Relocate ELF symbols

Question

How can I apply relocations of symbols in an elffile? I'm currently trying to archive this with pyelftools. Strangely, I could hardly find any information on how to do this, although some projects have to implement something like this (e.g. ida, angr, amoco, ...).

In other words: I want to know where elf imports will be located in virtual memory. Of course, the sections should adhere to these relocations.

Code snippets very appreciated.

Answer 1

So after some toying around with elftools, I found the following solution for import-symbols:

    for section in self._elf.iter_sections():
        if not isinstance(section, RelocationSection):
            continue
    symtable = self._elf.get_section(section['sh_link'])
    for relocation in section.iter_relocations():
        if not relocation['r_info_sym']:
            continue
        symbol = symtable.get_symbol(relocation['r_info_sym'])
        yield ImportSymbol(relocation['r_offset'], symbol.name)

The code is inspired by pyelftools readelf implementation

Although pyelftools offers some relocation handling, I've not yet seen a binary for which that actually works. It will complain it doesn't know some relocation types, but for the future, I'll post this snippet:

def _reloc_section(self, section):
    rh = RelocationHandler(self._elf)
    data = section.data()
    relocation_section = rh.find_relocations_for_section(section)
    if relocation_section:
        try:
            rh.apply_section_relocations(data, relocation_section)
        except Exception as e:
            print 'Could not relocate %s' % section.name
    return data