elfcopy

A tool for copying ELF binaries. The most notable feature is the implementation of the --redefine-syms from objcopy that allows for renaming dynamic symbols in a shared library.

99% of this code comes from the object crate's "elfcopy" example. As such, most of the credit goes to the original author, philipc.

Usage

# Usage: target/debug/elfcopy [--redefine-syms <file>] <infile> <outfile>
#
$ cargo run --redefine-syms mangle-list.txt libfoo.so liboutput.so

Where mangle-list.txt is of the form:

original_name new_name
original_name1 new_name1
original_name2 new_name2
...

Redefining symbols in a shared library

Renaming symbols in dynamic shared objects is difficult, there are a couple things that have to be done:

• Actually rename the symbol, updating the .dynstr section and setting the new offset in the symbol's entry in .dynsym. (medium difficulty)
• Regenerate the hashes for dynamic symbols (mostly easy)
• Recalculate all offsets (very hard)

Tools such as objcopy and ELF toolkits usually only implement reading dynamic ELF objects due to the difficulty of the third point, and writing or modifying is not allowed. The authors of the object crate bailed the rest of us out by actually having support for writing DSOs, and the elfcopy example was created to demonstrate this.

The base elfcopy reads a binary into a series of in_* variables that contain the string/symbol tables, section information, etc. These are then used to initialize out_* variables in the copy_file function to construct a copy of the data. The out_* data is then used to generate a valid ELF binary, including recalculating all memory offsets as part of generating.

The redefinition step occurs while filling in the out entries from the in data. Symbols are looked up in a table to find their redefined name, and that is inserted into out instead of the original name.