using BFD to load and reloc a simple file

using BFD to load and reloc a simple file

[thockin]

I am trying to load and relocate a simple ELF executable.  It's entirely
self contained, but needs to be loadable at an arbitrary 32 bit address.

I'm trying to use libbfd for this, but I am having trouble.  The load goes
fine, and it copies the ELF data into memory where I tell it to.  Very
nice and very easy.  When I call perform_relocation(), though, it goes
haywire.

First, my code:

int
load_and_reloc(char *filename, uint8_t *mem)
{
	bfd *payload;
	asection *sect;
	int i;
	long symsize;
	asymbol **symtab;

	bfd_init();

	/* open the payload */
	payload = bfd_openr(filename, NULL);
	if (!payload) {
		fprintf(stderr, "error: can't open %s\n", filename);
		return 1;
	}
	bfd_check_format(payload, bfd_object);

	/* find the magic section */
	sect = bfd_get_section_by_name(payload, ".aseg");
	if (!sect) {
		fprintf(stderr, "error: can't find .aseg section\n");
		return 1;
	}

	/* load the symbol table */
	symsize = bfd_get_symtab_upper_bound(payload);
	if (symsize <= 0) {
		fprintf(stderr, "error: can't get symbol table\n");
		return 1;
	}
	symtab = malloc(symsize);
	if (!symtab) {
		perror("malloc()");
		return 1;
	}
	bfd_canonicalize_symtab(payload, symtab);

	/* load the section into memory */
	bfd_get_section_contents(payload, sect, mem, 0, sect->size);
	bfd_set_section_vma(payload, sect, (unsigned)mem);

	/* do any relocations */
	{
		long rsize;
		arelent **relocs;
		long nrelocs;
		int j;
		char *err;

		rsize = bfd_get_reloc_upper_bound(payload, sect);
		relocs = malloc(rsize);
		if (!relocs) {
			perror("malloc()");
			return 1;
		}
		nrelocs = bfd_canonicalize_reloc(payload, sect, relocs, symtab);
		for (j = 0; j < nrelocs; j++) {
			bfd_perform_relocation(payload, relocs[j],
				mem+(i*1024*1024), sect, NULL, &err);
		}
	}
	return 0;
}

bfd_perform_relocation() eventually does:
	reloc_target_output_section = symbol->section->output_section;

The problem is that output_section is always NULL.  I don't want to output
a BFD, I want to relocate it in memory.

Is this possible?  Am I missing something important?  The BFD docs are
very good in many areas, but the relocation docs are missing plenty of
details.

Help?

Tim

p.s. I am not on this mailing list, can you plase CC: me on replies? :)

Re: using BFD to load and reloc a simple file

[Daniel Jacobowitz]

On Thu, Mar 02, 2006 at 01:15:18PM -0800, thockin@hockin.org wrote:
> I am trying to load and relocate a simple ELF executable.  It's entirely
> self contained, but needs to be loadable at an arbitrary 32 bit address.
> 
> I'm trying to use libbfd for this, but I am having trouble.  The load goes
> fine, and it copies the ELF data into memory where I tell it to.  Very
> nice and very easy.  When I call perform_relocation(), though, it goes
> haywire.

That's because this is a routine for the linker, not a routine for
in-place relocation.

> bfd_perform_relocation() eventually does:
> 	reloc_target_output_section = symbol->section->output_section;
> 
> The problem is that output_section is always NULL.  I don't want to output
> a BFD, I want to relocate it in memory.

You might be able to set the section to point at itself.  However, I'm
not sure if bfd_perform_relocation even does the operation you want it
to.

-- 
Daniel Jacobowitz
CodeSourcery

Re: using BFD to load and reloc a simple file

[thockin]

On Thu, Mar 02, 2006 at 04:18:00PM -0500, Daniel Jacobowitz wrote:
> On Thu, Mar 02, 2006 at 01:15:18PM -0800, thockin@hockin.org wrote:
> > I am trying to load and relocate a simple ELF executable.  It's entirely
> > self contained, but needs to be loadable at an arbitrary 32 bit address.
> > 
> > I'm trying to use libbfd for this, but I am having trouble.  The load goes
> > fine, and it copies the ELF data into memory where I tell it to.  Very
> > nice and very easy.  When I call perform_relocation(), though, it goes
> > haywire.
> 
> That's because this is a routine for the linker, not a routine for
> in-place relocation.

Is there an in-place variant?

> > bfd_perform_relocation() eventually does:
> > 	reloc_target_output_section = symbol->section->output_section;
> > 
> > The problem is that output_section is always NULL.  I don't want to output
> > a BFD, I want to relocate it in memory.
> 
> You might be able to set the section to point at itself.  However, I'm
> not sure if bfd_perform_relocation even does the operation you want it
> to.

I tried that and it stopped finding any relocations at all!  I did not
debug further.

I'm not sure that I am doing what I want to do most effectively.  The end
goal is a small self-contained executable with a few sections that will be
located into memory at some system-specific addresses (which can not be
known at build time).

I've succeeded in making a static binary that works for a given fixed
address.  Now I need to make it relocateable.  Linking with -r seems to
give me the binary I want (though I am not sure it would catch
unresolvable references anymore).

Would I be better off using -pie or some other method?

> -- 
> Daniel Jacobowitz
> CodeSourcery

-- 
Tim Hockin
thockin@hockin.org
Soon anyone who's not on the World Wide Web will qualify for a government 
subsidy for the home-pageless.

Re: using BFD to load and reloc a simple file

[David Daney]

thockin@hockin.org wrote:

> I'm not sure that I am doing what I want to do most effectively.  The end
> goal is a small self-contained executable with a few sections that will be
> located into memory at some system-specific addresses (which can not be
> known at build time).
> 
> I've succeeded in making a static binary that works for a given fixed
> address.  Now I need to make it relocateable.  Linking with -r seems to
> give me the binary I want (though I am not sure it would catch
> unresolvable references anymore).
> 
> Would I be better off using -pie or some other method?
> 
> 
You could take a look at the 2.6.x Linux kernel or the insmod program 
for kernel 2.4.x.  They both load and relocate elf objects and are GPL.

David Daney

Re: using BFD to load and reloc a simple file

[Daniel Jacobowitz]

On Thu, Mar 02, 2006 at 01:54:14PM -0800, thockin@hockin.org wrote:
> I've succeeded in making a static binary that works for a given fixed
> address.  Now I need to make it relocateable.  Linking with -r seems to
> give me the binary I want (though I am not sure it would catch
> unresolvable references anymore).
> 
> Would I be better off using -pie or some other method?

The best way to do this is link it as a shared object, and then
resolve dynamic relocations (of which there are many fewer).  You
can avoid BFD entirely.

-- 
Daniel Jacobowitz
CodeSourcery