Elfhack is a program to optimize ELF binaries for size and cold startup
speed.
Presently, it is quite experimental, though it works well for the target
it was created for: Firefox's libxul.so.
Elfhack currently only does one thing: packing dynamic relocations ;
which ends up being a quite complex task, that can be summarized this
way:
- Remove RELATIVE relocations from the .rel.dyn/.rela.dyn section.
- Inject a small code able to apply relative relocations "by hand"
after the .rel.dyn/.rela.dyn section.
- Inject a section containing relocative relocations in a different
and more packed format, after the small code.
- Register the small code as DT_INIT function. Make the small code call
what was initially the DT_INIT function, if there was one.
- Remove the hole between the new section containing relative
relocations and the following sections, adjusting offsets and base
addresses accordingly.
- Adjust PT_LOAD entries to fit new offsets, and add an additional
PT_LOAD entry when that is necessary to handle the discrepancy between
offsets and base addresses, meaning the section offsets may yet again
need adjustments.
- Adjust various DT_* dynamic tags to fit the new ELF layout.
- Adjust section headers.
- Adjust ELF headers.
See http://glandium.org/blog/?p=1177#relocations for some figures.
