Re: Appending program headers to existing ELFs?

[Nick Clifton <nickc@redhat.com>]

Hi Matheus,

> I'd like to embed arbitrary data into existing ELF executables
> in such a way that the data is automatically loaded onto memory
> and is reachable by the program at runtime.

I have to say that this does sound suspicious.  Why can't you add
the data at link time ?


> With objcopy it is already possible to embed arbitrary files
> to ELF files by placing them into sections:
> 
>      objcopy --add-section program.file1=file1 \
>              --add-section program.file2=file2 \
>              program program+files
> 
> However, those sections are unreachable when the program actually runs.

Hmm, could you create a new symbol for the section and reference the data
that way.  eg:

   objcopy \
     --add-section foo=file1 \
     --add-symbol foo_start=foo \
     --set-section-flags foo=alloc,load \
     a.out a.extra

This assumes that you can access the symbol table at runtime.  I have
no idea if this is true for kernel images however.


> So it seems to me that the solution is to add a new program header
> to the ELF executable instead of a new section. How can this be done
> with binutils tools?

At the moment this can only be done by the linker, and it will only work
if the input files are object files or relocatable binaries.  ie if a
final link has been performed then you are not going to be able to add
new program header entries using the binutils tools.  (At least not as
they are today).


> If it's not possible to do this with objcopy, can it be done with ld?
> Is it possible to add a placeholder program header table entry at link
> time so that it can be adjusted later by external tools to encompass the
> newly added sections without needing to relink the whole executable?

Possibly.  With a linker script you can layout a PHDRS structure which
describes the entries that should go into the program header.  So in
theory you can include an extra entry for use by later tools.  I do not
know if this has ever been done in practice however, so it may be that
there are unknown bugs waiting to be discovered.

Cheers
   Nick