Re: [Xen-devel] [GRUB2 PATCH v3 4/4] multiboot2: Add support for relocatable images

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Mar 11, 2016 at 05:13:19PM +0100, Vladimir 'phcoder' Serbinenko wrote:
> > > > +  if (relocatable)
> > > > +    {
> > > > +      if (base_addr > min_addr)
> > > > +       grub_multiboot_payload_eip += base_addr - min_addr;
> > > > +      else
> > > > +       grub_multiboot_payload_eip -= min_addr - base_addr;
> > > > +    }
> > > > +
> > > >
> > > Why is it relative to min_addr? Sounds like it should be just an offset
> >
> > Ugh... IIRC, it has meaning but I forgot what. I will check it.
> > However, this means that I must put comment here.
> >
>
> Is it possible that you have confused link address and minimal loading 
> address?

Yep, you are right. Fortunately it is quite easy to fix and probably do not
require any changes in Xen image.

> How is entry usually specified in ELF?

IIRC, there is no such thing per se. However, I think that
we should calculate link base address using following formula:

link_base_addr = ~0;

for (i = 0; i < ehdr->e_phnum; i++)
  link_base_addr = min(link_base_addr, phdr(i)->p_paddr);

> How do you suggest it should be done in mb headers?

I think that we can use multiboot_header_tag_address.load_addr
as link_base_addr.

Daniel

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