Re: Multiboot ELF Loader - Alignment

[Daniel Kiper]

Hey,

On Thu, Oct 21, 2021 at 08:46:06PM +0100, Chris Plant wrote:
> Hello,
>
> I'm continuing to play with Multiboot2 on ARM64 on a Raspberry Pi, and
> I've run into something which I'm trying to understand.

Why do you use Multiboot2 on ARM64?

> I have an ELF file for my kernel which has two segments (I have no idea
> why rust is giving me two segments, but it is).  If I boot directly
> into the ELF file from the Pi firmware it boots fine, but if I boot via
> GRUB I have issues with data corruption in .rodata which is in the
> second segment.  The first segment (.text) appears to load correctly.

Could you share the output from "readelf -Wa <kernel>"? Additionally,
how is your Multiboot2 header defined in the kernel?

> Digging into this, the ELF headers specify a load base address of
> 0x801060 for the second segment, but GRUB allocates and loads this to
> 0x802000.  If I change my linker to align the second segment onto
> 0x1000 everything works fine.

The Multiboot2 protocol does not tolerate "holes" in the image. You can
find good explanation what I mean by that here [1].

> Is this alignment to 0x1000 a defined behaviour of the GRUB allocator
> or Multiboot2?  I'm assuming it's not a problem with the ELF file as
> it's generated by usual means and runs fine otherwise.

I think you should take closer look at grub-core/loader/multiboot_elfxx.c
and especially lines starting from 258. It seems to me something around
here overwrites part of the image in the memory.

Daniel

[1] https://lists.xenproject.org/archives/html/xen-devel/2021-05/msg01003.html