This started as a playground for elf related stuff. I created a bash script to generate a tiny working elf file. Just using bash. Useless for anything else than learn how elf files work. But as it grows up started to look like a compiler. Then I started to define my own source file syntax. Maybe some day I can call it my own programming language. For now it is just a prototype.
In early tests it did work on native linux x86_64 and in a aarch64(osx, using ubuntu within docker container). Currently it is only working on x86_64. I pretend to update the aarch64 latter. But first I want to bootstrap it in x86_64 without using bash anymore.
Executable and Linkable Format(ELF) are the format used in all linux executables and libraries.
This repo is my try to write them from scratch. For fun and learning.
I did created a very simple debugger. Look at makefile. And you can use gdb to debug the execution. Here some tips
- Use objdump -x to get the start address put a break point at the start address:
br *0x010078- use si to step each instruction
- you can move the program counter to read any virtual address by using:
set $pc=0x00010168Print string at some address:
(gdb) p/x ($rdi)
$33 = 0x1013e
(gdb) x/s 0x1013e
0x1013e: "sample-code.gg"
argcis$rsp
(gdb) print *((int*)$rsp)
- when not in first backtrace, after a call like functions or procedures
argvis$rsp + 8
(gdb) print *((char**)($rsp + 8))
after copy to a mem address (mov %rsp, *0x000100b6; # 48892425b6000100)
(gdb) p *(void**)(*(void**)0x0100b6+8)
$43 = (void *) 0x3
(gdb) p *(char**)(*(void**)0x0100b6+16)
$44 = 0x7fffffffe6ff "/home/glaudiston/src/gelf/tests/test_arg_count.elf"
(gdb) p *(char**)(*(void**)0x0100b6+24)
$45 = 0x7fffffffe732 "abc"
(gdb) p *(char**)(*(void**)0x0100b6+32)
$46 = 0x7fffffffe736 "def"
There is some refs on the code. Read it.
https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html
https://anee.me/reversing-an-elf-from-the-ground-up-4fe1ec31db4a
https://reverseengineering.stackexchange.com/questions/1992/what-is-plt-got/1993#1993?newreg=360ad3611f2041ed84fa064d413f5de1
https://www.technovelty.org/linux/plt-and-got-the-key-to-code-sharing-and-dynamic-libraries.html
Maybe someday this can be a new lang... then here I go with some notes about it
- profiling/telemetry oriented
- debugging oriented
- binary updatable at real time
- high level user friendly interface
- allowing automation / text mode control
- portable, but what if emulated in any env ?
- what if a language can detect the target algorithm and suggest the best one ?
To discover bytecode one good way is create an asm file with the instruction, compile it and use ld with the --oformat binary:
echo "mov %r8, %r10" | as -al -o /dev/null -
GAS LISTING page 1
1 0000 4D89C2 mov %r8,%r10
to disasm a bytecode to the asm instruction:
echo -ne "\x48\x89\xc2" | ndisasm -b 64 -
00000000 4889C2 mov rdx,rax