Appending program headers to existing ELFs?

[Matheus Afonso Martins Moreira <matheus.a.m.moreira@gmail.com>]

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.

Linux passes a pointer to the program header table
of the running program via the auxiliary vector
which is how I plan on accessing the data.

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.
Given the address of the program header table, it is possible to obtain
a pointer to the ELF header. However, no progress can be made from
there since only the segments described by the program headers are
loaded by the kernel which excludes the sections and the data
I want to access. If I set the alloc flag on the new sections,
objcopy will even warn me about the fact they're not loaded.

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?

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?

I've been experimenting with linker scripts but with no success.