both systemd-boot and stub will be used, and you 
are correct about the stub part, because currently, the stub will lead 
the system to handover protocol and trigger the problem.

> I am trying to understand *why* this particular change works around
> the issue. Please elaborate.

When I removed the memset, and booted to efi_decompress_kernel, the
boot service crashed, and it indicated that the memory region from _bss
to the end of boot_heap cannot be overwritten. Upon inspecting the data 
in the BSS region found one thing is that the _bss address is not fixed on 
each boot (when the MOR bit is set to 1), and it changes randomly. 

For example, in normal boot the _bss address is 0xffee0000, if I set MOR 
to 1, then the address shifts to 0xff990000 or 0xff991000 or
 0xff993000. I cannot predict which will be the starting address for the 
next boot.

Since the entire BSS region was not cleaned, and it contains zeros and 
other data, so I tried to increase the boot_stack size by 0x3000 to 
cover the 'fragile' part. 

> My preliminary conclusion here is that your implementation of the EFI
> handover protocol (which I fail to understand where it comes from) is
> not allocating enough memory. This should be fixed on the bootloader
> side, as not clearing the BSS does not prevent this memory from being
> corrupted.

I understand that the handover protocol is nearing the end of its support 
and it seems I am only one experiencing the issue. However,
from the perspective of backward compatibility, I think this patch maybe 
useful.