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

From: Daniel Kiper <daniel.kiper@oracle.com>
To: Vladimir 'phcoder' Serbinenko <phcoder@gmail.com>
Cc: "jgross@suse.com" <jgross@suse.com>,
	"grub-devel@gnu.org" <grub-devel@gnu.org>,
	"eric.snowberg@oracle.com" <eric.snowberg@oracle.com>,
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	"roy.franz@linaro.org" <roy.franz@linaro.org>,
	"ning.sun@intel.com" <ning.sun@intel.com>,
	"david.vrabel@citrix.com" <david.vrabel@citrix.com>,
	"jbeulich@suse.com" <jbeulich@suse.com>,
	"stefano.stabellini@eu.citrix.com"
	<stefano.stabellini@eu.citrix.com>,
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	<richard.l.maliszewski@intel.com>,
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	"seth.goldberg@oracle.com" <seth>
Subject: Re: [GRUB2 PATCH v3 4/4] multiboot2: Add support for relocatable images
Date: Mon, 14 Mar 2016 12:38:30 +0100	[thread overview]
Message-ID: <20160314113830.GQ3500__17318.7077431523$1457955659$gmane$org@olila.local.net-space.pl> (raw)
In-Reply-To: <CAEaD8JN7AsoaG+cAdxbRbLN8BgDMS0W5JDU9UpFok081tyxx2A@mail.gmail.com>

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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