How to convert an ELF executable to C code? The generated C code need not be human-readable

Question

I have an ELF file that I would like to decompile into C code, and make simple changes to the resulting C code and rebuild it into an ELF.

The decompiled C code need not be fully human readable. Eg, if variables and function names come out obfuscated, it is okay.

Which tools can I use to accomplish this on Linux?

PS: If decompiling to C is not possible or is not easy, I'm willing to consider decompiling to assembly language, though tweaking the assembly source will be very difficult for me.

UPDATE: You may assume that I'm using the following C program to get my a.out ELF. Now, assume further that I've lost this original C source. So, I would now like to decompile it to (a possibly obfuscated) C source in which I'm at least able to change small things like the strings "world", "Hello", and "Bye", or be able to reverse the sense of the if statement, etc.

#include <stdio.h>
#include <string.h>

char buf[256];

const char *Hello = "Hello";
const char *Bye = "Bye";
const char *Who = "world";

char * greet(const char *greeting, const char *str) {
    strcpy(buf, greeting);
    strcat(buf, ", ");
    strcat(buf, str);
    strcat(buf, "!");
    return buf;
}

int main(int argc, char *argv[]) {
    int sayHello = 0;

    if(sayHello) {
        printf("%s\n", greet(Hello, Who));
    } else {
        printf("%s\n", greet(Bye, Who));
    }
    return 0;   
}

Answer 1

This will give you (almost) an assembly code translation:

objdump --disassemble <elf file>

I say almost because the output contains some annotations like binary file position markers and can't serve directly as input to an assembler, but it's close.

Answer 2

You write a simulator for the processor, then you run the elf instructions through your simulated processor. It's generally not too much of a task because even CISC processors have relatively small, contained instruction sets that perform simple operations.

When you've got that working, look at more efficient solutions, like outputting C code to match instructions.