Re: Multiboot2 on aarch64: Alignment of ELF Headers

[Hans Ulrich Niedermann <hun@n-dimensional.de>]

On Sat, 23 May 2020 12:21:27 +0100
Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:

> On Sat, 2020-05-23 at 12:43 +0200, Hans Ulrich Niedermann wrote:
> > On Fri, 22 May 2020 17:23:35 +0100
> > Chris Plant via Grub-devel <grub-devel@gnu.org> wrote:
> >   
> > > I'm continuing to work on Multiboot2 support on aarch64, and I'm
> > > looking at the alignment of the ELF headers which are passed
> > > through
> > > MB2.  
> > 
> > At the risk of me sounding stupid...
> > 
> > Having read the MB2 specs quite thoroughly in the past few months, I
> > still have no idea what "ELF headers" being "passed through MB2"
> > could
> > be about. The MB2 spec defines a MB2 header consisting of a four MB2
> > header magic fields and a set of MB2 header tags.
> > 
> > The only thing ELF related I could find there is the MB2 header
> > address
> > tag which duplicates some information from ELF (if OS image is in
> > ELF format) or contains address information (if OS image is in
> > non-ELF format). Is that what you are talking about?  
> 
> Yes, this is what I'm talking about.  I'm using the MB2 tag (type 9)
> to see where the ELF section headers are.

Ah. So the type 9 you are referring to is the "ELF-Symbols" boot
information tag (i.e. information the bootloader communicates to the OS
image), not an MB2 header tag (information the OS image communicates to
the bootloader).

I am catching up with you, finally.

> Apologies, I've not been clear enough and rambled too much.  The issue
> I see is nothing to do with grub's MB2 implementation, but instead in
> the way grub is loading the ELF section headers.
> 
> The ELF section headers appear to be loaded on a 4 byte alignment not
> an 8 byte alignment.  You can read the 4 byte members of the ELF
> header structure fine, but not the 8 byte members as they are aligned
> to 4 bytes.
> 
> So, my question is, should grub load the ELF headers to an 8 byte
> alignment, or should the OS be prepared for them to be aligned to 4
> byte?

The 64bit part of the ELF spec appears to specifically made such that
64bit values are aligned to 64bit boundaries (I have taken a look at
https://en.wikipedia.org/wiki/Executable_and_Linkable_Format for an
overview).

Therefore, loading an ELF64 image with its many 64bit values to an
address not aligned to 8 byte boundaries sounds to me like a probable
oversight going back to ELF32 days and thus a bug for ELF64, unless
someone has a very good argument to the contrary.

Uli