croemheld / elfconf

elfconf

Tool for manipulating ELF binaries

The elfconf tool was originally designed to patch ELF files from the elfboot project. The goal was to write the sectors from each stage into the ELF to avoid having to search for each stage in assembly.

elfconf is a simple CLI tool which can be used as follows:

Usage: elfconf {-h | -f <filename> -s <symbol> -v <value>}

The value written at the given symbol depends on the symbol size. Symbols in assembly have to specifically use the .size directive (GNU AS) in order to write the correct amount of bytes.

Examples

For the program global.c:

#include <stdio.h>

static int var = 5;

int main(void) {
        printf("Value of var: %d\n", var);
        return 0;
}

the executable global.o will return:

 $ ./global.o
Value of var: 5

By using elfconf, we want to set the value of the variable var to 1337:

 $ elfconf -f global.o -s var -v 1337

Subsequently, the program will now return:

 $ ./global.o
Value of var: 1337

Notes

• It is not possible to set variables in the .bss section of an ELF object, as this section is zeroed out when it is loaded into memory.
• The elfconf tool will only work on symbols that are defined in the particular ELF. It is not possible to manipulate undefined symbols.

Future work

The following features might be added in the near future:

• Specify size to write in arguments
• Modify values via binary operators, e.g. OR, AND, XOR, ...
• ... TBD