[GSOC2013] Dynamic object file loading

Chris Johns chrisj at rtems.org
Thu Apr 18 23:04:09 UTC 2013

Peng Fan wrote:
>     There are symbols in main.c that understand the tar file. Is
>     something wrong with them ?
>   I looked insight into the code and found that
>   the fs-root.tar is untared by setup_rootfs->Untar_FromMemory.
>   Is it becasue the tar version on my host pc does not match Untar_* in
>   I follow the compilation log to clarify how waf works and how
> fs-root.tar is compiled into rtld.look at the following:
> 1. shell-init libx.a -> fs-root.tar -> fs-root-tarfile.o

Yes using objcopy.

> 2. init.c.5.o main.c.5.o fs-root-tarfile.o -lrtl -> rtld.prelink

Link it together.

> 3. nm rtld.prelink -> rtld-gsyms.c

Get the symbols present in the base image.

> 4. init.c main.c rtld-gsyms.c xa.c x-long-name...-in-archive.c
>    -> x.rap

Create the RAP app.

> 5. shell-init libx.a x.rap -> fs-root.tar -> fs-root-tarfile.o

Build the tar file.

> 6. init.c.10.o main.c.10.o fs-root-tarfile.o  rtld-gsyms.10.o -lrtl -> rtld

Relink with the symbol table generated so the base image has its own 
symbol table.

> I also objdump the code to see if any address is wrong, but all is ok.
> Now what I doubt is only verison mismatch.


> A question that I want your answer, is the rtld.prelink only used for
> generating rtld-gsymcs.c? I think it is, but not sure.

Yes. You need to perform a link so the needed symbols are pulled into 
the kernel. Once you have these symbols you can generate a symbol table 
and then relink. The original base image has a weak symbol which is 
overridden with the real symbol table.

> When modified the rtems.py with "d.endswith('-rtems*eabi*' + version)",

The RTEMS tools are arm-rtems4.11. As far as I know there is no need for 
'eabi' to be present in the tools any more.

> It tells "error: libelf:elf_getdata: .ARM.attributes(xa.c.8.o): Invalid
> section descriptor"

This is the bug.

> I may dig into rtl-host code to figure out why and this may be part of
> the work that I should focus on

Thanks. It will be in the ELF section parsing somewhere.

>     The project need work to implement more architectures. I am
>     traveling at the moment and will be back later in the week. A
>     proposal to work on these other archs would be welcome.
> I want to implement arm support first. If time permits, I will implement
> mips too.
> Now I can not evaluate the workload whether I can finish one, two or
> more. I am preparing the proposal and it'll be finished soon.



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